atm_notes

8/3/2019 ATM_notes

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ATM NETWORKS

8/3/2019 ATM_notes


Switching techniques inCommunication N/Ws

• Switching : When N/W transferstream of data from source todestination it must assign a route(i.e; a sequence of links or channelsconnecting the src to the destn) andallocate a portion of the capacity or

bandwidth of each channel along theroute

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Circuit Switching

• In circuit switching, the route and BW allocated to the streamremain constant over the life time of the stream

• Capacity of each channel is divided into a number of fixed ratelogical channels called circuits

• Thus circuit switching involves assigning a route and one circuitin each link along the route

• Circuit switching has 3 phases – Connection setup phase – Data transfer phase – Connection teardown phase

• Switch computers maintain information about circuits that arebusy and routing tables• These computers execute algorithms for call admission policies

and routing strategies. Also, call duration and other statisticsneeded for administration, billing and maintenance arerecorded.

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Circuit Switching

• Ckt switching is easy to implement compared to otherswitching scemes

• Since stream is assigned a fixed rate ckt, capacity utilizationis low in the case of bursty data streams

• Therefore suitable for voice networks, but not for n/w sdesigned for data transfer

• Due to fixed bandwidth assignment, there is no queuingdelay in ckt s/w ing and we can guarantee the maximum end-to-end delay from src to destn.

• Guaranteed delay feature is useful in video conferencing andother real time applications.

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Packet Switching

• Data steram originating at src is divided into packets of fixed or variable size.

• Time interval between consecutive pkts may varydepending on burstiness of data stream.

• Pkt switching make use of a store-and-forward mechanism

• When bits in a pkt arrive at a switch or router, they areread into a buffer and after storing entire pkt, the switchroutes the pkt over one of its outgoing links

• Pkt remain s queued in the buffer until its outgoing linkbecomes idle.

• store-and-forward mechanism introduces random queuing

delay at each link, because the delay depends on the othertraffic sharing the same link• In pkt S/w ing Total delay at a switch is the queuing delay

plus the time taken to transmit the entire pkt

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Packet Switching

• Packet Switching nein two ways

– Datagram packet networks

– Virtual circuit packet networks

• Datagram Packet Networks

– In datagram packet networks, each packet within a stream is

independently routed – A routing table stored in the router(switch) specifies the outgoinglink for each destination

– The routing table may be static or may be periodically updated

– Each packet must contain bits denoting the address of the srcand destn, though it may be a significant overhead if theaverage packet size is small

– The overhead may be negligible if the pkt size is long, but thenthe transmission time and queuing delay will become large

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Virtual circuit packetNetworks• Similar to ckt switched networks, a fixed route

is selected before any data is transmitted in acall setup phase

• Unlike ckt switched networks, there is no notion

of fixed rate ckt or logical channel• All pkts belonging to the same data streamfollow this fixed route called virtual circuit.

• Each pkt must contain a bit string called virtualcircuit identifier , usually shorter than src and

destination address needed for datagrams• In virtual ckt packet networks call setup phase

takes time and creates a delay not present indatagram pkt networks

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Asynchronous TransferMode(ATM)

• Pkt and ckt switching N/Ws are suitable for messageand CBR traffic respectively

• ATM networks combines the good features of Pkt andckt switching

• ATM is a Packet switched, virtual circuit network• ATM N/W s were first developed in the mid 1980s with

the goal of designing a single networking technologythat would transport real time audio, video as well astext, e-mail and image files

• ATM N/Ws can provide services with a specific Qualityof Service (QoS) to meet the needs of all types of traffic.

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Main features of ATM

1. ATM provides connection oriented service, with data transferred over a virtualcircuit(VC)

2. Data is transferred in a 53-byte packet format called ATM cells. 5 byte header + 48 byte payload

3. Cells from different VCs that occupy same channel or link are statistically multiplexed.4. ATM switches may treat the cell streams in different VC connections unequally over

the same channel in order to provide different QoS.

Header Payload

5 Bytes 48Bytes

Basic ATM Cell Format

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ATM services

• ATM forum specifies Five categories of services that an ATM network can provide:

– Constant Bit Rate

– Variable Bit Rate- Real Time (VBR-RT) – Variable Bit Rate- Non Real Time (VBR-NRT)

– Available Bit Rate

– Unspecified Bit Rate

– Guaranteed Frame Rate

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ATM services –Parameters

• ATM services differ in the Parameters of trafficand the QoS that they specify.

• Parameters of traffic are defined by the Generalizedcell rate algorithm (GCRA) that controls arrival times

of cells. These parameters of traffic are – Peak cell rate

– Sustained cell rate

– Initial cell rate

– Cell delay variation tolerance

– Burst tolerance – Minimum cell rate

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ATM services –Parameters

• Parameters of QoS are:

– Cell Loss Ratio

– Cell Delay Variation

– Peak to peak cell delay variation

– Maximum cell transfer delay

– Mean cell transfer delay

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ATM Architecture

AAL

ATM

PHY

AAL

ATM

PHY

ATM

PHY

ATM

PHY

End system End system

ATM Switch ATM switch

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ATM Architecture

• ATM adaptation layer (AAL) is only present inthe edges of the ATM network

• AAL is analogous to the internet’s transportlayer

• On the sending side, AAL is passed datafrom the higher level application or protocol(such as IP if ATM is used to connect IPdevices)

• On receiving side it passes data upt o higherlayer protocol or application

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ATM Architecture

• AALs have been defined for different services – AAL for constant bit rate services

– AAL2 for variable bit rate services

– AAl5 for IP data gram transport

– AAL 3 for connection oriented streams

– AAL4 for connectionless messages

Among the services provided by AAL are errordetection and segmentation/reassembly

Unit o data handled by AAL is referred to as AALprotocol data unit

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AAL5 PDU

• PAD bits (0-47 bytes) ensures that PDU is an integer multiple of 48bytes, because PDU will be segmented to fit into 48 byte payloadsof the underlying ATM cells

• Length field indentifies the size of the PDU payload, so that PADbits can be removed by he receiver

• CRC field provides error detection using the same cyclic

redundancy check as in Ethernet

• Payload can be up to 65,535 bytes long

PDU payload

PAD Length CRC

0-65535 0-47 2 4

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ATM layer

• ATM layer lies at the heart of the ATMarchitecture

• ATM layer defines the structure of the

ATM cell and the meaning of the fieldswithin the cell

• The first 5 bytes of the cell constitute

the ATM header; remaining 48 bytesconstitute the ATM payload

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ATM cell header

• The fields in the ATM ccell have the following functions

• Virtual Channel identifier (VCI) Indicates the virtual channel to which the cell belongs

• Payload Type (PT):Indicates the type of the payload contained in the cell. There areseveral data pay load types, maintenance pay load types, and an idle cell payload type.PT field also includes a bit that serves to indicate the last cell in a fragmented AAL PDU

• Cell Loss Priority (CLP): a 1-bit field used as a priority indicator. When it is set to 0, thecell is high priority and interim switches must make every effort to forward the cellsuccessfully. When the CLP bit is set to 1, the interim switches sometimes discard the cellin congestion situations

• Header error control(HEC) byte Error detection bits that protect the cell header

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IP over ATM

• ATM network can be used to provide connectivity between IP devices

ATM backbone with four entry/exit points for internet IP traffic

Application layer

Transport layer

Network layer

AAL5

ATM layer

ATM physical layer

Link layer

physical layer

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IP over ATM

• ATM backbone can span very large area andmay have hundreds of ATM switches

• Most ATM backbones have permanent virtual

circuits(VCs) between each pair of entry/exitpoints

• Since Permanent VCs are used, ATM cells arerouted from entry to exit points without having

to establish and tear down VCs dynamically.

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IP over ATM

• Each router interface that connectsto the ATM n/w needs two addresses.An IP address and an ATM address.

This is similar to the IP and MACaddresses in an Ethernet interface

•

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IP over ATM

Issues: IP datagrams into

ATM AAL5 PDUs from IP addresses

to ATM addresses just like IP

addresses to802.3 MAC

addresses!

ATM

network

Ethernet

LANs

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Datagram Journey in IP-over-ATMNetwork

• at Source Host: – IP layer maps between IP, ATM dest address (using ARP)

– passes datagram to AAL5

– AAL5 encapsulates data, segments data into cells, passesto ATM layer

• ATM network: moves cell along VC to destination

• at Destination Host:

– AAL5 reassembles cells into original

datagram – if CRC OK, datagram is passed to IP

atm_notes

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