Wide Area Networking

Chapter 9

Copyright 2001 Prentice HallRevision 2: July 2001

2Wide Area Networks

WANs Link Sites (Locations) Usually sites of the same organization Sometimes, sites of different

organizations

WAN

Site A Site C

Site B

3Carriers

You can only install wires on your own property Called your customer premises

To send signals between sites or to customers, you must use a carrier

CarrierCustomerPremises

4Carriers

Carriers transport data and voice traffic between customer premises, charging a price for their services

Receive rights of way from the government to lay wires and radio links

Carrier

5Carrier Regulation

Traditionally, Carriers Have been Regulated Given rights of way Given monopoly protection from

competition In return, services normally must be tariffed

Tariff specifies exact terms of the service to be provided, and

Tariff specifies price to be charged

Prevents special deals, which would be inappropriate for a regulated monopoly

Regulators must approve price for reasonableness

6Carrier Regulation

There is a Strong Trend Toward Deregulation

Gradual removal of monopoly protections

Allows competition, so lower prices and more service options

Fewer services need to be tariffed, allowing price negotiation

7Carrier Regulation

Service Level Agreements (SLAs)

Even under competition, carriers may guarantee specific levels of service for certain service parameters in an SLAThroughputLatencyAvailabilityError Rates, etc.

Penalties are paid to customers if carrier fails to meet agreed-upon service levels

8High Cost of Long-Distance

LAN Communication is Inexpensive per Bit Transmitted So most LANs operate at 10 Mbps to a

few gigabits per second

Long-Distance Communication is Very Expensive per Bit Transmitted So Most WANs use low speeds Most WAN demand is 56 kbps to a few

Mbps

9Leased Lines

Leased Lines are Circuits (From Chapter 1) Often goes through multiple switches and

trunk lines Looks to user like a simple direct link

SwitchTrunkLine

LeasedLine

10Leased Lines

Leased lines Limited to point-to-point communication

Limits who you can talk to

Carriers offer leased lines at an attractive price per bit sent to keep high-volume customers

Leased Line

11Leased Line Meshes

If you have several sites, you need a mesh of leased lines among sites

Leased Line

Mesh

12Leased Line Speeds

Largest Demand is 56 kbps to a few Mbps

56 kbps (sometimes 64 kbps) digital leased lines DS0 signaling

T1 (1.544 Mbps) digital leased lines 24 times effective capacity of 56 kbps Only about 3-5 times cost of 56 kbps DS1 signaling

Fractional T1 Fraction of T1’s speed and price Often 128, 256, 384 kbps

13Leased Line Speeds

T3: is the next step 44.7 Mbps in U.S.

Europe has E Series E1: 2.048 Mbps E3: 34 Mbps

SONET/SDH lines offer very high speeds 156 Mbps, 622 Mbps, 2.5 Gbps, 10 Gbps

14SONET/SDH

Created as Trunk Lines for Internal Carrier Traffic As were other leased lines

The Trunk Line Breakage Problem Problem: unrelated construction products often

break carrier trunk lines, producing service disruptions

The most common cause of disruptions

X

15SONET/SDH Uses a Dual Ring

Normally, Traffic Travels in One Direction on One Ring

If Trunk Line Breakage, Ring is Wrapped; Still a Ring, So Service Continues

Switch

Normal Operation Wrapped

16Digital Subscriber Lines (DSLs)

Can Use DSLs Instead of Traditional Leased Lines Less expensive

HDSL (High-Speed DSL) Symmetrical: Same speed in each direction HDSL: 768 kbps (Half a T1) on a single twisted pair HDSL2: 1.544 Mbps (T1) on a single twisted pair

SHDSL (Super-High-Speed DSL) Can run at multiple rates up to 4.6

megabits/second Symmetrical

17Digital Subscriber Line

Normal Leased Lines Used Data Grade Wires High-quality, high-cost Two pairs (one in each direction)

DSLs Normally Use Voice Grade Copper Not designed for high-speed data So sometimes works poorly Usually one pair (ADSL, HDSL) Sometimes two pairs (HDSL2)

18Problems of Leased Lines

With many sites, meshes are expensive and difficult to manage

There are many leased lines between the sites Each site is likely to have several leased

lines connected to it

These leased lines tend to span long distances between sites

19Problems of Leased Lines

User firm must handle switching and ongoing management

Expensive because this requires planning and the hiring, training, and retention of a WAN staff

20T1 Leased Lines

Voice Requirements

Analog voice signal is encoded as a 64 kbps data stream (see Chapter 5)

8 bits per sample

8,000 samples per second

21T1 Leased Lines

T1 lines are designed to multiplex 24 voice channels of 64 kbps each

T1 lines use time division multiplexing (TDM) Time is divided into 8,000 frames per

secondOne frame for each sampling period

Each frame is divided into 24 8-bit slotsOne for each channel’s sample in that time

period(24 x 8) 192 bitsPlus one framing bit for 193 bits per frame

22T1 Leased Lines

Speed Calculation 193 bits per frame 8,000 frames per second 1.544 Mbps

Framing Bit One per frame 8,000 per second Used to carry supervisory information

(in groups of 12 or 24 framing bits)

23PSDNs

Public Switched Data Networks Designed for data rather than voice

Site-to-site switching is handled for you

You merely connect each site to the PSDN “cloud” (No need to know internal details)

PSDN

24PSDNs

Connect each site to the PSDN using one leased line Only one leased line per site Line only runs a short distance—to the

nearest PSDN access point

1 LeasedLine

PSDN

25PSDNs

Access Device Needed at Each Site Connects each site to access line Often a router Sometimes a device specific to a

particular PSDN Technology

PSDN

AccessDevice

26PSDNs

Point of Presence (POP) Place where you connect to the cloud May be several in a city May not have any POP close Need leased line to POP Separate from PSDN charges

LeasedLine PSDN

POP

27PSDNs in Perspective

Simpler than Networks of Leased Lines Less staffing Fewer leased lines and shorter distances

Less Expensive than Networks of Leased Lines Less staffing PSDN prices are very low PSDN is less expensive overall PSDNs are replacing many leased line

mesh networks

28Circuit-Switched PSDNs

End-to-End Capacity is Guaranteed If you need it, it is always there When you don’t need it, you still pay for

it Expensive for data traffic, which usually

has short bursts and long silences

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PSDN

29Packet-Switched PSDNs

Messages are divided into small units called packets

Short packets load switches more effectively than fewer long messages

30Packet Switched PSDNs

Packets are multiplexed on trunk lines Cost of trunk lines is shared Packet switching lowers transmission

costs Dominates PSDN service today

MultiplexedTrunk Line

31Packet Switched PSDNs: Virtual Circuits

All commercial packet switched PSDNs use virtual circuits Eliminates forwarding decisions for individual

packets Reduces switching load, so reduces switching

costs

VirtualCircuit

32Unreliable PSDNs

Most commercial PSDNs are Unreliable (Only obsolete X.25 PSDN technology

was reliable)

No error correction at each hop between switches

Reduces costs of switching

Note that both virtual circuits and unreliable service reduce switching costs

33PSDN Cost Savings

Packet Switching Reduces costs of transmission lines

through multiplexing

Virtual Circuits Reduces costs of switches because they

do not have to make decisions for each frame

Unreliability Reduces costs of switches because they

do not have to do error correction