Atm Module2

8/20/2019 Atm Module2

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ATM Basics, Version 1.6 © T.O.P. BusinessInteractive GmbH Page 1 of 18

2 ATM Architecture


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2 ATM Architecture...................................................................1 2.1 The B ISDN Reference Model ............................................3

2.2 Architecture – Overview ....................................................4

2.3 Physical Layer (1/2)............................................................5 2.3 Physical Layer (2/2)............................................................6 2.4 ATM Layer (1/2)...................................................................7 2.4 ATM Layer (2/2)...................................................................8

2.5 Virtual Paths and virtual channels (1/4) ...........................9 2.5 Virtual Paths and virtual channels (2/4) .........................10 2.5 Virtual Paths and virtual channels (3/4) .........................11

2.5 Virtual Paths and virtual channels (4/4) .........................12

2.6 Multi-Point Channels........................................................13 2.7 ATM Adaptation Layers (AALS) ......................................14 2.7.1 AAL Sub Layers.............................................................15

2.7.2 Different AAL-Types (1/3) .............................................16

2.7.2 Different AAL-Types (2/3) .............................................17 2.7.2 Different AAL-Types (3/3) .............................................18


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2.1 The B ISDN Reference Model

The ATM standards are closely connected with the CCITT's B-ISDN standards. The purpose

of B-ISDN is to offer ISDN services at a bandwidth of up to 622 Mbps and beyond. B-ISDNdefines transport systems which are mainly based on cell relay. ATM was developed as thesuitable technology for this. The CCITT B-ISDN standards are very incomplete as far as cellrelay is concerned, another reason for pushing forward ATM standardization over the lastfew years. ATM defines three layers:

• The Physical Layer (PHY), which includes the cables, bandwidths, interfaces, and soon.

• The ATM Layer (ATM), which controls cell use and structure, addressing, and thelike.

• The ATM Adaptation Layer (AAL) for application-specific use of the cell data area.The Management Layer is divided into two sublayers: the plane management and the layermanagement. The plane management contains all the functions of the ATM network andcoordinates the layers with each other. The layer management carries out functionsnecessary for network resource management and for operation and maintenance, theoperation and maintenance functions (OAM).


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2.2 Architecture – Overview

ATM consists of three main layers: the Physical Layer, the ATM Layer and the ATM Adaptation Layer. The ATM Adaptation Layer (AAL) receives payload data from the higherlayers. Its complex functionality allows payload for a wide variety of services to be placed onthe ATM network. The AAL consists of two sublayers which we'll discuss later on. The ATMLayer is responsible for cell switching, cell multiplexing and cell transport, allowing severalservices to be transported over one physical connection simultaneously. The Physical Layerdefines the physical interfaces and the framing structure of a physical connection. It consistsof two sublayers.


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2.3 Physical Layer (1/2)

Let's start our closer examination of the three main layers with the Physical Layer. Thisdefines the physical interfaces and frame structure for an ATM network segment. The twosublayers are the Transmission Convergence (TC) sublayer and the Physical MediaDependent (PMD) sublayer.The TC sublayer is the higher of the two. It generates and recognizes the frames, i.e. the bitstream's logical structure. At the sender, the cell stream is integrated into the bit stream. Atthe receiving end, the cell stream is extracted from the bit stream and passed on to the ATMLayer above. At the sender, the TC sublayer inserts empty cells to adapt the ATM cells'service bit rate to the transmission system's general bit rate. At the receiving end, the emptycells are removed again.


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2.3 Physical Layer (2/2)

The PMD sublayer is the bottommost layer. It carries out functions depending on thephysical medium. It provides bit transmission and bit allocation. Here the line coding and,if needed, electro-optical conversion is carried out. A signal type suitable for the mediumis generated and received for bit synchronization. Bit synchronization information isinserted at the sender and extracted at the receiving end. The ATM Forum specified amultitude of physical standards for both private and Telco use.


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2.4 ATM Layer (1/2)

The ATM Layer is concerned with cell transport both at the sender and at the receiver. At

the sender, it receives payload from a higher layer, adds a header, and passes thegenerated 53 byte cells on to the Physical Layer. Conversely, the ATM Layer at thereceiver is handed over 53 byte cells from the Physical Layer, removes the header, andpasses the remaining 48 bytes on to a higher layer. The ATM Layer does not know thedifferent types of traffic data. However, it distinguishes the quality of service, based onthe information received during connection set-up.


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2.4 ATM Layer (2/2)

Another function of the ATM Layer is Generic Flow Control (GFC). The GFC function is only

defined at the user network interface. There is no internal flow control within the ATMnetwork, i.e. at the Network Node Interfaces, otherwise the network couldn't cope with thehigh transmission rates. Instead ATM controls the data input rate at the user networkinterface to be able to limit the data rate transmitted to the network. At the sender, the ATM Layer bundles the cells from individual virtual circuits into oneasynchronous cell stream. At the receiver, the cells are separated according to their virtualcircuits, so that the connection's integrity, as far as cell order is concerned, is guaranteed bythe ATM Layer.


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2.5 Virtual Paths and virtual channels (1/4)

The ATM Layer fits the header and the payload, which it receives from the AAL, together to

form a 53 byte cell which then must be transmitted over the Physical Layer. Data which is tobe transmitted between two endpoints across a network can pass through a number ofnetwork elements on its way across the network. For this purpose, logical connections areset up between network elements, until a logical route between the sender and the receiverhas been established. This route is called a virtual circuit. Virtual circuits always existbetween the endpoints of a transmission.


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ATM supports both Switched Virtual Circuits (SVCs) and Permanent Virtual Circuits (PVCs),which we'll discuss later on. These are identified by addressing information in the relevantcontrol fields of the ATM cell header. The cell header contains a Virtual Channel Identifier(VCI) and a Virtual Path Identifier (VPI).The VCI associates a cell with a specific virtual channel. Virtual channels exist between twonetwork elements, but they shouldn't be pictured as separate physical links. The VPIidentifies a group of virtual channels that take the same path across the network. A group ofvirtual channels can be assigned to one virtual path. Several of these virtual paths can betransmitted over one physical link.


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Virtual paths are groups of virtual channels which interconnect two endpoints. The VPs are

able to bundle payload data directed to the same destination, making its transport moreefficient. The VCI and the VPI values are allocated during the first connection set-up. Thiscan be done dynamically, like for the SVCs, or manually, like for the PVCs.The current value of the VCIs is only relevant to a specific ATM segment. The VCI for aparticular virtual circuit can be different in different ATM segments. Cells can be copied withinswitches, so that virtual multicast transmissions become possible. Different types of datatraffic can be mixed on the transmission path. Thus voice, data and video can be transmittedover different virtual circuits.


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Several virtual channels between two or more ATM network elements can exist on the same

physical links, sharing the bandwidth available for this connection. Each cell contains enoughaddress information to enable the receiving switch to identify the next switch for the cell. ATMallows cells carrying different types of data to be transmitted on the same physical link. Itmust be considered that PVCs are set up manually by the network administrator, which canbe very time-consuming and uncomfortable. Virtual circuits can also be configureddynamically as required. But this calls for an automated ATM addressing mechanism andsignalling.


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2.6 Multi-Point Channels

ATM doesn't only allow normal bidirectional point-to-point channels, but also unidirectionalpoint-to-multipoint channels. These support only unidirectional traffic – from the sender, orthe root, to several receivers, also known as leaves. Traffic back to the sender, or betweenmulticast receivers, must use different channels.User Network Interface, or UNI3, allows the sender to add receivers. The channel is first setup to one receiver only. The sender can add or remove receivers at any time. There is nospecial status for the first receiver. Receivers can also actively disconnect from the sender.The User Network Interface UNI 4.1 allows end stations to join an existing multipointchannel. This is called Leaf Initiated Join (LIJ). The sender must define that LIJ is allowed atconnection set-up, and decide whether it wants other receivers to join before it gives apermission.


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2.7 ATM Adaptation Layers (AALS)

The highest layer defined in ATM is the ATM Adaptation Layer. This lies above the ATMLayer and supports different service classes, which are required due to the different types ofpayload data. At the sender, the AAL segments the payload frames from higher layers, e.g.Ethernet frames, into 48 byte cells. At the receiver, the AAL reassembles the payload framesfrom the cells received by the ATM Layer. Then it passes them on to the higher protocollayers. The AAL contains the protocols needed to provide end-to-end transmission ofpayload data.


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2.7.1 AAL Sub Layers

The AAL consists of two sublayers: the Segmentation and Reassembly Sublayer (SAR) andthe Convergence Sublayer (CS). The SAR layer is responsible for converting the format ofthe payload units into 48 byte payload cells. SAR is also responsible for detecting lost cellsor duplicates, as well as for padding incomplete cells. To do this, each single SAR ProtocolDatagram Unit (SAR PDU) is provided with specific control fields.The recovery of lost or misordered cells is the responsibility of the Convergence Sublayer(CS). The CS carries out service functions. This involves error recovery, if needed. Since ATM is able to transport a great variety of payload types, the AAL contains differentadaptation protocols that operate simultaneously.


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2.7.2 Different AAL-Types (1/3)

The ATM Adaptation Layer for specific applications only takes effect at the endpoints of anetwork connection, not at the switches, since switches use the information in the cell headerfor routing. Since ATM transmits many different types of payload, and these types of payloadshow different transmission characteristics – thus requiring different qualities of service –there are several AAL Layers defined.


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For ATM networks, there are seven different adaptation protocols currently specified. AAL0 isa null protocol for payload that has already been packetized into 48 byte cells. AAL1supports connection-oriented, constant bit rate services which are transmitted under real-time conditions and with short delays. This protocol is suitable for coded voice and real-timevideo services. It is used when voice data is transported over public ATM networks. AAL2,like AAL1, supports connection-oriented and real-time services, but uses a variable data rate.Real-time applications such as High Definition Television (HDTV) use this protocol.


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AAL3/4 has been defined to transmit computer data over ATM. It supports variable bit rateconnectionless and connection-oriented services, i.e. applications that don't need real time.The complexity of this protocol is not taken up to its full extent. Its functions are currentlytaken over by AAL5. AAL5 is an older AAL protocol which has been introduced to substitute AAL3/4, because AAL3/4 was considered unsuitable for some applications. AAL5 is also known as Simple andEfficient Adaptation Layer (SEAL) and is regarded as the preferred method ofinterconnecting data- and LAN-based systems such as workstations on a network. AAL5 cantransport datagrams of up to 64 kilobyte; its overhead is 10% less in size than that of AAL3/4.The Service Specific Connection Orientated Protocol (SSCOP) is the interface for thetransmission of signalling messages to AAL5.

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