CCNA4_M2_WAN_Technologies (1130789)

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CCNA – Semester4Module 2WAN TechnologiesObjectives• Identify the devices used in a WAN and liststandards• Differentiate between packet-switched and circuitswitched WAN technologies• Describe equipment involved in the implementationof various WAN services• Compare and contrast WAN design modelsWAN Technology OverviewParts of a WAN serviceModem• In order for the local loop to carry data, a devicesuch as a modem is needed to prepare the data fortransmission.DTE and DCE• Devices that put dataon the local loop arecalled data circuitterminating equipment,or data communicationsequipment (DCE).• The customer devices that pass the data to the DCE arecalled data terminal equipment (DTE).WAN Line Types and BandwidthWAN DevicesWAN Devices: Modem••A modem is needed if the local loop is analog rather thandigital.Modems transmit data over voice-grade telephone lines bymodulating and demodulating the signal.WAN Devices: CSU/DSU• The communications linkneeds signals in anappropriate format.• For digital lines, a channelservice unit (CSU) and adata service unit (DSU) arerequired.• The CSU/DSU may also bebuilt into the interface cardin the router.WAN standard organizationsWAN standard• WANs use the OSI reference model, but focusmainly on Layer 1 and Layer 2.• WAN standards typically describe both physicallayer delivery methods and data link layerrequirements, including physical addressing,flow control, and encapsulation.WAN physical layer•Describe how to provide electrical, mechanical, operational,and functional connections to the services provided by acommunications service provider.Physical ConnectorsWAN data link layer• The data link layer protocols define how data isencapsulated for transmission to remote sites, andthe mechanisms for transferring the resultingframes.WAN encapsulation••••The choice of encapsulation protocols depends on the WANtechnology and the equipment.Most framing is based on the HDLC standard.The address field is not needed for WAN links, which are almostalways point-to-point.The control field indicates the frame type:– Unnumbered frames carry line setup messages.– Information frames carry network layer data.– Supervisory frames control the flow of information frames and request dataretransmission in the event of an error.WAN Data-link ProtocolsCircuit Switching• Fixed capacity may not beefficient due to accessbursts.• Generally an expensiveway of moving data.• The data bits areautomatically delivered tothe far end because thecircuit is alreadyestablished.Packet SwitchingPacket Switching• Two approaches:connectionless orconnection-oriented.• Connectionless systemscarry full addressinginformation in eachpacket.• Connection-orientedsystems predeterminethe route for a packet,and each packet needonly carry an identifier.WAN link optionsWAN TechnologiesAnalog dialup••••When intermittent, low-volume data transfers are needed, modemsand analog dialed telephone lines provide low capacity anddedicated switched connections.The advantages are simplicity, availability, and low implementationcost.The upper limit is around 33 kbps.

The rate can be increased toaround 56 kbps if the signal is coming directly through a digitalconnection.The disadvantages are the lowdata rates and a relatively longconnection time, not proper forvoice or video traffic.ISDN• The connection uses 64 kbps bearer channels(B) for carrying voice or data and a signaling,delta channel (D) for call set-up.• ISDN is used to provide additional capacity orbackup for leased-line.• ISDN tariffs are based on a per-B channel basisand are similar to those of analog voiceconnections.ISDN• Basic Rate Interface (BRI) ISDN is intended for the homeand small enterprise.• For larger installations, Primary Rate Interface (PRI)ISDN is available.Leased line• The dedicated capacity gives no latency or jitterbetween the endpoints.• These dedicated circuits are generally priced based onbandwidth required and distance between the twoconnected points.X.25•••Packet-switched networks using shared lines to reduce costs,tariffs are based on the amount of data .Switched or permanent virtual circuits can be establishedthrough the network and data packets are subject to delay.X.25 networks are usually low capacityFrame Relay••••Frame Relay is a much simpler protocol with no error or flowcontrolThe simplified handling of frames leads to reduced latencyMost Frame Relay connections are PVCs rather than SVCsIt can operate at much higher bit rate that provides permanentshared medium bandwidth connectivity that carries both voiceand data traffic.ATM• ATM provide very low latency and jitter at much higherbandwidths.• ATM has data rates beyond 155 Mbps, capable oftransferring voice, video, and data.• It is built on a cell-based architecture rather than on aframe-based architecture.

ATM cells are always a fixedlength of 53 bytes including 5 bytes header.DSL••••Digital Subscriber Line (DSL) technology is a broadband technologyUse existing twisted-pair telephone lines to transport high-bandwidthdata.Broadband refers to a technique which uses multiple frequencieswithin the same physical medium to transmit data.The transfer rates are dependent on the actual length of the localloop and the type and condition of its cabling.Cable modem• Network access is available from some cabletelevision networks allows greater bandwidththan telephone local loop.• Enhanced cable modems enable two-way, highspeed data transmissions up to 6.5 times that ofT1 leased lines.• It is an attractive medium for transferring largeamounts of digital information quickly, includingvideo clips, audio files, and large amounts ofdata.WAN DesignWAN design• Approaching the design in a systematic manner canlead to superior performance at a reduced cost.• It is necessary to know what data traffic must be carried,its origin, and its destination.Traffic Characteristics• For each pair of end points and for each traffic type,information is needed on the various trafficcharacteristics.Steps in WAN design••Several modifications maybe necessary before adesign is finalized.Continued monitoring andre-evaluation are alsorequired after installation ofthe WAN to maintainoptimal performance.Designing a WAN• Designing a WAN essentially consists of thefollowing:– Selecting an interconnection pattern or layout for the linksbetween the various locations– Selecting the technologies for those links to meet theenterprise requirements at an acceptable costWAN topologies• Knowing the various end points allows theselection of a topology or layout for the WAN.• The topology will be influenced by geographicconsiderations but also by requirements suchas availabilityWAN Technologies• With the end points and the links chosen, the necessarybandwidth can be estimated.• Type of traffic on the links may have varyingrequirements for latency and jitter.• With the bandwidth availability already determined,suitable link technologies must be selected.Three-layer design model• A systematic approach is needed when manylocations must be joined.

A hierarchical solutionwith three layers offers many advantages.• This three-layer model follows the hierarchicaldesign used in telephone systems.Advantages of the hierarchical approachAdvantages of the hierarchical approachThree-layer design modelOther layered design models• Many networks do not require the complexity of afull three-layer hierarchy. Simpler hierarchies maybe used.Summary• Identify the devices used in a WAN and list standards• Packet-switched and circuit-switched WANtechnologies• Overview of analog dialup, ISDN, leased line, X.25,Frame Relay, cable modem and ATM services• Steps in WAN design and WAN design requirements• Advantages offered with a three-layer hierarchical WANdesignCCNA4 – Module2.

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