Cisco Notes

Cisco Notes

http://cisco.netacad.net

ACE 5, Semester 1; 3/12/04
CCNA Networking Basics v2.1.4

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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:

Printed circuit board (PCB): thin insulating board on which chips (integrated circuits) and other electronic components are placed and interconnected by conducting paths.
Microprocessor: integrated circuit that contains a CPU.
MOBO: is the main printed circuit board of a microcomputer.
Bus: collection of wires thru which data is transmitted from 1 part of a comp. to another.
(RAM): device for data storage; volatile.
(ROM): memory which data has been prerecorded; cannot be removed; only be read.
System Unit: is the main part of a PC; chassis, mproc, main memory, bus, and ports.

Backplane Components:

Backplane: large circuit board that contains sockets for expansion cards.
NIC: expansion board inserted into a comp. to connect to a network.
Parallel Port: interface that is capable of transferring more than one bit simultaneously.
Serial port: interface in which only one bit is transmitted at a time.
Mouse port: port designed for connecting a mouse to a PC.

Information Flow:

Boot instructions are used to initialize the computer and are stored in ROM
Software applications are stored in RAM after they have been loaded from disk or other storage media
RAM and ROM store data and instructions for rapid access by the CPU through the bus
Application information is stored in RAM while applications are being used
Saved information is information that flows from RAM to some form of storage device.
Exported information is data that flows from RAM and the CPU, through the bus and expansion slots, to the printer, video card, sound card, or network card

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:

NIC, including jumpers, plug-and-play software, and erasable programmable read-only memory (EPROM). EPROM: type of memory that retains its contents until it is exposed to ultraviolet light.
NIC diagnostics, including the vendor-supplied diagnostics and loopback test.
Resolve hardware resource conflicts: IRQ, I/O Base Address, and DMA (direct memory address: used to transfer data from RAM to a device without going through the CPU)

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

Binary 0 represented by 0 volts of electricity (0 = 0.5 volts)
Binary 1 represented by +5 volts of electricity (1 = +5 volts)

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

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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.

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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:

Breaks network communication into smaller parts
Standardizes network components to allow multiple vendor development/support
Allows different types of network HW and SW to communicate with each other
Prevents changes in 1 layer from affecting the other layers - so they can develop quickly
Breaks network communication into smaller parts to make learning it easier

Layered Network Model:

Reduces complexity
Standardizes interfaces
Facilitates modular engineering
Ensures interoperable technology
Accelerates evolution
Simplifies teaching and learning

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:

Build the data
Package the data for end-to-end transport
Add the network IP address to the header
Add the data link layer header and trailer
Convert to bits for transmission

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

http://www.ietf.org/

TCP/IP Model: Created by U.S. Department of Defense (DoD)

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

Both have layers
Both have application layers, though they include very different services
Both have comparable transport and network layers
Packet-switched (not circuit-switched) technology is assumed
Networking professionals need to know both

Differences

TCP/IP combines the presentation and session layer issues into its application layer
TCP/IP combines the OSI data link and physical layers into one layer
TCP/IP appears simpler because it has fewer layers, however this is a misconception. The OSI reference model, with its less complex and multiple layers, is simpler to develop and troubleshoot.
TCP/IP protocols are the standards around which the Internet developed, so the TCP/IP model gains credibility just because of its protocols. In contrast, typically networks are not built on the OSI protocol, even though the OSI model is used as a guide.

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?

Session

NOT: data link, network, or presentation

2) Which of the following is the Layer 4 PDU?

Segment

NOT: bit, frame, or packet

3) Which layer of the OSI model is responsible for reliable end-to-end network communications?

Transport

NOT: application, network, physical

4) Which of the following best describes the function of the presentation layer?

It ensures that data is readable by the receiving system

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?

Media access, ordered delivery of frames, and physical addressing

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:

TFTP

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:

Media access controls

NOT: voltage levels, media connection types, or maximum transmission distances

9) Which OSI model layer provides packet encapsulation service to Layer 4?

Network layer

NOT: data link layer, physical layer, or transport layer

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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:

Bus topology uses a single backbone segment (length of cable). All the hosts connect to it directly.
Ring topology connects one host to the next and the last host to the first. This creates a physical ring of cable.
Star topology connects all cables to a central point. This point is usually a hub or switch.
Extended star topology links individual stars together by linking the hubs/switches. This will extend the length and size of the network.
Hierarchical topology is created similar to an extended star. Instead of linking the hubs/switches together, the system is linked to a computer. The computer controls the traffic on the topology.
Mesh topology is used when there can be absolutely no break in communications. The control systems of a nuclear power plant are a good example. As you can see in Figure [2], each host is connected to all other hosts. This also reflects the design of the Internet, which has multiple paths to any one location.

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.

Encapsulation: data to be sent must be put in manageable, traceable packages Encapsulation begins in the application, presentation, and session layers, where data is prepared for transmission by the creation of a common data format. The transport layer breaks up the data into manageable size units called segments. The data link layer further encapsulates the packet and creates a frame. It adds the source and destination local (MAC) address to the frame..
http://searchnetworking.techtarget.com/sDefinition/0,,sid7_gci212060,00.html (Encapsulation)
http://www.cs.mun.ca/~donald/bsc/node13.html
http://searchnetworking.techtarget.com/sDefinition/0,,sid7_gci212736,00.htm (packets)
http://www.ecse.rpi.edu/Homepages/shivkuma/teaching/sp99/i02_fnd/sld006.htm (Encapsulation Slide)
http://www.erg.abdn.ac.uk/users/gorry/course/intro-pages/encapsulation.html (encapsulation of PDUs)

Data
Segment
Packet
frame, which encapsulates the packet

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

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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