Cisco Notes
ACE 5, Semester 1; 3/12/04
CCNA Networking Basics v2.1.4
------------------------------------
CCNA1 CH 1:
Networking Basics
Transistor: amplifies a signal or opens and closes a circuit.
Integrated circuit (IC): made of
semiconductor material; contains many transistors and performs a specific task.
Resistor: made of material which
restricts the flow of electric current.
Capacitor: electronic component that
stores energy in the form of an electrostatic field; it consists of two
conducting metal plates separated by an insulating material.
(LED) is a semiconductor device
which emits light when a current passes through it.
Personal Computer Subsystems:
Backplane Components:
Information Flow:
NIC: network
interface card communicates with the network through a serial connection, and with the computer through a parallel connection. Each card requires
an:
·
interrupt request
(IRQ)
·
input/output (I/O)
address
·
device driver
software to work with OS
I/O address:
a location in I/O address space that is used to uniquely select the auxiliary
device and communicate with it. In DOS-based systems, upper memory refers to
the memory area between the first 640 kilobytes (KB) and 1 megabyte (MB) of
RAM.
In order to perform the installation,
you should have the following resources:
EPROM (electrically erasable programmable
read-only memory): is user-modifiable read-only memory (ROM)
that can be erased and reprogrammed (written to) repeatedly through the
application of higher than normal electrical voltage. Unlike EPROM
chips, EEPROMs do not need to be removed from the computer to be modified.
However, an EEPROM chip has to be erased and reprogrammed in its entirety, not
selectively. It also has a limited life - that is, the number of times it can
be reprogrammed is limited to tens or hundreds of thousands of times. In an
EEPROM that is frequently reprogrammed while the computer is in use, the life
of the EEPROM can be an important design consideration.
Command-line network diagnostic utilities:
WINIPCFG.EXE
98/ME
IPCONFIG.EXE:
Win NT/2000/XP/Server
IFCONFIG: Linux, Mac
8 bits =1 byte = a single character of
data (ASCII)
1 byte represents a single addressable
storage location.
2134 = 2134/2, record remainder, keep dividing until
8 binary digits reslut
10110 = (1 x 24 = 16)
+ (0 x 23 = 0) + (1 x 22 = 4) + (1 x 21
= 2) + (0 x 20 = 0) = 22 (16 + 0 + 4
+ 2 + 0) à
22
LAN = 10m to 1km
WAN = 100km to 100,000km
LAN Devices: Router, Bridge, Ethernet
Switch, ATM Switch, HUB
WAN Devices: Router, Comm. Server, Modem CSU/DSU TA/NT1,
WAN Bandwidth Switch
Typical Media/Bandwidth/Max. Physical Distance:
50-Ohm Coaxial Cable = 10-100Mbps =
185m = Ethernet 10Base2, ThinNet
50-Ohm Coaxial Cable = 10-100Mbps =
500m = Ethernet 10Base5, ThickNet
Cat 5 UTP = 10Mbps = 100m = Ethernet
10BaseT
Cat 5 UTP = 100Mbps = 100m = Ethernet
100BaseT, Fast Ethernet
Multimode (62.5/125um) = 100Mbps =
2000m = Optical Fiber 100BaseFX
Singlemode (9/125um core) = 1000Mbps
(1Gbps) = 3000m = Optical Fiber 1000BaseLX
Wireless = 11Mbps = few 100meters
Frame-Relay = 56k to 1544kbps
T1 = 1.544Mbps
T3 = 44.736Mbps
E1 = 2.048Mbps
E3 = 34.368Mbps
STS-1 (OC-1) = 51.840Mbps
STS-3 (OC-3) = 155.251Mbps
STS-48 (OC-48) = 2.488320 Gbps
Best Download T = S/BW
Typical Donwload T = S/P
BW = max theoretical bandwidth of
“slowest link” between source and destination
P = actual throughput at the moment of
transfer (bps)
T = Time for a file transfer to occur
(s)
S = File size in bits
-------------------------------------
Base 2:
2^7 = 128 ; 2^6 = 64
Binary to Decimal Conversion:
Convert 1 0 1 1 0 to decimal:
(0 x 2^0) + (1 x 2^1) + (1 x
2^2) + (0 x 2^3) + (1 x 2^4) =
0 + 2 + 4 + 0 + 16 à 22
Decimal to Binary Conversion:
Convert
the decimal number 192 to a binary
number.
|
192/2 |
= |
96 |
with a
remainder of |
0 |
|
96/2 |
= |
48 |
with a
remainder of |
0 |
|
48/2 |
= |
24 |
with a
remainder of |
0 |
|
24/2 |
= |
12 |
with a
remainder of |
0 |
|
12/2 |
= |
6 |
with a
remainder of |
0 |
|
6/2 |
= |
3 |
with a
remainder of |
0 |
|
3/2 |
= |
1 |
with a
remainder of |
1 |
|
1/2 |
= |
0 |
with a
remainder of |
1 |
Write down all the remainders,
backwards, and you have the binary number 11000000.
----------------------------------------------
CCNA1 CH 2:
The OSI Model
2.1 General Model
Of Communication
2.2 the OSI Reference Model
2.3 Comparison of
the OSI Model and the TCP/IP Model
OSI Model: 1984:
primary model for network
communications; framework for how information travels throughout a network;
used to visualize how information, or data packets, travel from applications like
spreadsheets and documents, through a network medium.
7 Layers of the OSI Model; provides these advantages:
Layered Network Model:
http://www.rad.com/networks/1994/osi/osi.htm
7 Layers of OSI Model:
Layer 7: application layer
Layer 6: presentation layer
Layer 5: session layer
Layer 4: transport layer
Layer 3: network layer
Layer 2: data link layer
Layer 1: physical layer
Layer 7: The Application Layer
Closest to the user; provides network services to the user's apps; differs from
the other layers in that it doesn’t provide services to any other OSI
layer, but rather, only to applications outside the OSI model (i.e. spreadsheet
apps, word processing apps, and bank terminal programs); establishes the
availability of intended communication partners, synchronizes and establishes
agreement on procedures for error recovery and control of data integrity.
-think of browsers
Layer 6: The Presentation Layer
Ensures that the information that the application layer of one system sends out
is readable by the application layer of another system; translates between multiple data formats by using a common format;
- think of a common data format
Layer 5: The Session Layer
Establishes, manages, and terminates sessions
between two communicating hosts; provides its services to the presentation
layer; synchronizes dialogue between
the two hosts' presentation layers and manages their data exchange; offers provisions for efficient data transfer, class
of service, and exception reporting
of session layer, presentation layer, and application layer problems.
- think of dialogues and conversations.
Layer 4: The Transport Layer
Segments data from the sending
host's system and reassembles the
data into a data stream on the receiving host's system. The boundary between
the transport layer and the session layer can be thought of as the boundary
between application protocols and data-flow
protocols. Whereas the top 3 layers are concerned with application issues, the
lower 4 layers are concerned with data transport issues.
Attempts to provide a data transport
service that shields the upper layers from transport implementation details. Issues
such as how reliable transport between two hosts are accomplished. In providing
communication service, the transport layer establishes, maintains, and
properly terminates virtual circuits. In providing reliable service, transport error detection-and-recovery and information flow control are used.
- think of
quality of service, and reliability
Layer 3: The Network Layer
a complex layer that provides connectivity and path selection between two host
systems that may be located on geographically separated networks; concerned
with IP addresses.
- think of
path selection, routing, and logical addressing
Layer 2: The Data Link Layer
Provides reliable transit of data across a physical link; concerned with physical (as opposed to logical) addressing, network topology, network
access, error notification, ordered delivery of frames, and flow control.
- think of frames
and media access control
Layer 1: The Physical Layer
Defines the electrical, mechanical, procedural, and functional specifications
for activating, maintaining, and deactivating the physical link between end systems;
characteristics such as voltage levels, timing of voltage changes, physical
data rates, maximum transmission distances, physical connectors, and other,
similar, attributes are defined by physical layer specifications.
- think of
signals and media
Encapsulation
wraps data with the necessary protocol information before network transit.
Therefore, as the data packet moves down through the layers of the OSI model,
it receives headers, trailers, and other information.
Encapsulation Process:
Protocol data units (PDU): During this
process, the protocol of each layer exchanges information, called (PDUs),
between peer layers. Each layer of communication on the source computer
communicates with a layer-specific PDU
TCP/IP Model: Created
by
Application Layer
Transport Layer
Internet Layer
Network Access Layer
Application:
FTP, TFTP, HTTP, SMTP, DNS, TELNET, SNMP
Presentation/Session: Very little
foucs
Transport: TCP
Network: IP
Data Link/Physical: Ethernet, LAN
TCP/IP Model:
Application Layer (FTP/HTTP/SMTP/DNS/TFTP)
The designers of TCP/IP felt that the higher level protocols should include the
session and presentation layer details. This layer handles high level protocols, issues of representation, encoding, and
dialog control. The TCP/IP combines all issues related to application into one
layer, and assures this data is properly packaged for the next layer.
Transport Layer (TCP/UDP)
Deals with the quality of service issues of reliability,
flow control, and error correction. One of its protocols,
TCP, provides excellent and flexible ways to create reliable, well-flowing,
low-error network communications. TCP is a connection-oriented
protocol; dialogues between source and destination while packaging application
layer information into units called segments. Connection-oriented does not mean
that a circuit exists between the communicating computers (see circuit
switching); mean Layer 4 segments travel
back and forth between two hosts to acknowledge the connection exists logically
for some period (packet switching).
Internet Layer (IP)
Sends source packets from any network on the internetwork and has them arrive
at the destination independent of the path and networks they took to get there.
IP protocol governs this layer; best
path determination and packet switching occur at this layer. Think of it in
terms of the postal system (you do not know how it gets there [multiple
routes], but you do care that it arrives).
Network Access Layer (Internet, Your LAN, Many LANs
and WANs)
Also called the host-to-network layer;
concerned with all of the issues that an
IP packet requires to actually make a physical link; includes the LAN and WAN
technology details, and all the details in the OSI physical and data link
layers.
TCP (Transmission Control Protocol): FTP, HTTP,
SMTP, DNS, TFTP
UDP (User Datagram Protocol)S: DNS, TFTP
If you compare the OSI model and the
TCP/IP model, you will notice that they have similarities and differences.
Examples include:
Similarities
Differences
TCP/IP Model vs. OSI Model:
TCP/IP:
Protocols = Application +
Transport Layers
Networks = Internet + Network
Access Layers
OSI:
Application Layers =
Application, Presentation, Session
Data Flow Layers = Transport,
Network, Data Link, Physical
Lab Exercise:
Match OSI Layer Protocols and Devices
Application Layer: FTP, HTTP, Redirector
Presentation Layer: JPEG, Encryption , EBCDIC, ASCII
Session Layer: Dialogue Control, NFS, Checkpoint,
Synchronization
Transport Layer: Sliding Windows, Acknowledgment, Sequencing,
Segment
Network Layer: IP Address, Packet, Router
Data Link layer: MAC Address, LAN Topologies, Switch, Frame
Physical Layer: Cabling, Hub, Repeater, Bits
http://www.ethermanage.com/ethernet/ethernet.html
Chapter 2 Quiz:
1) Which layer of the OSI
model establishes, maintains, and terminates connections between applications?
NOT: data link, network, or presentation
2) Which of the following is the Layer 4 PDU?
NOT: bit, frame, or packet
3) Which layer of the OSI model is responsible
for reliable end-to-end network communications?
NOT: application, network, physical
4) Which of the following best describes the
function of the presentation layer?
NOT: it manages data exchange between layer
entities
NOT: it provides connectivity and path selection
between two end systems
NOT: it is responsible for the reliable network
connection between end nodes
5) Which of the following describes the function
of the data link layer?
NOT: best path selection
NOT: establishment and maintenance of virtual
circuits
NOT: data exchange between presentation layer entities
6) All of the follwing protocols use the
services provided by TCP except:
NOT: FTP, HTTP, or SMTP
7) Which application is common to both TCP and
UDP in the TCP/IP reference model?
NOT: FTP, HTTP, or SMTP
8) All of the following are defined by physical
layer specifications EXCEPT:
NOT: voltage levels, media connection types, or maximum transmission distances
9) Which OSI model layer provides packet
encapsulation service to Layer 4?
NOT: data link layer, physical layer, or transport
layer
-----------------------
CCNA1 CH 3:
Networking Basics
Local Area Networks (LANs)
3.1 Basic LAN
Devices
3.2 Evolution of
Network Devices
3.3 Basics of Data
Flow Through LANs
3.4 Building LANs
Topology:
Logical topology: how the hosts communicate across the medium. The two most
common types of logical topologies are broadcast and token passing.
Broadcast topology
simply means that each host sends its data to all other hosts on the network
medium
Token-passing
controls network access by passing an electronic token sequentially to each
host
Media:
Token Ring
FDDI Ring
Ethernet Line
Serial Line
5-4-3 Rule,
when extending LAN segments. This rule states that you can connect five network
segments end-to-end using four repeaters but only three segments can have hosts
(computers) on them.
The first classification is active or passive hubs. Most modern
hubs are active; they take energy from a power supply to regenerate network
signals. Some hubs are called passive
devices because they merely split the signal for multiple users, like using a
"Y" cord on a CD player to use more than one set of headphones.
Passive hubs do not regenerate bits, so they do not extend the cable length.
They simply allow two or more hosts to connect to the same cable segment.
Another classification of hubs is intelligent or dumb. Intelligent hubs
have console ports, which means they can be programmed to manage network
traffic. Dumb hubs simply take an incoming networking signal and repeat it to
every other port without the ability to do any management.
The role of the hub in a Token Ring
network is played by a Media Access Unit
(MAU). Physically it resembles a hub, but token-ring technology is very
different, as you will learn later. In FDDIs, the MAU is called a concentrator.
MAUs are also Layer 1 devices.
Bridge: is a Layer 2(Data Link) device designed
to connect two LAN segments. The purpose of a bridge is to filter traffic on a
LAN, to keep local traffic local, yet allow connectivity to other parts
(segments) of the LAN for traffic that has been directed there. the bridge keeps track of which MAC addresses
are on each side of the bridge and makes its decisions based on this MAC address list. What really defines a bridge is its Layer 2
filtering of frames and how this is actually accomplished.
Layer
2 Devices: NIC, Bridge, Switch,
Switch: a multiport bridge,
just as a hub is called a multiport repeater; switches make decisions based on
MAC addresses and hubs do not make decisions; "switching" data only
out the port to which the proper host is connected; hub will send the data out
all of its ports;
AUI is a transceiver
that converts one type of signal or connector to another. To connect, for
example, a 15-pin AUI interface to an RJ-45 jack
Router: operates at the OSI network Layer 3; the router to make
decisions based on network addresses as opposed to individual Layer 2 MAC
addresses; can also connect different
Layer 2 technologies, such as Ethernet, Token-ring, and FDDI. Purpose is to examine the Layer 3 addresses
of incoming packets, choose the best path for them through the network, and
then switch them to the proper outgoing port.
Segment: identifies the Layer 1 media that provide the common path
for data transmission in a LAN. Each time a Layer 2 or Layer 3 device is used
to extend the length or manage data on the media a new segment is created.
Cisco commonly defines a segment as
a collision domain.
Third definition for segment describes a Layer 4 PDU (Protocol Data Unit).
Important
Dates:
1890: Bell invents telephone
1901: Marconi’s first transatlantic
wireless transmission
1920s: AM Radio
1939: FM Radio
1940s: WWII spurs radio and microwave
development
1947: Shockley, Barden and Brittain
invent the solid-state (semiconductor) transistor
1948: Claude Shannon publishes “A Theory
of Electronic Communication”, perhaps the most important paper on communication
1950s: Invention of Integrated Circuits
1960s: Mainframe Computing
1962: Paul Baran at RAND works on
“packet switching” networks
1967: Larry Roberts publishes first
paper on ARPANET
1969: ARPANET established at UCLA, UCSB,
U-Utah, and Stanford
1972: Ray Tomlinson creates program to
send messages
1970s: Widespread use of digital
integrated circuits; advent of digital personal computers
1973: Bob Kahn and Vint Certf begin work
on what later becomes TCP/IP
1982: The term Internet is assigned to a
connected set of networks
1980s: Widespread use of personal
computers and Unix-based mini-computers
1982: ISO releases OSI Model and
protocols; the protocols die but the model is very influential
1984: Domain Name Service introduced
1991: Tim Berners-Lee develops code for
WWW
1993: Mosaic, the first GUI browser, is
uintroduced
1994: Netscape Navigator introduced
1990s (Late): Internet users doubling
everty 6 months
1998: Cisco hits 70% of sales via
internet, Networking Academies launched
1999: Major corporations race toward the
video, voice and data convergence
Layers
1 – 7: hosts and servers operate at this
layer; clouds
Layer 1 Devices: Transceivers, repeaters, hubs
Passive Layer 1 components: patch cables, patch panels, and other
interconnection components; NICs
Layer 2 Devices: NICs, Bridges, Switches
Layer 3 Devices: Routers
NICs
are considered Layer 2 devices since
they are the location of the MAC address. However, since they often handle
signaling and encoding they are also Layer 1 devices. Bridges and switches are
considered Layer 2 devices because they use Layer 2 (MAC address) information
to make decisions on whether or not to forward frames. They also operate on
Layer 1 in order to allow bits to interact with the media.
Routers
(layers 1,2,3) are considered Layer 3
devices because they use Layer 3 (network) addresses to choose best paths and
to switch packets to the proper route. Router interfaces operate at Layers 2
and 1 as well as Layer 3. Clouds, which may include routers, switches, servers,
and many devices we have not yet introduced, involve Layers 1-7.
Chapter 3 Quiz:
1) Which protocol data units are forwarded by a router?
Answer:
packets NOT: bits, frames,
segments
2) What device performs the role of
a hub in a token-ring network?
Answer:
MAU NOT: router, switch, repeater
3) What is the topology if one
central hub has four hubs connected to each of those four hubs has four
workstations attached to it?
Answer: an extended star NOT:
bus, ring, star
4) Which of the following is a
reason that hubs are considered Layer 1 devices?
Answer:
They deal only with bits NOT: they encode data as bits, they control access
to the shared media, they perform parity checks on the bit stream
5) Which statements regarding
switches is correct?
Answer: Switches combine the connectivity
of a hub with the traffic regulation of a bridge
6) Which networking device can make
traffic forwarding decisions based IP addressing?
Answer:
Router NOT: bridge, hub, MAU
7) A ‘networking cloud’ symbol can
be used to represent all of the following EXCEPT:
Answer:
a single device such as a WAN switch or router NOT: devices at all seven layers of the OSI model,
another network – a collection of networks – or the entire internet; or a large
group of details that are not pertinent to a situation – or description – at a
given time.
8) Which function performed by a NIC
is classified as a Layer 2 activity?
Answer:
Controlling a host’s access to the network medium NOT:
encapsulating data into segments, encoding bits as electrical signals,
or using network addresses to direct data delivery
OSI MODEL:
http://www.geocities.com/SiliconValley/Monitor/3131/ne/osimodel.html
Protocol Stacks:
http://www.lex-con.com/osimodel.htm
Webopedia OSI:
http://www.webopedia.com/quick_ref/OSI_Layers.asp
http://www.tomewing.com/radio.html
http://labmice.techtarget.com/articles/securingwin2000.htm
j
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CCNA 1: Networking Basics CH 4
Chapter 4
Layer 1 – Electronics and
Signals
4.1 Basics of Electricity
4.2 Basics of Digital Multimeters
4.3 Basics of Signals and Noise in Communications Systems
4.4 Basics of Encoding Networking Signals
Coulomb's Law - Opposite charges attract; like charges repel
Bohr's model - Protons are positive
charges, and electrons are negative charges; There is more than 1 proton in the
nucleus
Electrical conductors (conductors): materials through which electronics flow.
Electrostatic discharge (ESD): A
static discharge can randomly damage chips, data, or both
Semiconductors: materials where the amt. of electricity they conduct can be
precisely controlled. (Silicon,
carbon, germanium, and the alloy, gallium arsenide)
Alternating Current (AC): a
ways in which current flows. Alternating current and voltages vary with
time, by changing their polarity, or direction; flow is in one direction, then
reverses its direction, and repeats the process. AC voltage is positive at one
terminal, and negative at the other, and then it reverses its polarity. This
process repeats itself continuously.
Direct Current (DC): the other way in which current flows; always flows in the
same direction, and DC voltages always have the same polarity. One terminal is
always positive, and the other is always negative; hey do not change or reverse;
can be found in flashlight batteries, car batteries, and as power for the
microchips on the motherboard of a computer.
Voltage (electromotive force (EMF)): an electrical force, or pressure, that occurs when
electrons and protons are separated. Voltage
is created by the separation of charges, which means that voltage measurements
must be made between two points.
Electrical
current (current): the flow of charges that is created when
electrons move.
Resistance [ohm (Ω).]: Materials through which current flows
providing opposition to movement of electrons; generally used when referring to
DC voltages.
Impedance [Z]: measure of the combined opposition to the flow
of AC and DC current flow; a general term, and is the measure of how the flow
of electrons is resisted, or impeded.
Ohm's
Law [I = V/R] : the amount of
current that will flow through a piece of conductor when a voltage is applied
to it.
resistance = R =
Ohms
current = I = V/R.
AC
and DC electrical systems: the flow of electrons is always from a
negatively charged source to a positively charged source
Short Circuit: Conducting path
Open Circuit: Discontinuity in conducting path
Ground
(reference point, or 0 (zero) volts
level)
Multimeter: test
equipment used for measuring voltage, current, resistance, and possibly other
electrical quantities and displaying the value in numeric form; has two
wires; the black wire is referred to as the ground (reference ground) A negative
terminal on a battery is also referred to as 0 volts, or reference ground
Oscilloscope: graphs the electrical waves, pulses, and patterns. It has
an x-axis that represents time, and a y-axis that represents
voltage.
power lines: form
of alternating current (AC); usually delivered to a pole-mounted transformer.
The transformer reduces the high voltages used in the transmission to the 120
or 240 volts used by typical consumer electrical appliances
Jean Baptiste Fourier: a special sum of sine waves of
harmonically related frequencies, which are multiples of some basic frequency,
could be added together to create any wave pattern.
One binary digit or word (bit or pulse)
Signal reference
ground
Optical signals à
Binary 0 = encoded as a low-light,
or no-light, intensity (darkness)
Binary 1= encoded as a higher-light
intensity (brightness), or other
more complex patterns.
Wireless signals
à
Binary 0 = short burst of waves; B
Binary 1 =longer burst of waves, or
another more complex pattern.
Factors
affecting a single bit:
round trip time, (RTT): The time it takes the bit to travel from one end of the
medium and back again is referred to as the. Assuming no other delays,
the time it takes the bit to travel down the medium to the far end is RTT/2.
Reflection:
for electrical signals; when voltage pulses, or bits, hit a discontinuity, some energy can be reflected; optical
signals reflect whenever they hit a discontinuity in the glass fiber, such as
when a connector is plugged into a device
Cross talk:
electrical noise on the cable originates from signals on other wires in the
cable.
Near-end cross talk (NEXT):
when two wires are near each other and untwisted, energy from one wire can wind
up in an adjacent wire and vice versa. This can cause noise at both ends of a
terminated cable.
Thermal noise:
caused by the random motion of electrons; unavoidable but usually relatively
small compared to the signals.
AC Power and reference ground noises: crucial problems in networking: electricity carried to
appliances and machines by wires concealed in walls, floors, and ceilings
AC line noise coming from a nearby
video monitor or hard disk drive can be enough to create errors in a computer
system
Electromagnetic Interference
(EMI)/Radio Frequency Interference (RFI):
External sources of electrical impulses that affect the quality of electrical
signals include lighting, electrical motors, and radio systems.
Shielding and cancellation
Dispersion: when the signal broadens in time; caused by the type of
media involved
Jitter: Clock pulses
cause the CPU to calculate, the data to be stored in memory, and the NIC to
send bits. If the clock on the source host is not synchronized with the
destination, which is quite likely, timing will occur
Latency (Delay): Einstein's theory of relativity states, "nothing can
travel faster than the speed of light in a vacuum (3.0 x 108
meters/second)". Wireless networking signals travel at slightly less than
the speed of light in vacuum. Networking signals on copper media travel in the
range of 1.9x108 m/s to 2.4x108 m/s. Networking signals
on optical fiber travel at approximately 2.0x108 m/s
Collision: occurs when two bits from two different communicating
computers are on a shared medium at the same time
Modulation: taking a wave and changing, or modulating it so that it
carries information.
AM (amplitude modulation) -
the amplitude, or height, of a carrier sine wave is varied to carry the message
FM
(frequency modulation) - the frequency of the carrier wave is varied
to carry the message
PM
(phase modulation) - the phase, or beginning and ending points of a
given cycle, of the wave is varied to carry the message
TTL
(transistor-transistor logic) encoding
is the simplest. It is characterized by a high signal and a low signal (often
+5 or +3.3 V for binary 1 and 0 [zero] V for binary 0 [zero]). In optical
fibers, binary 1 might be a bright LED or laser light, and binary 0 (zero),
dark or no light. In wireless networks, binary 1 might mean a carrier wave is present
and binary 0 (zero), no carrier at all.
Chapter 4 QUIZ:
1) What is required for electrons
to flow?
A: a closed loop of conductors
2) Which of the following describes
attenuation?
A: a loss of signal strength
3) Which is a cause of crosstalk?
A: a poorly terminated network cabling
not: loss of a signal’s ground reference
4) Which material is considered an
electrical semiconductor?
A: Silicon
5) Which describes
A: Bits are represented by transitions in voltage
6) What must occur before Layer 2
devices can process a signal that has been transmitted on their LAN segment?
A: the signal must be converted from voltage to bits
7) Which of the following is a
design goal when planning Ethernet networks?
A: localizing and minimizing the number
of collisions
8) What does the ground plane
provide in a computer circuit board?
A: signal reference ground
CCNA1 CH 5:
Layer 1 – Media, Connections, and Collisions
5.1 Most Common
LAN Media
5.2 Cable
Specification and Termination
5.3 Making and
Testing Cable
5.4 Layer 1 -
Components and Devices
5.5 Collisions and
Collision Domains in Shared Layer Environments
5.6 Basic
Topologies Used in Networking
Screened UTP (ScTP) [AKA Foil Twisted Pair (FTP)] hybrid STP/UTP; essentially UTP wrapped in a metallic foil
shield, or "screen’; usually 100 or 120 Ohm
cable; UTP has an external diameter of approximately .43
cm.
UTP: UTP cable has four pairs of either 22 or 24
gauge copper wire; 100 ohms; ext. diameter of .43 cm
STP: shielding, cancellation, twisted wires; each
pair is wrapped in metal foil; the four pairs wrapped in metallic braid or
fiol; reduces electrical noise
(coupling, cross-talk, EMI, RFI); 150 Ohm cable.
Coaxial
cable: 500 ohm (thinnet/cheapernet);
consists of a hollow outer cylindrical conductor that surrounds a single inner
wire made of two conducting elements. A copper conductor located in the center
of the cable. Surrounding it is a layer of flexible insulation. Over this
insulating material is a woven copper braid or metallic foil that acts as the
second wire in the circuit, and as a shield for the inner conductor; outside
diameter of only .35 cm.
Fiber-optic cable: a
networking medium capable of conducting modulated light; consists of two fibers
encased in separate sheaths; the light guiding parts of an optical fiber are
called the core and the cladding. The core is usually very pure
glass with a high index of refraction.
When the core glass is surrounded by a cladding layer of glass or plastic with
a low index of refraction, light can be trapped in the fiber core. This process
is called total internal reflection,
and it allows the optical fiber to act like a light pipe, guiding light for
tremendous distances, even around bends.
Solid-state laser light
Cladding/core: light guiding parts of an optical fiber.
Refraction
Total internal reflection: When the core glass
is surrounded by a cladding layer of glass or plastic with a low index of
refraction, light is trapped in the fiber core;
allows the optical fiber to act like a light pipe.
Wireless
IEEE 802.11
WLANS typically use:
Radio Waves = 900MHz
Microwaves = 2.4 GHz
Infrared Waves = 820 nanometers
Speed of light (electromagnetic waves) = c
Frequency x Wavelength
= c
Low Frequency electromagnetic waves
= long wavelength (distance from one peak to
next)
High Frequency electromagnetic waves
= short wavelength
TIA/EIA Standards:
-568A = Commercial Building Telecomm Cabling
Standard
standards for horizontal cabling,
which defines horizontal cabling as cabling that runs from a telecommunications
outlet to a horizontal cross-connect
For shielded twisted-pair cable, the TIA/EIA-568-A standard calls for two pair
150 ohm cable. For unshielded-twisted pair, the standard calls for four pair
100 ohm cable. For fiber-optic, the standard calls for two fibers of 62.5/125
multimode cable
the maximum distance for cable runs in horizontal cabling is 90 meters
patch cords or cross-connect jumpers located at the horizontal cross-connect
cannot exceed 6 m in length. TIA/EIA-568-A also allows 3 m for patch cords that
are used to connect equipment at the work area. The total length of the patch
cords and cross-connect jumpers used in the horizontal cabling cannot exceed 10
m. A final specification for horizontal cabling contained in TIA/EIA-568-A
requires that all grounding and bonding must conform to TIA/EIA-607.
-569A =
contains specifications governing
cable performance. It calls for running two cables, one for voice and one for
data, to each outlet. Of the two cables, the one for voice must be four-pair
UTP
five categories in the specifications. These are category 1 (CAT 1), category 2
(CAT 2), category 3 (CAT 3), category 4 (CAT 4), and category 5 (CAT 5) cabling.
-570A = Residential and Light Comm Telecomm Wiring
Standard
-606 = Adminn Standard for the Telecomm Infrastructure
of Commercial Buildiings
-607 =
fork-like tool called a punch down tool
Patch panels
Repeaters are
internetworking devices that exist at the physical
layer (Layer 1) of the OSI model
(HUB) Multiport repeaters combine
connectivity with the amplifying and retiming properties of repeaters
Fluke 620 LAN CableMeter
Shared media environment - when multiple hosts have access to the same medium
Extended shared media
environment - special type of shared
media environment - networking devices can extend the environment so that it
can accommodate multiple-access, or more users
Point-to-point network
environment - shared
networking environment - one device is connected to only one other device via a
link, such as a phone line
Collision domain: The area
within the network where the data packets originated and collided, and includes
all shared media environments. One wire may be connected to another wire
through patch cables, transceivers, patch panels, repeaters, and even hubs. All
of these Layer 1
interconnections are part of the collision domain.
Competition for the medium (contention)
Attempting to develop a wireless
communication system for the islands of
To ensure that a repeated 10BASE-T network will function properly,
the following condition must be true: (repeater delays + cable delays + NIC
delays) x 2 < maximum round-trip delay.
When this delay limit is exceeded, the
number of late collisions dramatically increase.
Late
collision - when a collision happens after the first 64 bytes of the frame
are transmitted.
The size of collision domains can be
reduced by using intelligent networking
devices that break up the domains. (bridges, switches, and routers. This
process is called segmentation)
The
chipsets in NICs are not required to retransmit automatically when a late
collision occurs. These late collision frames add delay referred to as consumption
delay. As consumption delay and latency increase, network
performance decreases.
Ethernet 5-4-3-2-1 rule - 5 sections
of the network, 4 repeaters or hubs, 3 sections of
the network are ”mixing” section (populated
w/ hosts) 2
sections are link sections (for link
purposes), and 1 large collision domain.
A bridge
can eliminate unnecessary traffic on a busy network by dividing a network into
segments and filtering traffic based on the station address.
Cellular topology
consists of circular or hexagonal areas, each of which has an
individual node at its center.
Mesh topology - every node is linked directly to every other node.
Irregular network topology - here is no obvious pattern to the links and nodes.
Tree topology - similar to the extended star topology, the primary
difference being that it does not use one central node.
Extended star topology - repeats a star topology, except that each node that
links to the center node is, also, the center of another star.
Star topology - has a central node with all links to other nodes radiating
from it and allows no other links. The
flow of information is hierarchical.
Dual ring topology - consists of two concentric rings,
each of which is linked only to its adjacent ring neighbor. The two rings are
not connected.
Ring
topology - a single closed ring consisting of nodes and links, with
each node connected to only two adjacent nodes.
Bus
topology has - all of its nodes connected directly to one link, and
has no other connections between nodes.
Cellular
topology - consists of circular or hexagonal areas, each of which
has an individual node at its center. The
cellular topology is a geographic area that is divided into regions (cells) for the purposes of
wireless technology. Sometimes the
receiving nodes move (for example, car cell phone), and sometimes the sending
nodes move (for example, satellite communication links). The disadvantages are that signals are
present everywhere in a cell. The signals are susceptible to disruptions
(man-made and environmental) and to security violations.
Chapter 6: Layer 2 Concepts
6.1 LAN Standards
6.2 Hexadecimal
Numbers
6.3 MAC Addressing
6.4 Framing
6.5 Media Access
Control (MAC)
Data Link Layer 2: Provides
access to networking media and physical transmission across the media - enables
the data to locate its intended destination on a network; handles error
notification, network topology, and flow control; concerned with physical (as
opposed to network/ logical) addressing, network topology, line discipline (how
end systems will use the network link), error notification, ordered delivery of
frames, and flow control.
Layer 2 LLC to communicate w/ upper level
layers
Layer 2 Addressing
(naming) process
Layer 2 framing to organize/group bits
Layer 2 MAC (media access control) to
control who will transmit
LAN Specification:
Ethernet; IEEE 802.2; IEEE 802.3, 10Base-T, Token Ring/IEEE 802.5, FDDI
IEEE 802.2:
IEEE 802.3, 10Base-T, Token Ring/IEEE
802.5, FDDI
LLC: participates in the
encapsulation process; defined by the
IEEE 802.2 specification.
The LLC PDU (LLC
packet) takes the network protocol data, an IP packet, and adds more
control information to help deliver the IP packet; adds two addressing
components of the 802.2 specification, the Destination Service Access Point (DSAP) and the Source Service
Access Point (SSAP). This repackaged IP packet then travels to the MAC sublayer for handling by the
required specific technology for further encapsulation and data. The LLC manages communications between
devices over a single link on a network; supports both connectionless and
connection-oriented services; IEEE 802.2 defines a number of fields in the data link layer frames that enable multiple higher layer protocols
to share a single physical data link.
Layer
2 Concepts:
Hex: shorthand method for representing the 8-bit bytes that
are stored in the computer system; chosen to represent identifiers because
easily represents the 8-bit byte by using only 2 hexadecimal symbols.
MAC
addresses: 48 bits; expressed as twelve
hexadecimal digits; referred to as burned-in
addresses (BIAs) because they are burned into ROM and are copied into
RAM when the NIC initializes; they have no structure, and are considered flat
address spaces.
1st six hexadecimal digits: (administered
by the IEEE) identify the manufacturer or vendor and thus comprise the Organizational Unique Identifier (OUI).
2nd six hexadecimal digits: comprises the interface serial number, or another value (administered by the
specific vendor) MAC addresses are sometimes
--------------------------
The position of each symbol, or digit, in a hex number represents the base
number 16 raised to a power, or exponent, based on its position. Moving from
right to left, the first position represents 160, or 1; the
second position represents 161, or 16; the third position, 162,
or 256; and so on.
Example: 4F6A = (4
x 163)+ (F[15] x 162)+ (6 x 161)+
(A[10] x 160) = 20330 (decimal)
--------------------
Remainder method: the decimal number
is repeatedly divided by the base number (16). The remainder is then converted
each time into a hex number.
Example: Convert the decimal number 24032 to hex.
|
24032/16 |
= |
1502, with a remainder of 0 |
|
1502/16 |
= |
93, with a remainder of 14 or E |
|
93/16 |
= |
5, with a remainder of 13 or D |
|
5/16 |
= |
0, with a remainder of 5 |
By collecting all the remainders backward,
you have the hex number 5DE0.
----------------
Convert hexadecimal numbers to decimal numbers by multiplying the hex
digits by the base number of the system (Base 16) raised to the exponent of the
position.
Example: Convert the hex number 3F4B to a decimal number.
(Work from right to left.)
|
|
|
16203 |
= decimal equivalent |
|
B(11) x 160 |
= |
11 |
|
|
4 x 161 |
= |
64 |
|
|
F(15) x 162 |
= |
3840 |
|
|
3 x 163 |
= |
12288 |
|
----------------------------
Converting binary to hexadecimal and hexadecimal to binary is an
easy conversion. The reason is that Base 16 (hexadecimal) is a power of Base 2
(binary). Every four binary digits (bits) are equal to one hexadecimal digit.
The conversion looks like this:
|
Binary |
|
Hex |
Binary |
|
Hex |
|
0000 |
= |
0 |
1000 |
= |
8 |
|
0001 |
= |
1 |
1001 |
= |
9 |
|
0010 |
= |
2 |
1010 |
= |
A |
|
0011 |
= |
3 |
1011 |
= |
B |
|
0100 |
= |
4 |
1100 |
= |
C |
|
0101 |
= |
5 |
1101 |
= |
D |
|
0110 |
= |
6 |
1110 |
= |
E |
|
0111 |
= |
7 |
1111 |
= |
F |
If there is a binary number that looks like 01011011, it is broken into two groups of four bits (work from right to left). These look like this: 0101 and 1011. When converting these two groups to hex, they look like 5 and B. So converting 01011011 to hex is 5B. To convert hex to binary do the opposite. Convert hex AC to binary. First convert hex A which is 1010 binary and then convert hex C which is 1100 binary.
No matter how large the binary number, the same conversion is always applied. Start from the right of the binary number and break the number into groups of four. If at the left end of the number it does not evenly fit into a group of four, add zeros to the left end until it is equal to four digits (bits). Then convert each group of four to its hex equivalent. Here is an example:
|
000100100010111110111110111001001 |
converts to: |
||||||||
|
0001 |
0010 |
0100 |
0101 |
1111 |
0111 |
1101 |
1100 |
1001 |
converts to: |
|
1 |
2 |
4 |
5 |
F |
7 |
D |
C |
9 |
so: |
|
|
|
||||||||
|
000100100010111110111110111001001 Binary = 1245F7DC9 hex |
|||||||||
As stated before hex works in exactly the opposite way. Each hex digit converts to four binary digits (bits). For example:
|
AD46BF |
converts to: |
||||||||
|
|
|
|
A |
D |
4 |
6 |
B |
F |
converts to: |
|
|
|
|
1010 |
1101 |
0100 |
0110 |
1011 |
1111 |
so: |
|
|
|
||||||||
|
AD46BF
hex converts to 101011010100011010111111 binary |
|||||||||
AD46BF hex converts to
101011010100011010111111 binary
That is the conversion for binary to hexadecimal
and from hexadecimal to binary.
Data
à Segment à Packet à Frame à Bit
Frame format diagram: which is based on voltage versus time graphs. They are
read from left to right, just like an oscilloscope graph. The frame format
diagram shows different groupings of bits (fields) that perform other
functions.
Analogies
for Data Frames:
·
A picture frame
marks the borders of a painting;
A data frame shows the borders of encapsulated data
·
A shrink-wrapped
pallet is the last step before heavy objects are shipped;
Framing is the last packaging before data is
transmitted on the medium
·
Video is conveyed
as a series of still images called frames;
Data (info) is conveyed as a series of data frames
Single
Generic Frame: has sections
called fields; each field is composed
of bytes. The names of the fields
are as follows:
Padding Bytes:
extra data sometimes added so that the frames have a min length for timing
purposes. LLC bytes are also included with the data field in the IEEE standard
frames.
Frame
Check Sequence (FCS) field: contains a number that is
calculated by the source computer and is based on the data in the frame. When
the destination computer receives the frame, it recalculates the FCS number and
compares it with the FCS number included in the frame. If the two numbers are
different, an error is assumed, the frame is discarded, and the source is asked
to retransmit.
3
primary ways to calculate the Frame Check Sequence number:
3 Analogies for Media Access Control
·
Stopping at a
tollbooth
·
Waiting in a ticket
line
·
Speaking in a
meeting
Tollbooth Analogy
A tollbooth controls multiple lanes of vehicles crossing a bridge. Vehicles gain
access to the bridge by paying a toll. The
vehicle is the frame,
the bridge is the shared medium, and paying the fee at the tollbooth is the protocol that allows access to the bridge.
Ticket Line Analogy
Imagine waiting in line to ride a roller coaster at an amusement park. The line
is necessary to ensure order. There are a specified maximum number of people
that can fit into the roller coaster car at one time. Eventually, as the line
moves, tickets are purchased, and people sit in the car. In this analogy, the
people are the data,
the cars are the frames,
the roller coaster tracks are the shared medium, and the protocol is the waiting in line and
presentation of the ticket.
Meeting Analogy
Imagine being at a meeting table, along with the other members of a large
talkative group. There is one shared medium, the space above the meeting table
(air), through which signals (spoken words) are communicated. The protocol for
determining access to the medium is that the first person that speaks, when
everyone quiets down, can talk as long as he/she wishes, until finished. In
this analogy, the words of the individual members are the packets, the air above the meeting
table is the medium,
and the first person to speak in the meeting is the protocol.
Nondeterministic
MAC protocols: use a first-come,
first-served (FCFS) approach. In the late
1970s, the
CSMA/CD: Everyone on the system listens for quiet, at which time it
is OK to transmit. However, if two people talk at the same time, a collision
occurs, and neither person can transmit. Everyone else on the system also hears
the collision, waits for silence, and then tries to transmit.
Deterministic - 1
Describes a system whose time evolution can be predicted exactly. Contrast
{probabilistic}. 2 Describes an {algorithm} in which the correct next step
depends only on the current state. This contrasts with an algorithm involving
{backtracking} where at each point there may be several possible actions and no
way to chose between them except by trying each one and backtracking if it
fails.
Updated On : 9/22/1995
Common
Layer 2 technologies: Token Ring, FDDI,
and Ethernet. All three specify Layer 2 issues (LLC, naming, framing, and MAC),
as well as Layer 1 signaling components and media issues.
Chapter
6 QUIZ:
1) Which manages communication between
a specific Layer 2 LAN technology and network layer protocols? Answer: LLC
2) What is the hexadecimal equivalent
of the decimal number 2766? Answer: ACE
3) What is the decimal equivalent of
the hex number FAD? Answer: 4013
4) What is the hex equivalent of the
binary number 11000011? Answer: C3
5) How does a receiving host detect
that there has been an error during transmission of a frame? Answer: It compares
the FCS included in the frame to the FCS that it recalculates
6) What is the purpose of media access
control? Answer:
It determines which workstation on a shared medium LAN
is allowed to transmit data
7) Which is an example of a
non-deterministic LAN technology? Answer:
Ethernet
8) Which is a drawback of the CSMA/CD
media access control protocol? Answer:
Collisions can decrease network performance
9) Which describes MAC addresses? Answer: The
1st 6 hex digits identify the manufacturer and the last 6 digits
identify the device
10) All statements are true regarding data link layer LAN specifications
EXCEPT: Answer: Hierarchical addressing is used to identify the network to
which the device belongs
Related
Chapter 6 Links:
FAQs for OUIs:
http://standards.ieee.org/faqs/OUI.html
Data
Link Layer:
http://cs.nmhu.edu/osimodel/datalink/
MAC Sublayer:
http://www.100vg.com/white/mac.htm
CHAPTER 18 The Network
18.3 Ethernet Frame
http://wks.uts.ohio-state.edu/sysadm_course/html/sysadm-326.html
Hex
Explained:
http://chem.csustan.edu/JTB/help/HEX/hex-def.htm
TechWeb
– The Business Technology Network:
http://www.techweb.com/encyclopedia/
MAC:
http://www.ecs.umass.edu/ece/wireless/ECE671/hw6/node1.html
-------------------------------------------------------------
CCNA1 CH 7: Technologies
7.1 The Basics of
Token Ring
7.2 Basics of FDDI
7.3 Ethernet and
IEEE 802.3
7.4 Layer 2
Devices
7.5 Effects of
Layer 2 Devices on Data Flow
7.6 Basic Ethernet
10Base-T Troubleshooting
Tokens
Tokens = 3 bytes; consist of a start delimiter,
an access control byte, and an end delimiter.
Start delimiter alerts each station to the
arrival of a token or data/command frame; also includes signals that
distinguish the byte from the rest of the frame by violating the encoding
scheme used elsewhere in the frame.
Access control byte contains the priority and reservation field, and a token
and monitor bit.
Token bit distinguishes a token from a data/command frame.
Monitor bit determines whether a frame is continuously
circling the ring.
End delimiter signals the end of the token or
data/command frame; contains bits that indicate a damaged frame, and a frame
that is the last of a logical sequence.
Data/command frames vary in size depending on the size of the information
field. Data frames carry information for upper
layer protocols. Command frames contain control
information and have no data for upper layer protocols.
Data/command frames: a frame control byte follows the access control
byte. The frame control byte indicates whether the frame contains data or
control information. In control frames, this byte specifies the type of control
information.
Following the frame control byte are
two address fields that identify destination and source stations. As with IEEE
802.5, their addresses are 6 bytes in length.
The data field follows the address field. The length of this field is limited by
the ring token that holds the time. Thus it defines the maximum time a station
may hold the token.
Following the data field is the frame check sequence (FCS) field. The source station fills this field with a
calculated value dependent on the frame contents. The destination station
recalculates the value to determine whether the frame has been damaged in
transit. The frame is discarded if it has been damaged. As with the token, the
end delimiter completes the data/command frame.
Each station can hold the token for a
maximum period of time, depending on the specific technology that has been
implemented
When a token is passed to a host that
has information to transmit, the host seizes the token and alters one
particular bit
token releases
Token Ring networks have no collisions
Unlike CSMA/CD
networks, such as Ethernet, token passing
networks are deterministic
- you can calculate the maximum time
that will pass before any end station will be able to transmit. (good for
factory environments where any delay must be predictable)
Token Ring frames have two fields that
control priority, the priority field and the reservation field (priority can be set on a per station basis)
Only stations with a priority equal
to, or higher than, the priority value contained in a token can seize that token. Stations that raise the priority level of a
token must reinstate the previous priority when their transmission has been
completed
mechanisms for detecting and
compensating for network faults:
1.
select one station
in the Token Ring network to be the active monitor
2.
2
Active Monitor: acts as a centralized source of timing
information for other ring stations and performs a variety of ring maintenance
functions; one function is to remove continuously circulating frames from the
ring and regenerate a new one.
The IBM Token Ring network star
topology contributes to the overall network reliability. Active multi-station access units (MSAUs) can
see all information in a Token Ring network and allows them to check for
problems, and to selectively remove stations from the ring whenever necessary.
Beaconing:
detects and tries to repair network faults. When a station detects a serious
problem with the network (e.g. cable break) it sends a beacon frame. The beacon frame defines a failure domain which includes the station that is
reporting the failure, its nearest active upstream
neighbor (NAUN), and everything in between. Beaconing initiates a
process called autoreconfiguration,
where nodes within the failure domain automatically perform diagnostics in an
attempt to reconfigure the network around the failed areas. (Physically, MSAUs
can accomplish this through electrical reconfiguration)
Devices on network polled to see if
they need to transmit data
Signal encoding: way of combining both clock and data information into a stream of
signals.
Differential
MLT-3: Used on Fast Ethernet (100Base-TX) networks
Other Binary Encoding Schemes: 4B/5B, 8B/10B, AMI, Bipolar, AMI,
Pseudoternary, B&Zs and HDB3
The 4/16 Mbps Token Ring networks use
differential Manchester encoding; A 1 bit is represented by no polarity change
at the start of the bit time and a 0 bit is represented by a polarity change at
the start of the bit time
Patch cables connect MSAUs to other
adjacent MSAUs. Lobe cables connect MSAUs to stations. MSAUs include bypass
relays for removing stations from the ring
UTP Token Ring Hub
4-Pair Horizontal Cabling
Patch Cord
Patch Panel
ANSI X3T9.5 standards committee produced FDDI and submitted to the ISO
FDDI: frequently used as a backbone, or high speed computer connectivity to
LAN.
Four
Specifications of FDDI:
FDDI
Frame Format: Preample, Start
delimeter, Frame control, end delimeter
FDDI
frame fields:
FDDI supports real-time allocation of
network bandwidth; provides this support by defining two types of traffic,
synchronous and asynchronous
FDDI uses an encoding scheme called 4B/5B
- 4 bits of data are sent as a 5 bit code. The
signal sources in FDDI transceivers are LEDs or lasers; FDDI specifies a 100
Mbps, token passing, dual ring LAN that uses a fiber-optic transmission medium.
Advantages
of Optical Fiber:
Modes: thought of as
bundles of light rays entering the fiber at a particular angle.
Single-mode
fiber allows only one mode of light to propagate through the
fiber; capable of higher bandwidth and greater cable run distances, than
multimode fiber - often used for inter-building
connectivity; generally uses lasers opposed to LEDs.
Multimode fiber
allows multiple modes of light to propagate through the fiber. Multiple
modes of light may travel different distances, depending on their entry angles.
This causes them to arrive at the destination at different times, a phenomenon
called modal
dispersion; MMF is often used for intra-building connectivity;
uses LEDs as the light-generating devices opposed to Lasers.
FDDI specifies the use of dual rings; each ring travels in opposite directions;
rings consist of two or more point-to-point connections between adjacent
stations. One of the two FDDI rings is called the primary
ring. It is used for data transmission. The other ring is called the secondary ring, and it is generally used as a back up.
Class B (single
attachment stations (SAS)),
attach to one ring.
Class A (dual attachment stations (DAS)),
attach to both rings. SASs are attached to the primary ring through a concentrator,
which provides connections for multiple SASs. The concentrator ensures that a
failure, or power down, of any given SAS, does not interrupt the ring. This is
particularly useful when PCs, or similar devices that frequently power on and
off, connect to the ring; each FDDI DAS has two ports, designated A and B.
These ports connect the station to the dual FDDI ring. Therefore, each port
provides a connection for both the primary and the secondary ring.
Ethernet: well suited to applications where a local communication
medium must carry sporadic,
occasionally heavy traffic at high peak data rates; origins in 1960s at University of
Hawaii; CSMA/CD was developed here; Xerox
Corporation's Palo Alto Research Center (PARC)
developed the first experimental Ethernet system in the early 1970s; used as the basis for the (IEEE) 802.3 spec
released in 1980.
After 1980 IEEE 802.3 spec, Digital
Equipment Corporation, Intel Corporation, and Xerox Corporation developed/released
an Ethernet spec, V 2.0 - substantially compatible with IEEE 802.3. Together,
Ethernet and IEEE 802.3 currently maintain the greatest market share of any LAN
protocol. Today, the term Ethernet is often used to refer to all SMA/CD LAN
that generally conform to Ethernet specs.
Ethernet and IEEE 802.3 specify
similar technologies. Backoff algorithms determine when the colliding
stations can retransmit.
Both Ethernet and IEEE 802.3 LANs are broadcast networks - every station can see all of the
frames, regardless of whether they are the intended destination of that data.
Each station must examine the received frames to determine if they are the
destination. If so, the frame is passed to a higher layer protocol within the
station for appropriate processing.
Differences between Ethernet and IEEE
802.3 LANs are subtle. Ethernet provides services corresponding to Layer 1 and
Layer 2 of the OSI reference model. IEEE 802.3 specifies the Layer 1, and the
channel access portion of the Layer 2, but does not define a LLC protocol. Both
Ethernet and IEEE 802.3 are implemented through hardware.
The Ethernet
and IEEE 802.3 frame fields:
Ethernet is a shared-media
broadcast technology
the access method CSMA/CD used in Ethernet performs three functions:
Networking devices are able to tell
when a collision has occurred because the amplitude of the signal on the
networking media will increase. When a collision occurs, each device that is
transmitting will continue to transmit data for a short time - done to ensure
that all devices see the collision. Once this happens, those devices that were
attempting to transmit when the collision was detected will invoke an algorithm. After those devices have backed off for a certain period of time (different
for each device), any device can attempt to gain access to the networking media
once again. When data transmission resumes on the network, the devices that
were involved in the collision do not have priority to transmit data.
Ethernet is a broadcast
transmission medium - means all devices on a network can see all data
that passes along the networking media - only the device whose MAC address and IP address matches the destination MAC address and destination IP
address carried by the data will copy the data - then it checks the data packet
for errors. If the device detects errors, the data packet is discarded. The
destination device will not notify the source device of whether the packet
arrived successfully or not. Ethernet is connectionless
network architecture and is referred to as a best-effort
delivery system.
Signal
encoding: way of combining both clock and data info into a stream of signals over a
medium - Manchester encoding define a 0 as a signal that is high for the first
half of the period and low for the second half. It defines a 1 as a signal that
is low for the first half of the period and high for the second half.
10BASE-T
transceivers: designed to send/receive
signals over a segment that consists of four wires. One pair of wires for
transmitting data, and one pair of wires for receiving data
In a LAN with a star topology - networking media is run from a central
hub out to each device - resembles spokes radiating from the hub of a wheel.; considered
the easiest to design and install; If one run of networking media is broken or
shorted, then only the device attached at that point is out of commission, the
rest of the LAN will remain functional; it also increases the amount of
networking media required; Single-point of failure in a hub
Active hub connects the networking
media as well as regenerates the signal. In
Ethernet where hubs act as multiport repeaters, they are sometimes referred to
as concentrators.
Passive hub: device used to connect
networking media and does not regenerate a signal.
TIA/EIA-568-A : topology that is
to be used for horizontal cabling, must be a
star topology - mechanical termination for each telecommunications
outlet/connector is located at the patch panel in the wiring closet. Every
outlet is independently and directly wired to the patch panel. (Max Hor. Cabl.
UTP = 90m)(Patch Cords = 3m)(Max Patch Length @ horizontal cross-connect = 6m)
The maximum distance for a run of
horizontal cabling, that extends from the hub to any workstation, is 100 m. (actually 99 m. - commonly rounded up to 100
m.) This figure includes the 90 meters for the horizontal cabling, the 3 meters
for the patch cords, and the 6 meters for the jumpers at the horizontal
cross-connect. Horizontal cabling runs in a star topology radiate out from the
hub, much like the spokes of a wheel. This means that a LAN that uses a star
topology could cover the area of a circle with a radius of 100 m.
There will be times when the area to
be covered by a network will exceed the TIA/EIA-568-A specified maximum length
that a simple star topology can accommodate. For example, envision a building
where the dimensions are 200 m x 200 m. A simple star topology that adhered to
the horizontal cabling standard specified by TIA/EIA-568-A could not provide
complete coverage for that building.
Extending the length of the networking
media beyond the TIA/EIA-568-A specified maximum length.
If a signal travels beyond the specified maximum distance, there is no
guarantee that it will be readable when it reaches the NIC.
Extended Star Topology: use of
internetworking devices (repeaters) that compensate for the attenuation of the
signal
NICs are the physical connections from
workstations to the network. Network cards all require an IRQ, an I/O address,
and upper memory addresses for DOS and Windows
95/98. Three factors to consider:
NICs perform important Layer 2 data link layer functions, such as the
following:
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Switching is a technology that
alleviates congestion in Ethernet LANs by reducing traffic and increasing
bandwidth. Switches, also referred to as LAN switches, often replace shared
hubs and work with existing cable so there is minimal disruption of existing
networks during installation. All switching and routing equipment
perform two basic operations:
Like bridges, switches connect LAN
segments, use a table of MAC addresses to determine the segment on which a
frame needs to be transmitted, and reduce traffic. Switches operate at much
higher speeds than bridges. They can support new functionality, such as
virtual LANs. An Ethernet switch has many
benefits, such as allowing many users to communicate in parallel through the
use of virtual circuits and dedicated network segments in a collision-free
environment. This maximizes the bandwidth available on the shared medium.
Another benefit is that moving to a switched LAN environment is very cost
effective because existing hardware and cabling can be reused. Finally,
network administrators have great flexibility in managing the network through
the power of the switch and its LAN-configuration software. LAN switches are considered
multiport bridges with no collision domain. You can think of each switch port
as a micro bridge. This process is called microsegmentation. Data is
exchanged at high speeds by switching the frame to its destination. By
reading the destination MAC address Layer 2 information, switches can achieve
high speed data transfers, much like a bridge does. The frame is sent to the
port of the receiving station prior to the entire frame entering the switch.
This leads to low latency levels and a high rate of speed for frame
forwarding. Ethernet switching increases the
bandwidth available on a network. It does this by creating dedicated network
segments, or point-to-point connections, and connecting these segments in a
virtual network within the switch. This virtual network circuit exists only
when two nodes need to communicate. This is called a virtual circuit
because it exists only when needed, and is established within the switch. Even though the LAN switch reduces
the size of collision domains, all hosts connected to the switch are still in
the same broadcast domain. Therefore, a broadcast from one node will still be
seen by all other nodes connected through the LAN switch. Switches are data
link layer devices that, like bridges, enable multiple physical LAN segments
to be interconnected into single larger network. Similar to bridges, switches
forward and flood traffic based on MAC addresses. Because switching is
performed in hardware instead of in software, it is significantly faster. You
can think of each switch port as a micro bridge. This process is called microsegmentation.
Thus each switch port acts as a separate bridge and gives the full bandwidth
of the medium to each host. |
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Ethernet LANs that use a bridge for
segmenting the LAN provide more bandwidth per user because there are fewer
users on the segments than there are when compared to the entire LAN. The
bridge allows only those frames that have destinations outside the segment to
pass through. Bridges learn a network's segmentation by building address tables
that contain the physical address of each network device, as well as the port
to use to reach the device. Bridges differ from routers because they are Layer
2 devices, and are, therefore, independent of Layer 3 protocols. Bridges pass
on data frames, regardless of which Layer 3 protocol is used, and are
transparent to the other devices on the network.
Bridges increase the latency
(delay) in a network by 10-30%. This latency is due to the decision making that
is required of the bridge, or bridges, when transmitting data to the correct
segment. A bridge is considered a store and forward device because it must
receive the entire frame and compute the cyclic redundancy check (CRC) before
forwarding can take place. The time it takes to perform these tasks can slow
network transmissions, thus causing delay.
A LAN that uses a switched Ethernet
topology creates a network that performs as though it had only two nodes, the
sending node and the receiving node. These two nodes share 10 Mbps bandwidth
between them, which means nearly all bandwidth is available for the
transmission of data. A switched Ethernet LAN allows a LAN topology to work
faster and more efficiently than a standard Ethernet LAN can, because it uses
bandwidth so efficiently. In a switched Ethernet implementation, the available
bandwidth can reach close to 100%.
It is important to note that even
though 100% of the bandwidth may be available, Ethernet networks perform best
when kept under 30-40% of full capacity. This limitation is due to the media
access method of Ethernet, CSMA/CD. Bandwidth usage that exceeds the
recommended limitation results in increased collisions. The purpose of LAN
switching is to ease bandwidth shortages and network bottlenecks, such as that
occurring between a group of PCs and a remote file server. A LAN switch is a
high speed multiport bridge that has one port for each node, or segment, of the
LAN. A switch segments a LAN into micro segments, thereby creating collision
free domains from one formerly larger collision domain.
Switched Ethernet is based on standard
Ethernet. Each node is directly connected to one of its ports, or to a segment
that is connected to one of the switch's ports. This creates a 10 Mbps
connection between each node and each segment on the switch. A computer
connected directly to an Ethernet switch is its own collision domain and
accesses the full 10Mbps. As a frame enters a switch it is read for the source
and/or destination address. The switch then determines which switching action
will take place based on what is learned from the information in the frame. If
the destination address is located on another segment, the frame is then
switched to its destination.
Routers are more advanced than typical
bridges. A bridge is passive (transparent) at the network layer and operates at
the data link layer. A router operates at the network layer, and bases all of
its forwarding decisions on the Layer 3 protocol address. It accomplishes this
by examining the destination address on the data packet, then looking in its
routing table for forwarding instructions. Routers create the highest level of
segmentation because of their ability to make exact determinations of where to
send the data packet.
Because routers perform more functions
than bridges, they operate with a higher rate of latency. Routers must examine
packets to determine the best path for forwarding them to their destinations.
Unavoidably, this process takes time and introduces latency.
The teaching topology contains
examples of segmentation by bridges, switches, and routers. Also in the
teaching topology, many different parts of the network are brought together by
the main router. The bridge divides the E1 Ethernet network into two segments.
Traffic is filtered at the bridge, reducing potential collisions and the
physical extent of the collision domain. Therefore, the bridge breaks the E1
Ethernet network into two segments: the first segment has the repeater and
hosts K, L, M, N on it; the second segment has hosts O and P on it. This remains,
however, a broadcast domain. The repeater extends the collision domain rather
than segmenting it.
The main switch divides the E0
Ethernet network into multiple network segments with each having guaranteed
full bandwidth. The workgroup switch divides the workgroup segment into more
segments. Also note that the switches provide high connectivity to their
unshared bandwidth. The hub does not segment its part of the network. The hub
and all the devices attached to it, all the way up to the main switch port,
remain a collision domain. The router segments the entire LAN into two Ethernet
subnetworks, which are segmented, and a Token Ring and FDDI subnetwork, which
by their nature, have no collision domains.
There are many approaches to network
troubleshooting. The first is to work up through the layers of the OSI model.
This method isolates problems that can masquerade as other problems. Time can
be wasted troubleshooting a browser that does not function properly, only to
find that the computer is not connected to the network. It is best to start
troubleshooting at Layer 1. Ask yourself whether things are plugged in and
connected before you go to the next higher level, with its more complicated
issues. An effective troubleshooting approach by OSI layer is summarized in the
graphic.
Links Chapter
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