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