Wide Area NetworksWide Area Networks

2

Types of Traditional Telephone Types of Traditional Telephone CircuitsCircuits

Dial-Up Service (Any-to-Any)

Leased Lines Point-to-point only Cheaper for high volumes of use

Leased Line

SwitchedDial-UpService

Seattle

Washington, D.C.

3

Leased Line ServiceLeased Line Service

Customer Premises A Customer Premises B

SwitchingOffice

SwitchingOffice

SwitchingOffice

TrunkLine

TrunkLine

LocalLoop Local

Loop

Leased LinesMay Pass ThroughMultiple Switches,

Even MultipleCarriers

4

Leased LinesLeased Lines

Limited to 2 points

Cheaper than dial-up on high-volume routes

Companies can build enterprise networks from meshes of leased lines between sites

LeasedLine

Corporate-ownedSwitch

See this document for price example

5

Data NetworksData Networks

Data Networking Alternatives Use the telephone network and modems (slow) Lease lines, add own switching (complex)

Data Networks Optimized for data transmission Customer only has to connect to the data network Carrier handles transmission, switching, management Shown as cloud to indicate lack of need to know details Two types: circuit-switched and packet-switched

Data Network

6

Circuit-Switched Data NetworksCircuit-Switched Data Networks

Switched for any-to-any communication

Just dial the number of the party being called

Very flexible

7

Circuit-Switched Data NetworksCircuit-Switched Data Networks

Dedicated Capacity Circuit is maintained during the duration of the call

Capacity is always available

You must pay for this constant capacity

Most data transmission is burst, with long silences between transmission

Utilization of the line may be as low as 5%

So circuit-switched services is inherently expensive

8

Circuit-Switched Data NetworksCircuit-Switched Data Networks

ISDN Usually circuit-switched Both voice and data Two 64 kbps B channels to the desktop Sometimes can combine into a 128 kbps circuit About twice as expensive as a telephone line Needs a terminal adapter to connect computer, phone Reasonably inexpensive, reasonably fast, not popular

ISDN

9

Packet-Switched Data NetworksPacket-Switched Data Networks

Messages are Broken into Small Pieces (Packets)

Flow through the network more easily than long messages, like sand in an hourglass

Packet

10

Packet SwitchesPacket Switches

Packet Switched Networks have Switches Route the packets through the network

21

6

5

4

7

3Switch

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Packet Switching is EfficientPacket Switching is Efficient

Packets from several stations multiplexed over trunk lines between switches No costly dedicated transmission capacity

11 22Trunk Line

12

Error Checking in Packet-Switched Data Error Checking in Packet-Switched Data NetworksNetworks

The Process

Sender transmits the packet

Sender maintains the packet in memory

Receiver checks the packet for errors

If there is an error, asks for a retransmission

Sender retrieves from memory, retransmits

21

13

Error Checking in Packet-Switched Data Error Checking in Packet-Switched Data NetworksNetworks

Considerations in Adds delay (latency) every time it is done

Places a heavy load on the switch, lowering throughput

Not often needed, because there are very few errors on modern transmission lines.

14

Reliable Packet-Switched Data NetworksReliable Packet-Switched Data Networks

Check for Errors at Each Hop Have reduced throughput Have latency (delays)

21 3 4 5

ErrorCheck

ErrorCheck

ErrorCheck

ErrorCheck

ErrorCheck

15

Unreliable Packet-Switched Data Unreliable Packet-Switched Data NetworksNetworks

No Error Check at Each Packet Switch Check only once, at receiving host Low latency, load on switches

21 3 4 5

ErrorCheckNo Error Checks at Switches

16

Unreliable ServiceUnreliable Service

Most Packet Switched Networks Today are Unreliable

Little Need: Error rates are low with modern lines, switches

Reduces delays: critical for some applications

Low load on the switches for high throughput

Better to check once, on the receiving host, than at every switch

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Connectionless ServiceConnectionless Service

Routing Decision for each packet at each switch Places a heavy load on switches Unnecessary work: subsequent packets usually travel

same path, because conditions rarely change between packets

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6

5

4

7

3Decision

DecisionDecision

Decision

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Connection-Oriented ServiceConnection-Oriented Service

Routing decision is made once, at start of connection

21

6

5

4

7

3Decision

DecisionDecision

Decision

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Connection-Oriented ServiceConnection-Oriented Service

First decision establishes a path (virtual circuit) All subsequent packets follow the virtual circuit

21

6

5

4

7

3

Virtual Circuit

20

Connection-Oriented Packet-Switched Connection-Oriented Packet-Switched Data NetworksData Networks

All Commercial Packet Switched Networks are Connection-Oriented Reduces loads on the switches for higher throughput

Lower latency because of less work at each switch

When marketers say “packet switched,” they now automatically include the concept of connection orientation

21

Connections in Packet-Switched Data Connections in Packet-Switched Data NetworksNetworks

Permanent Virtual Circuits (PVCs) Established for long durations Set up weeks or months ahead of time If your firm has four sites, need 6 PVCs Makes packet switched networks like network of leased

lines

Site 1Site 1

Site 3Site 3 Site 4Site 4

Site 2Site 2PVC

22

Connections in Packet-Switched Data Connections in Packet-Switched Data NetworksNetworks

Switched Virtual Circuits

Established at call setup

Only available in some packet switched networks

Will provide the any-to-any flexibility of circuit-switched data networks AND the efficiency of connection-oriented packet switching

23

OSI LayeringOSI Layering

Connectionless Service OSI Layer 3 (Networking) Routing across a series of packet switches Alternative Routing

Connection-Oriented Service OSI Layer 2 (Data Link) Reduces network to a single path Loses flexibility of alternative routing after virtual

circuit is established