EEC4113Data Communication &

Multimedia SystemChapter 7: Network Layer

by Muhazam Mustapha, October 2011

Learning Outcome

• By the end of this chapter, students are expected to be able to explain switching and routing issues related to wide area network (WAN)

Chapter Content

• Circuit Switching

• Packet Switching– Virtual Circuit– Datagram

Switching

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Switching Network

• Long distance transmission is typically done over a network of switched nodes

• Nodes not concerned with content of data

• End devices are stations– Computer, terminal, phone, etc.

• A collection of nodes and connections is a communications network

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Node

• Data routed by being switched from node to node

• Nodes may connect to other nodes only, or to stations and other nodes

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Node

• Node to node links usually multiplexed

• Network is usually partially connected– Some redundant connections are

desirable for reliability

• Two different switching technologies– Circuit switching

– Packet switching

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Circuit Switching

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Circuit Switching

• Originated in public telephone networks

• Well suited to analog transmission of voice signal

• Dedicated communication path between two stations

• Three phases– Establish– Transfer– Disconnect

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Circuit Switching

• Must have switching capacity and channel capacity to establish connection

• Must have intelligence to work out routing

• Inefficient– Channel capacity dedicated for duration of

connection– If no data, capacity wasted

• Set up (connection) takes time

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Circuit Switching

• Once connected, transfer is transparent

• Developed for voice traffic (phone)

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Telecom Components

• Subscriber– Devices attached to network

• Subscriber line– Link between subscriber and network

• Also called Local Loop or Subscriber Loop

– Almost all Local Loops are TPW (twisted pair wire)

– Range from Few km up to tens of km

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Telecom Components

• Exchange– Switching center in the network– End office specific switching center that

supports subscribers

• Trunks– Branches between exchanges– Multiplexed

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Telecom Components

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Blocking & Non-Blocking

• Blocking– A network may not be able to connect stations

because all paths are in use (more stations than path)

– Used on voice systems• Short duration calls

• Non-blocking– Permits all stations to connect (in pairs) at

once (at least as many paths as stations)– Used for some data connections

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Space Division Switching

• Developed for analog environment, but carried over into digital

• Signal paths are physically separate

• Each connection requires dedicated path (crossbar switch)

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Crossbar Switch

• Number of crosspoints grows as square of number of stations

• Loss of crosspoint prevents connection– Inefficient use of crosspoints– If all stations connected, only a few

crosspoints in use

• Non-blocking

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Crossbar Switch

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Output Lines

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Multistage Switch

• Reduced number of crosspoints

• More than one path through network– Increased reliability

• More complex control

• May be blocking

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3 Stage Switch Example

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Signaling in Circuit Switching

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Control Signals

• Transmission of dialed number

• Call can not be completed indication

• Call ended indication

• Signal to ring phone

• Billing info

• Equipment and trunk status info

• Diagnostic info

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Control Signal Sequence• Both phones on hook• Subscriber lifts receiver (off hook)• End office switch signaled• Switch responds with dial tone• Caller dials number• If target not busy, send ringer signal to target subscriber• Feedback to caller

– Ringing tone, engaged tone, unobtainable

• Target accepts call by lifting receiver• Switch terminates ringing signal and ringing tone• Switch establishes connection

– Call placed

• Connection release when either subscriber hangs up

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In Channel Signaling

• Use same channel for signaling control and call– Requires no additional transmission facilities

• Inband– Control signals have same electromagnetic

properties (frequency) as voice signal– Can go anywhere a voice signal can– Impossible to set up a call on a faulty speech

path

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In Channel Signaling

• Out of band– Voice signals do not use full 4kHz bandwidth– Narrow signal band within 4kHz used for

control– Can be sent whether or not voice signals are

present– Need extra electronics– Slower signal rate (narrow bandwidth)

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Shortcomings of In Channel Signaling

• Limited transfer rate

• Delay between entering address (dialing) and connection

• Overcome by use of common channel signaling

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Common Channel Signaling

• Control signals carried over paths independent of voice channel

• One control signal channel can carry signals for a number of subscriber channels

• Common control channel for these subscriber lines

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Common Channel Signaling

• Associated Mode– Common channel closely tracks interswitch

trunks

• Disassociated Mode– Additional nodes (signal transfer points)– Effectively two separate networks

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Common Channel Signaling

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Circuit Switching Shortcomings

• Inefficient for data because of idle time

• Provides for transmission at constant rate – must transmit and receive at same data rate. Limits versatility

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Packet Switching

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Packet Switching Basic Operation• Data transmitted in small packets

– Typically 1000 octets (8 bit byte)– Longer messages split into series of packets– Each packet contains a portion of user data

plus some control info

• Control info– Routing (addressing) info

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Packet Switching Basic Operation• Packets are received, stored briefly

(buffered) and passed on to the next node– Store and forward

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Advantages

• Line efficiency– Single node to node link can be shared by

many packets over time– Packets queued and transmitted as fast as

possible

• Data rate conversion– Each station connects to the local node at its

own speed– Nodes buffer data if required to equalize rates

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Advantages

• Packets are accepted even when network is busy– Delivery may slow down

• Priorities can be used

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Switching Technique

• Station breaks long message into packets

• Packets sent one at a time to the network

• Packets handled in two ways– Datagram– Virtual circuit

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Datagram

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Datagram

• Each packet treated independently

• Packets can take any practical route

• Packets may arrive out of order

• Packets may go missing

• Up to receiver to re-order packets and recover from missing packets

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Datagram

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Virtual Circuit

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Virtual Circuit

• Preplanned route established before any packets sent

• Call request and call accept packets establish connection (handshake)

• Each packet contains a virtual circuit identifier instead of destination address

• No routing decisions required for each packet

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Virtual Circuit

• Clear request to drop circuit

• Not a dedicated path

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Comparisons

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Virtual Circuits vs Datagram

• Virtual circuits– Network can provide sequencing and error

control– Packets are forwarded more quickly

• No routing decisions to make

– Less reliable• Loss of a node loses all circuits through that node

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Virtual Circuits vs Datagram

• Datagram– No call setup phase

• Better if few packets

– More flexible• Routing can be used to avoid congested parts of

the network

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Circuit vs Packet SwitchingCircuit Switched

• Bandwidth guaranteed• Circuit capacity not

reduced by other network traffic

• Circuit costs independent of amount of data transmitted, resulting in wasted bandwidth

Packet Switched• Bandwidth dynamically

allocated on as-needed basis

• May have concurrent transmissions over physical channel

• May have delays and congestion

• More cost-effective, offer better performance

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Timing Comparison

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Packet Size Issue

Transmission time reduced due to parallel processing

Transmission time increased due to fixed header size

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