Data communications

Data CommunicationsChapter 6

What is a Computer Network?• A computer network is a

collection of computers that communicate with one another over transmission lines.

• Three basic types of networks are:

– Local area networks (LANs)–connects computers that reside in a single geographic location on the premises of the company that operates the LAN.

– Wide area networks (WANs)–connects computers at different geographic sites.

– Internets–a network of networks

Major Network Types

Local Area Networks

• A local area network (LAN) is a group of computers connected together on a single company site.

• Usually the computers are located within a half mile or so of each other, although longer distances are possible.– The key distinction, however, is that all of the computers are

located on property controlled by the company that operates the LAN.

• Computers and printers are connected via a switch, which is a special-purpose computer that receives and transmits messages on the LAN.

© 2007 Prentice Hall, Inc. 5

Local Area Networks (Continued)

• Each device on a LAN (computer, printer, etc.) has a hardware component called a network interface card (NIC) that connects the device’s circuitry to the cable.

– The NIC works with programs in each device to implement Layer 1 and Layer 2 protocols.

– Each NIC has a unique identifier, which is called the (MAC) media access control address.

• The computers, printers, switches, and other devices on a LAN are connected using one of two media.

– Most connections are made using unshielded, twisted pair (UTP) cable.

– A device called an RJ-45 connector is used to connect the UTP cable into NIC devices on the LAN.


Local Area Networks (Continued)

• The connection between switches can use UTP cable, but if they carry a lot of traffic or are far apart UTP cable may be replaced by optical fiber cables.

– The signals on such cables are light rays, and they are reflected inside the glass core of the optical fiber cable.

– Optical fiber cable uses special connectors called ST and SC connectors.

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Figure 5-6 Local Area Network


IEEE 802.3 or Ethernet Protocol

• The committee that addresses LAN standards is called the IEEE 802 Committee.– Thus, IEEE LAN protocols always start with the number 802.

• Today, the world’s most popular protocol for LAN is the IEEE 802.3 protocol.

– This protocol standard, also called Ethernet, specifies hardware characteristics such as which wire carries which signals.

– It also describes how messages are to be packaged and processed for transmission over the LAN.

– Ethernet operates at Layers 1 and 2 of the TCP/IP-OSI architecture.


IEE 802.3 or Ethernet Protocol (Continued)

• Most personal computers today are equipped with an onboard NIC that supports what is called 10/100/1000 Ethernet.– These products conform to the 802.3 specification and allow for

transmission at a rate of 10, 100, or 1,000 Mbps.– Communications speeds are expressed in bits, whereas memory

sizes are expressed in bytes.


LANs with Wireless Connections

• Wireless connections have become popular with LANs.

• The NIC for wireless devices have been replaced by wireless NIC (WNIC).– For laptop computers, such devices can be cards that slide into the

PCMA slot or they can be built-in, onboard devices.

• Several different wireless standard exist

– As of 2005, the most popular is IEEE 802.11g– The current standard, 802.11g allows speeds of up to 54 Mbps.– The WNICs operate according to the 802.11 protocol and connect to an

access point (AP).


Figure 5-10


Figure 5-12 Summary of LAN and WAN Networks


Wide Area Networks

• A wide area network (WAN) connects computers located at physically separated sites.– A company with offices in Detroit and Atlanta must use a WAN to

connect the computers together.– Because the sites are physically separated, the company cannot

string wire from one site to another.

• An Internet service provider (ISP) has three important functions:– It provides you with a legitimate Internet address.– It serves as your gateway to the Internet.– It receives the communication from your computer and passes

them on to the Internet, and it receives communication from the Internet and passes them on to you.


Connecting the Personal Computer to an ISP: Modems

• Home computers and those of small businesses are commonly connected to an ISP in one of three ways:– Using a regular telephone line– Using a special telephone line called a DSL line– Using cable TV line

• All three ways require that the digital data in the computer be converted to an analog, or wavy, signal.

• A device called a modem, or modulator/demodulator performs this conversion.

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Figure 5-14 Personal Computer (PC) Internet Access


DSL Modems

• A DSL modem is the second modem type.

• DSL stands for digital subscriber line.

• DSL modems operate on the same lines as voice telephones and dial-up modems.– They operate so that their signals do not interfere with voice

telephone service.– They provide much faster data transmission speeds than dial up

modems.– They always maintain a connection.


DSL Modems (Continued)

• DSL data transmission and telephone conversations can occur simultaneously.

• DSL modems use their own Layer 1 and Layer 2 protocols for data transmission.

• DSL lines that have different upload and download speeds are called asymmetric digital subscriber lines (ADSL).

• Symmetrical digital subscriber lines (SDSL) offers the same speed in both directions.


Cable Modems

• A cable modem is the third modem type.

• Cable modems provide high-speed data transmission using cable television lines.

• At the maximum, users can download data up to 10 Mbps and can upload data at 256 kbps.

• Narrowband lines typically have transmission speeds less than 56 kbps.

• Broadband lines have speeds in excess of 256 kbps.


Figure 5-15 Wide Area Network Using Leased Lines


Networks of Leased Lines

• A WAN connects computers located at geographically distributed company sites.

• The lines that connect these sites are leased from telecommunication companies that are licensed to provide them.

• A variety of access devices connect each site to the transmission.– These devices are typically special-purposed computers.– The particular devices required depend on the line used and other

factors– Sometimes switches and routers are employed.


Figure 5-16 Transmission Line Types, Uses, and Speeds


Public Switched Data Network

• Another WAN alternative is a public switched network (PSDN), a network of computers and leased lines that is developed and maintained by a vendor that leases time on the network to other organizations.

• When using a PSDN, each site must lease a line to connect to the PSDN network.– The location at which this occurs is called a point of presence

(POP); it is the access point into the PSDN.– Once a site has connected to the PSDN POP, the site obtains

access to all other sites connected to the PSDN.


Public Switched Data Network (Continued)

• PSDNs save the setup and maintenance activities when using leased lines.– They also save costs because a company does not have to pay for

the entire network.– The company can just pay for the traffic that it sends.

• Three Layer 1 and 2 protocols are used with PSDNs:– Frame Relay can process traffic in the range of 56 kbps to 40

Mbps.– Asynchronous transfer mode (ATM) can process speeds from 1 to

156 Mbps (can handle both voice and data).– Ethernet, the protocol developed for LANs

Figure 5-17 Wide Area Network Using PSDN


Virtual Private Network

• Virtual private network (VPN) is the fourth WAN alternative.

• A VPN uses the Internet or a private internet to create the appearance of private point-to-point connections.

• A VPN uses the public Internet to create the appearance of a private connection.

• A connection called a tunnel, is a virtual pathway over a public or shared network from the VPN client to the VPN server.


Virtual Private Network (Continued)

• VPN communications are secure.– The VPN client software encrypts, or codes, the original messages

so that its contents are hidden.

• Virtual private networks offer the benefit of point-to-point leased lines, and they enable remote access, both by employees and by any others who have been registered with the VPN server.

Example of Remote Access Using a VPN

Example of a WAN using VPN

Criteria for Comparing WANS• Many different computer

networking alternatives are available, each with different characteristics.

• There are three types of costs that need to be considered.

– Setup costs include the costs of acquiring transmission lines and necessary equipment, such as switches, routers, and access devices.

– Operational costs include lease fees for lines and equipment, charges of the ISP, the cost of ongoing training, etc.

– Maintenance costs include those for periodic maintenance, problem diagnosis and repair, and mandatory upgrades.

Criteria Continued• There are six considerations with regard to performance:

– Speed– Latency (delays during busy periods)– Availability– Loss rate– Transparency (user involvement in operation)– Performance guarantees

• Other criteria to consider when comparing network alternatives include the growth potential (greater capacity) and the length of contract commitment

Data communications

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