Wide Area Networking Chapter 9 Copyright 2001 Prentice Hall Revision 2: July 2001

2 Wide Area Networks zWANs Link Sites (Locations) yUsually sites of the same organization ySometimes, sites of different organizations WAN Site A Site C Site B

3 Carriers zYou can only install wires on your own property yCalled your customer premises zTo send signals between sites or to customers, you must use a carrier Carrier Customer Premises

4 Carriers zCarriers transport data and voice traffic between customer premises, charging a price for their services zReceive rights of way from the government to lay wires and radio links Carrier

5 Carrier Regulation zTraditionally, Carriers Have been Regulated yGiven rights of way yGiven monopoly protection from competition yIn return, services normally must be tariffed xTariff specifies exact terms of the service to be provided, and xTariff specifies price to be charged xPrevents special deals, which would be inappropriate for a regulated monopoly xRegulators must approve price for reasonableness

6 Carrier Regulation zThere is a Strong Trend Toward Deregulation yGradual removal of monopoly protections yAllows competition, so lower prices and more service options yFewer services need to be tariffed, allowing price negotiation

7 Carrier Regulation zService Level Agreements (SLAs) yEven under competition, carriers may guarantee specific levels of service for certain service parameters in an SLA xThroughput xLatency xAvailability xError Rates, etc. yPenalties are paid to customers if carrier fails to meet agreed-upon service levels

8 High Cost of Long-Distance zLAN Communication is Inexpensive per Bit Transmitted ySo most LANs operate at 10 Mbps to a few gigabits per second zLong-Distance Communication is Very Expensive per Bit Transmitted ySo Most WANs use low speeds yMost WAN demand is 56 kbps to a few Mbps

9 Leased Lines zLeased Lines are Circuits (From Chapter 1) yOften goes through multiple switches and trunk lines yLooks to user like a simple direct link Switch Trunk Line Leased Line

10 Leased Lines zLeased lines yLimited to point-to-point communication xLimits who you can talk to yCarriers offer leased lines at an attractive price per bit sent to keep high-volume customers Leased Line

11 Leased Line Meshes zIf you have several sites, you need a mesh of leased lines among sites Leased Line Mesh

12 Leased Line Speeds zLargest Demand is 56 kbps to a few Mbps z56 kbps (sometimes 64 kbps) digital leased lines yDS0 signaling zT1 (1.544 Mbps) digital leased lines y24 times effective capacity of 56 kbps yOnly about 3-5 times cost of 56 kbps yDS1 signaling zFractional T1 yFraction of T1’s speed and price yOften 128, 256, 384 kbps

13 Leased Line Speeds zT3: is the next step y44.7 Mbps in U.S. zEurope has E Series yE1: Mbps yE3: 34 Mbps zSONET/SDH lines offer very high speeds y156 Mbps, 622 Mbps, 2.5 Gbps, 10 Gbps

14 SONET/SDH zCreated as Trunk Lines for Internal Carrier Traffic yAs were other leased lines zThe Trunk Line Breakage Problem yProblem: unrelated construction products often break carrier trunk lines, producing service disruptions yThe most common cause of disruptions X

15 SONET/SDH Uses a Dual Ring zNormally, Traffic Travels in One Direction on One Ring zIf Trunk Line Breakage, Ring is Wrapped; Still a Ring, So Service Continues Switch Normal OperationWrapped

16 Digital Subscriber Lines (DSLs) zCan Use DSLs Instead of Traditional Leased Lines yLess expensive zHDSL (High-Speed DSL) ySymmetrical: Same speed in each direction yHDSL: 768 kbps (Half a T1) on a single twisted pair yHDSL2: Mbps (T1) on a single twisted pair zSHDSL (Super-High-Speed DSL) yCan run at multiple rates up to 4.6 megabits/second ySymmetrical

17 Digital Subscriber Line zNormal Leased Lines Used Data Grade Wires yHigh-quality, high-cost yTwo pairs (one in each direction) zDSLs Normally Use Voice Grade Copper yNot designed for high-speed data ySo sometimes works poorly yUsually one pair (ADSL, HDSL) ySometimes two pairs (HDSL2)

18 Problems of Leased Lines zWith many sites, meshes are expensive and difficult to manage zThere are many leased lines between the sites yEach site is likely to have several leased lines connected to it zThese leased lines tend to span long distances between sites

19 Problems of Leased Lines zUser firm must handle switching and ongoing management yExpensive because this requires planning and the hiring, training, and retention of a WAN staff

20 T1 Leased Lines zVoice Requirements yAnalog voice signal is encoded as a 64 kbps data stream (see Chapter 5) y8 bits per sample y8,000 samples per second

21 T1 Leased Lines zT1 lines are designed to multiplex 24 voice channels of 64 kbps each zT1 lines use time division multiplexing (TDM) yTime is divided into 8,000 frames per second xOne frame for each sampling period yEach frame is divided into 24 8-bit slots xOne for each channel’s sample in that time period x(24 x 8) 192 bits xPlus one framing bit for 193 bits per frame

22 T1 Leased Lines zSpeed Calculation y193 bits per frame y8,000 frames per second y1.544 Mbps zFraming Bit yOne per frame y8,000 per second yUsed to carry supervisory information (in groups of 12 or 24 framing bits)

23 PSDNs zPublic Switched Data Networks yDesigned for data rather than voice ySite-to-site switching is handled for you yYou merely connect each site to the PSDN “cloud” (No need to know internal details) PSDN

24 PSDNs zConnect each site to the PSDN using one leased line yOnly one leased line per site yLine only runs a short distance—to the nearest PSDN access point 1 Leased Line PSDN

25 PSDNs zAccess Device Needed at Each Site yConnects each site to access line yOften a router ySometimes a device specific to a particular PSDN Technology PSDN Access Device

26 PSDNs zPoint of Presence (POP) yPlace where you connect to the cloud yMay be several in a city yMay not have any POP close yNeed leased line to POP ySeparate from PSDN charges Leased Line PSDN POP

27 PSDNs in Perspective zSimpler than Networks of Leased Lines yLess staffing yFewer leased lines and shorter distances zLess Expensive than Networks of Leased Lines yLess staffing yPSDN prices are very low yPSDN is less expensive overall yPSDNs are replacing many leased line mesh networks

28 Circuit-Switched PSDNs zEnd-to-End Capacity is Guaranteed yIf you need it, it is always there yWhen you don’t need it, you still pay for it yExpensive for data traffic, which usually has short bursts and long silences A bcd efg PSDN

29 Packet-Switched PSDNs zMessages are divided into small units called packets yShort packets load switches more effectively than fewer long messages

30 Packet Switched PSDNs zPackets are multiplexed on trunk lines yCost of trunk lines is shared yPacket switching lowers transmission costs yDominates PSDN service today Multiplexed Trunk Line

31 Packet Switched PSDNs: Virtual Circuits zAll commercial packet switched PSDNs use virtual circuits yEliminates forwarding decisions for individual packets yReduces switching load, so reduces switching costs Virtual Circuit

32 Unreliable PSDNs zMost commercial PSDNs are Unreliable y(Only obsolete X.25 PSDN technology was reliable) yNo error correction at each hop between switches yReduces costs of switching yNote that both virtual circuits and unreliable service reduce switching costs

33 PSDN Cost Savings zPacket Switching yReduces costs of transmission lines through multiplexing zVirtual Circuits yReduces costs of switches because they do not have to make decisions for each frame zUnreliability yReduces costs of switches because they do not have to do error correction