Comp net 2

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Characteristics of WANs

Covers large geographical areas

Circuits provided by a common carrier

Consists of interconnected switching nodes

Traditional WANs provide modest capacity

• 64000 bps common

• Business subscribers using T1 service – 1.544 Mbps common

Higher-speed WANs use optical fiber and transmission technique known as asynchronous transfer mode (ATM)

• 10s and 100s of Mbps common

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Wide Area Networks

Alternative technologies

• Circuit switching

• Packet switching

• Frame relay

• Asynchronous Transfer Mode (ATM)

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Characteristics of LANs

Like WAN, LAN interconnects a variety of devices

and provides a means for information exchange

among them

Traditional LANs

• Provide data rates of 1 to 20 Mbps

High-speed LANS

• Provide data rates of 100 Mbps to 10 Gbps

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Differences between LANs and WANs

Scope of a LAN is smaller

• LAN interconnects devices within a single

building or cluster of buildings

LAN usually owned by organization that owns the

attached devices

• For WANs, most of network assets are not owned

by same organization

Internal data rate of LAN is much greater

Example Networks

The Internet

Connection-Oriented Networks: ATM

Ethernet

Wireless LANs: 802:11

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Architecture of the Internet

Overview of the Internet

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ATM Virtual Circuits

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ATM cell

The ATM Reference Model

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Ethernet

Architecture of the original Ethernet

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Wireless LANs

(a) Wireless networking with a base station.(b) Ad hoc networking.

Modems

(a) A binary signal

(b) Amplitude modulation

(c) Frequency modulation

(d) Phase modulation

The Local Loop: Modems, ADSL, and Wireless

Digital Subscriber Lines

Bandwidth versus distanced over category 3 UTP for DSL.

Digital Subscriber Lines

Operation of ADSL using discrete multitone modulation

A typical ADSL equipment configuration.

Wireless Local Loops

Frequency Division Multiplexing

Wavelength Division Multiplexing

Time Division Multiplexing

The T1 carrier (1.544 Mbps)

Time Division Multiplexing

Multiplexing T1 streams into higher carriers

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Techniques Used in Switched Networks

Circuit switching

• Dedicated communications path between two

stations

• E.g., public telephone network

Packet switching

• Message is broken into a series of packets

• Each node determines next leg of transmission for

each packet

Circuit Switching

(a) Circuit switching.

(b) Packet switching.

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Data sent out of sequence

Small size (packets) of data at a time

Packets passed from node to node between source

and destination

Used for terminal to computer and computer to

computer communications

Packet Switching

Message Switching

(a) Circuit switching (b) Message switching (c) Packet switching

Packet Switching

A comparison of circuit switched and packet-switched networks.

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Effect of Packet Size on Transmission

Effect of Packet Size on Transmission time

Propagation and Transmission Delay

Propagation Delay = Distance/Propagation speed

Transmission Delay = Message size/bandwidth bps

Latency = Propagation delay + Transmission delay +

Queueing time + Processing time

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Performance comparison

Circuit switching Packet switching

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Variable vs. Fixed-Length Packets

No Optimal Length

if small: high header-to-data overhead

if large: low utilization for small messages

Fixed-Length Easier to Switch in Hardware

simpler

enables parallelism

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Network Criteria

Performance

Depends on Network Elements

Measured in terms of Delay and Throughput

Reliability

Failure rate of network components

Measured in terms of availability/robustness

Security

Data protection against corruption/loss of data due to:

• Errors

• Malicious users