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Chapter 10

Circuits Switching and Packet Switching

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Content

•  Switched communication networks •  Circuit switching networks •  Circuit-switching concepts •  Packet-switching principles •  X.25 (mentioned but not covered) •  Frame relay (mentioned but not covered)

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Switching Networks

•  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

•  Data routed by being switched from node to node

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Nodes

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

•  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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Simple Switched Network

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Circuit Switching

•  Dedicated communication path between two stations

•  Three phases –  Establish –  Transfer –  Disconnect

•  Must have switching capacity and channel capacity to establish connection

•  Must have intelligence to work out routing

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Circuit Switching (II)

•  Inefficient – Channel capacity dedicated for duration of

connection –  If no data, capacity wasted

•  Set up (connection) takes time •  Once connected, transfer is transparent •  Developed for voice traffic (phone)

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Telecommunications Components •  Subscriber

–  Devices attached to network •  Subscriber line

–  Local Loop or subscriber loop –  Connection to network –  Few km up to few tens of km

•  Exchange –  Switching centers –  End office - supports

subscribers •  Trunks

–  Branches between exchanges –  Multiplexed

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Circuit Establishment

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Circuit Switching Concepts •  Digital Switch

–  Provide transparent signal path between devices

•  Network Interface •  Control Unit

–  Establish connections •  Generally on demand •  Handle and acknowledge

requests •  Determine if destination is

free •  Construct path

–  Maintain connection –  Disconnect

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Blocking or Non-blocking

•  Blocking –  A network is unable to connect stations because all

paths are in use –  A blocking network allows this –  Used on voice systems

•  Short duration calls

•  Non-blocking –  Permits all stations to connect (in pairs) at once –  Used for some data connections

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Space Division Switching

•  Developed for analog environment •  Separate physical paths •  Crossbar switch

–  Number of crosspoints grows as square of number of stations

–  Loss of crosspoint prevents connection –  Inefficient use of crosspoints

•  All stations connected, only a few crosspoints in use

–  Non-blocking

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Space Division Switch

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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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Time Division Switching

•  Modern digital systems rely on intelligent control of space and time division elements

•  Use time division multiplexing to achieve switching.

•  Two popular methods –  Time slot interchange –  TDM bus

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Packet Switching Principles •  Circuit switching designed for

voice –  Resources dedicated to a

particular call –  Much of the time a data

connection is idle –  Data rate is fixed

•  Both ends must operate at the same rate

•  Data transmitted in small packets –  Typically 1000 octets –  Longer messages split into

series of packets –  Each packet contains a portion

of user data plus some control information

•  Control information –  At least, routing (addressing)

information •  Packets are received, stored

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

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Use of Packets

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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

•  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

•  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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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

•  Clear request to drop circuit •  Not a dedicated path

What is the difference between the virtual circuit switching we

just saw and real circuit switching?

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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

•  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 Switching

•  Performance –  Propagation delay –  Transmission time –  Node delay

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Packet Size

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Circuit v Packet Switching

•  performance depends on various delays –  propagation delay –  transmission time –  node delay

•  range of other characteristics, including: –  transparency –  amount of overhead

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Chapter 11

Asynchronous Transfer Mode (ATM)

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ATM Cell

•  ATM divides all data into small, fixed-size cells.

•  Each cell contains exactly 53-octets. •  5 octets of header •  48 octets of data

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An ATM Cell

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ATM Logical Connections

•  Connection oriented service •  VCC (Virtual Channel Connection)

–  Logical connection in ATM –  A VCC is set up between two end users through the

network and a variable rate, full-duplex flow of fixed-size cells is exchanged over the connection.

•  VPC (Virtual Path Connection) –  A bundle of VCCs that have the same endpoints.

•  VPI (VP Identifier) •  VCI (VC Identifier)

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TP: physical Transmission Path

TPs, VPs, and VCs

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UNI: User Network Interface NNI: Network network Interface

Architecture of an ATM Network

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Example of VPs and VCs

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ATM switch changes the VPI/VCI in each cell it handles.

Routing with a VPC Switch

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A Conceptual View of a VPC Switch

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ATM Layers (ATM Adaptation Layer)

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ATM Header

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Header Fields

•  GFC (General Flow Control) –  for flow control

•  PT (Payload Type) •  CLP (Cell Loss Priority)

–  0: higher priority, should not be discarded … –  1: lower priority

•  HEC (Header Error Control) – CRC: X8+X2+X+1

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CBR: Constant Bit Rate VBR: Variable Bit Rate ABR: Available Bit Rate UBR: Unspecified Bit Rate

Service Classes

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Service Classes and Capacity of Network

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ATM Applications (I)

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ATM Applications (II)

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ATM Applications (III)