EEC4113 Data Communication & Multimedia System Chapter 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 CO1

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 CO1

Node Data routed by being switched from node to node Nodes may connect to other nodes only, or to stations and other nodes CO1

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 CO1

Circuit Switching CO1

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 CO1

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 CO1

Circuit Switching Once connected, transfer is transparent Developed for voice traffic (phone) CO1

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 CO1

Telecom Components Exchange –Switching center in the network –End office specific switching center that supports subscribers Trunks –Branches between exchanges –Multiplexed CO1

Telecom Components CO1

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 CO1

Space Division Switching Developed for analog environment, but carried over into digital Signal paths are physically separate Each connection requires dedicated path (crossbar switch) CO1

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 CO1

Crossbar Switch Input Lines Output Lines CO1

Multistage Switch Reduced number of crosspoints More than one path through network –Increased reliability More complex control May be blocking CO1

3 Stage Switch Example CO1

Signaling in Circuit Switching CO1

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 CO1

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 CO1

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 CO1

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) CO1

Shortcomings of In Channel Signaling Limited transfer rate Delay between entering address (dialing) and connection Overcome by use of common channel signaling CO1

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 CO1

Common Channel Signaling Associated Mode –Common channel closely tracks interswitch trunks Disassociated Mode –Additional nodes (signal transfer points) –Effectively two separate networks CO1

Common Channel Signaling CO1

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 CO1

Packet Switching CO1

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 CO1

Packet Switching Basic Operation Packets are received, stored briefly (buffered) and passed on to the next node –Store and forward CO1

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 CO1

Advantages Packets are accepted even when network is busy –Delivery may slow down Priorities can be used CO1

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 CO1

Datagram CO1

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 CO1

Datagram CO1

Virtual Circuit CO1

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 CO1

Virtual Circuit Clear request to drop circuit Not a dedicated path CO1

Comparisons CO1

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 CO1

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 CO1

Circuit vs Packet Switching Circuit 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 CO1

Timing Comparison CO1

Packet Size Issue Transmission time reduced due to parallel processing Transmission time increased due to fixed header size CO1