Aynchronous Transfer Mode

– Atm Essay, Research Paper

Introduction

Aynchronous Transfer Mode (ATM) has been accepted universally as the transfer mode of choice for Broadband Integrated Services Digital Networks(BISDN). ATM can handle any kind of information i.e. voice, data, image, text and video in an integrated manner. ATM provides a good bandwidth flexibility and can be used efficiently from desktop computers to local area and wide area networks. ATM is a connection- oriented packet switching technique in which all packets are of fixed length i.e. 53 bytes(5 bytes for header and 48 bytes for information). No processing like error control is done on the information field of ATM cells inside the network and it is carried transparently in the network.

ATM meets the following objectives for BISDN networks.

1) Supports all existing services as well as emerging services in the future

2) utilizes network resources very efficiently

3)minimizes the switching complexity

4)minimizes the processing time at the intermediate nodes and supports very high transmission speeds.

5)minimizes the number of buffers required at the intermediate nodes to bound the delay and the complexity of buffer management

6)guarantees performance requirements of existing and emerging applications.

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Basic Concepts in ATM

Information Transfer

Routing

ATM Resources

ATM Cell Identifiers

Throughput

Quality Of Service

Usage Parameter Control

Flow Control

Information Transfer: ATM is a fast packet oriented transfer mode based on asynchronous time division multiplexing and it uses fixed length(53 bytes) cells. Each ATM cell consists of 48 bytes for information field and 5 bytes for header. The header is used to identify cells belonging to the same virtual channel and thus used in appropriate routing. Cell sequence integrity is preserved per virtual channel. ATM Adaptation layers(AAL) are used to support various services and provide service specific functions. This AAL specific information is contained in the information field of the ATM cell. Basic ATM cell structure can be shown as follows.

Look on the internet for the cell structure!

Routing: ATM is a connection oriented mode. The header values(i.e. VCI and VPI etc.) are assigned during the connection set up phase and translated when switched from one section to other. Signaling information is carried on a separate virtual channel than the user information. In routing, there are two types of connections i.e. Virtual channel connection(VCC) and Virtual path connection(VPC). A VPC is an aggregate of VCCs. Switching on cells is first done on the VPC and then on the VCC.

ATM Resources: ATM is connection-oriented and the establishment of the connections includes the allocation of a VCI i.e. virtual channel identifier and/or VPI i.e. virtual path identifier and also includes the allocation of the required resources on the user access and inside the network. These resources, expressed in terms of throughput and quality of service, can be negotiated between user and network either before the call-set up or during the call.

ATM Cell Identifiers:ATM cell identifiers, i.e. Virtual Path Identifier, Virtual Channel Identifier and Payload Type Identifier(PTI) are used to recognise an ATM cell on a physical transmission medium. VPI and VCI are same for cells belonging to the same virtual connection on a shared transmission medium.

Throughput: Peak Cell Rate(PCR) can be defined as a Throughput parameter which in turn is defined as the inverse of the minimum interarrival time T between two consecutive basic events and T is the peak emission interval of the ATM connection. PCR applies to both constant bit rate(CBR) and variable bit rate(VBR) services for ATM connections. It is an upper bound of the cell rate of an ATM connection and there is another parameter sustainable cell rate(SCR) allows the ATM network to allocate resources more efficiently.

Quality Of Service: Quality of Service (QOS) parameters include cell loss, the delay and the delay variation incurred by the cells belonging to the connection in an ATM network. QOS parameters can be either specified explicitly by the user or implicitly associated with specific service requests. A limited number of specific QOS classes will be standardized in practice.

Usage Parameter Control In ATM, excessive reservation of of resources by one user affects traffic for other users. So the throughput must be policed at the user-network interface by a Usage Parameter Control function in the network to ensure that the negotiated connection parameters per VCC or VPC between network and subscriber is maintained by each other user. Traffic parameters describe the desired throughput and QOS in the contract. The traffic parameters are to be monitered in real time at the arrival of each cell. CCITT recommends a check of the peak cell rate(PCR) of the high priority cell flow(CLP = 0) and a check of the aggregate cell flow(CLP = 0+1), per virtual connection.

Flow Control In order to control the flow of traffic on ATM connections from a terminal to the network, a General Flow Control(GFC) mechanism is proposed by CCITT at the User to Network Interface(UNI). This function is supported by GFC field in the ATM cell header. Two sets of procedures are associated with the GFC field i.e. Uncontrolled Transmission which is for use in point-to-point configurations and Controlled Transmission which can be used in both point-to-point and shared medium configurations.

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ATM standards

BISDN Protocol Reference Model(PRM) for ATM:

BISDN PRM consists of 3 planes.

1) a User plane for trnasporting user information.

2) a Control plane which is responsible for call control and and connection control functions and it contains mainly signalling information.

3)a Management plane which contains layer management functions and plane management functions.

There is no defined(or standardized) relationship between OSI layers and BISDN ATM protocol model layers. But the following relations can be found. The Physical layer of ATM is almost equivalent to layer 1 of the OSI model and it performs bit level functions. The ATM layer can be equivalent of the lower edge of the layer 2 of the OSI model. The ATM Adaptation Layer performs the adaptation of OSI higher layer protocols.

The BISDN Protocol Reference Model sublayers and functions can be shown by the following figure.

Physical layerFunctions

As shown in the above figure, Physical Layer is divided into two sub-layers. 1.Physical Medium(PM) sub-layer 2.Transmission Convergence(TC) sub-layer

The PM sub-layer contains only the Physical Medium dependent functions. It provides bit transmission capability including bit alignment.It performs Line coding and also electrical/optical conversion if necessary.Optical fiber will be the physical medium and in some cases, coaxial and twisted pair cables are also used.

It includes bit timing functions such as the generation and reception of waveforms suitable for the medium and also insertion and extraction of bit timing information.

The TC sub-layer mainly does five functions as shown in the figure.The lowest function is generation and recovery of the transmission frame.

The next function i.e. transmission frame adaptation takes care of all actions to adapt the cell flow according to the used payload structure of the transmission system in the sending direction. It extracts the cell flow from the transmission frame in the receiving direction. The frame can be a synchronous digital hierarchy (SDH) envelope or an envelope according to ITU-T Recommendation G.703.

Cell delineation function enables the receiver to recover the cell boundaries.Scrambling and Descrambling are to be done in the information field of a cell before the transmission and reception respectively to protect the cell delineation mechanism.

The HEC sequence generation is done in the transmit direction and its value is recalculated and compared with the received value and thus used in correcting the header errors. If the header errors can not be corrected, the cell will be discarded.

Cell rate decoupling inserts the idle cells in the transmitting direction in order to adapt the rate of the ATM cells to the payload capacity of the transmission system. It suppresses all idle cells in the receiving direction. Only assigned and unassigned cells are passed to the ATM layer.

ATM layer functions

ATM layer is the layer above the physical layer. As shown in the figure, it does the 4 functions which can be explained as follows.

Cell header generation/extraction: This function adds the appropriate ATM cell header(except for the HEC value) to the received cell information field from the AAL in the transmit direction. VPI/VCI values are obtained by translation from the SAP identifier. It does opposite i.e. removes cell header in the receive direction. Only cell information field is passed to the AAL.

Cell multiplex and demultiplex: This function multiplexes cells from indiv- idual VPs and VCs into one resulting cell stream in the transmit direction. It divides the arriving cell stream into individual cell flows w.r.t VC or VP in the receive direction.

VPI and VCI translation:This function is performed at the ATM switching and/or cross-connect nodes. At the VP switch, the value of the VPI field of each incoming cell is translated into a new VPI value of the outgoing cell. The values of VPI and VCI are translated into new values at a VC switch.

Generic Flow Control(GFC): This function supports control of the ATM traffic flow in a customer network. This is defined at the B-ISDN User-to-network interface (UNI).

ATM Adaptation Layer Functions(AAL):

AAL is divided into two sub-layers as shown in the figure. 1.Segmentation and reassembly(SAR) 2.Convergence sublayer(CS)

SAR sublayer: This layer performs segmentation of the higher layer information into a size suitable for the payload of the ATM cells of a virtual connection and at the receive side, it reassembles the contents of the cells of a virtual connection into data units to be delivered to the higher layers.

CS sublayer: This layer performs functions like message identification and time/clock recovery. This layer is further divided into Common part conver- gence sublayer(CPCS) and a Service specific convergence sublayer(SSCS) to support data transport over ATM. AAL service data units are transported from one AAL serv- ice access point(SAP) to one or more others through the ATM network. The AAL users can select a given AAL-SAP associated with the QOS required to transport the AAL-SDU. There are 5 AALs have been defined, one for each class of service. The service classification for AAL can be shown by the following figure.

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ATM switching

ATM Switching is also known as fast packet switching. ATM switching node transports cells from the incoming links to outgoing links using the routing information contained in the cell header and information stored at each switching node using connection set-up procedure. Two functions at each switching node are performed by a connection set up procedure. 1) A unique connection identifier at the incoming link and the link identifier and a unique connection identifier at the outgoing link are defined for each connection. 2) Routing tables at each switching node are set up to provide an association between the incoming and outgoing links for each connection. VPI and VCI are the two connection identifiers used in ATM cells.

Thus the basic functions of an ATM switch can be stated as follows. Routing(space switching) which indicates how the information is internally routed from the inlet to outlet. queueing which is used in solving contention problems if 2 or more logical channels contend for the same output. And final function is header translation that all cells which have a header equal to some value j on incoming link are switched to outlet and their header is translated to a value k. There are various Switching networks existing and available from various manufacturers and research institutes for ATM switch architecture.

Queueing disciplines in an ATM switching element:

There are mainly 3 different buffering strategies available determined by their physical location as follows.

Input queueing: In this, the contention problem is solved at the input buffer of the inlet of the switching element. Each inlet contains a dedicated buffer which is used to store the incoming cells until the arbitration logic decides to serve the buffer. The switching transfer medium then switches the ATM cells from the input queues to the outlet avoiding an internal contention. The arbitration logic can be as simple as round-robin or can be complex such as taking into account the input buffer filling levels. However, this scheme has Head of Line(HOL) blocking problem i.e. if two cells of two different inlets contend for the same output, one of the cells is to be stopped and this cell blocks the other cells in the same inlet which are destined for different outlet. This queueing discipline can be shown by the following figure.

Output Queueing: In this queueing discipline, queues are located at each outlet of the switching element and the output contention problem is solved by these queues. The cells arriving simultaneously at all inlets destined for the same output are queued in the buffer of the outlet. The only restriction is that the system must be able to write N cells in the queues during one cell time to avoid the cell loss where N is the total no. of inlets of the switch.In this mechanism, no arbitration logic is required as all the cells can be switched to their respective output queue. The cells in the output queue are served on FIFO basis to maintain the integrity of the cell sequence. The following figure illustrates this mechanism.

Central Queueing: In this scheme, the queueing buffers are shared between all inlets and outlets. All the incoming cells are stored in the central queue and each outlet chooses the cells which are destined for it from this central memory. Since cells for different outlets are merged in this central queue, FIFO discipline is not followed in reading and writing of this queue. Cells can be written and read at random memory locations and this needs a complex memory management system for this scheme. The following figure shows this mechanism.

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Switching Networks can be classified as follows.

Single-stage networks are characterized by a single stage of switching elements which are connected to the inputs and outputs of a switching network.

Extended switching matrix

Funnel-type network

Shuffle exchange network

Multi-stage networks contain several stages which are interconnected by a certain link pattern are sub-divided as follows.

Single-path networks(Banyan networks)

Multiple-path networks

This page is still under construction.

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Performance Issues:

There are 5 parameters that characterize the performance of ATM switching systems. They are 1)Throughput 2)Connection Blocking Probability 3)Cell Loss Probability 4)Switching Delay 5)Jitter on the delay

Throughput:This can be defined as the rate at which the cells depart the switch measured in the number of cell departures per unit time. It mainly depends on the technology and dimensioning of the ATM switch. By choosing a proper topology of the switch, the throughput can be increased.

Connection Blocking probability: Since ATM is connection oriented, there will be a logical connection between the logical inlet and outlet during the connection set up phase. Now the connection blocking probability is defined as the probability that there are not enough resources between inlet and outlet of the switch to assure the quality of all existing as well as new connection.

Cell Loss Probability: In ATM switches, when more cells than a queue in the switch can handle will compete for this queue, cells will be lost. This cell loss probability has to be kept within limits to ensure high reliability of the switch. In Internally Non-Blocking switches, cells can only be lost at their inlets/outlets. There is also possibility that ATM cells may be internally misrouted and they reach erroneously on another logical channel. This is called Cell Insertion Probability.

Switching Delay: This is the time to switch an ATM cell through the switch. The typical values of switching delay range between 10 and 1000Micro Secs. This delay has two parts. 1.Fixed Switching Delay and it is because of internal cell transfer through the hardware. 2.Queueing delay and this is because of the cells queued up in the buffer of the switch to avoid the cell loss.

Jitter on the Delay: This is denoted as the probability that the delay of the switch will exceed a certain value. This is called a quantile and for example, a jitter of 100Micro secs at a 10exp-9 quantile means the probability that the delay in the switch is larger than 100Micro secs. is smaller than 10exp-9.

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User-Network Interface(UNI) & Protocols

B-ISDN UNI Reference Configuration:

Two elements can be used to describe a reference configuration of the User-Network access of B-ISDN. They are 1)Functional groups 2) Reference points. The following figure gives the reference configuration.

B-NT1 functions are similar to Layer1 of the OSI Reference model and some of the functions are 1)Line Transmission Termination 2)Transmission Interface handling 3)OAM functions.

B-NT2 functions are similar to layer1 and higher layers of the OSI model. Some functions of B-NT2 are 1) Adaptation functions for different interface media and topology 2) Multiplexing & demultiplexing and concentration of traffic 3) Buffering of ATM cells 4) Resource allocation & Usage parameter control 5) Signalling protocol handling 6) Interface handling 7)Switching of internal connections. SB and TB indicate reference points between the terminal and the B-NT2 and between B-NT2 and B-NT1 respectively.

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ATM Signalling:

The Signalling capability for ATM Networks has to satisfy the following functions.

1. set up, maintain and release ATM virtual channel connections for information transfer.

2. Negotiate the traffic characteristics of a connection(CAC algorithms are considered for these functions.)

Signalling functions may also support multi-connection calls and multi-party calls. Multi-connection call requires the establishment of several connections to set up a composite call comprising various types of traffic like voice,video, image and data. It will also have the capability of not only removing one or more connections from the call but also adding new connections to the existing ones. Thus the network has to correlate the connections of a call. A multi-party call contains several connections between more than two end-users like conferencing calls.

Signalling messages are conveyed out-of band in dedicated signalling virtual channels in broadband networks. There are different types of signalling virtual channels that can be defined at the B-ISDN user-to-network interface. They can be described as follows. 1. Meta-signalling virtual channel is used to establish, check and release point-to-point and selective broadcast signalling virtual channels. It is bidirectional and permanent. 2.Point-to-point signalling channel is allocated to a signalling endpoint only while it is active. These channels are also bidirectional and are used to establish, control and release VCCs to transport user information. In a point-to- multipoint signalling access configuration, meta-signalling is needed for managing the signalling virtual channels.

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ATM Networks

ATM for LANs: Traditional Local Area Networks(LANs) like Ethernet, Token Ring and Token Bus are limited in speed(10Mb) and thus are limited to particular type of (mainly data) application. For multimedia applications, the bandwidth requirement is high and the information is a combination of voice, video and data and it requires a transfer mode capable of transporting and switching these different types of information. ATM satisfies this requirement for LANs and ATM Forum was created to specify the interfaces for ATM LANs. ATM LANs may be used to interconnect multiple LANs and also to connect to powerful workstations and servers. ATM Forum is a non-profit organization founded in 1991 and is actively involved in the definition of ATM LAN and also in the eventual deployment of a universal BISDN network. A simple ATM LAN can be shown as follows.

ATM allows transmission capacities upto 622 Mbit/s which is enough for most LAN applications. ATM LANs are expected to be mainly star configured and they have simpler network management functions. Deployment of ATM in LANs will also stimulate faster deployment of the public BISDN.

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Traffic Control in ATM Networks

There are many functions involved in the traffic control of ATM networks which are given as follows.

1. Connection Admission Control: This can be defined as the set of actions taken by the network during the call set-up phase to establish whether a VC/VP connection can be made. A connection request for a given call can only be accepted if sufficient network resources are available to establish the end-to-end connection maintaining its required quality of service and not affecting the quality of service of existing connections in the network by this new connection.

There are two classes of parameters which are to be considered for the connection admission control. They can be described as follows.

A. Set of parameters that characterize the source traffic i.e. Peak cell rate, Average cell rate, burstiness and peak duration etc.

B. Another set of parameters to denote the required quality of service class expressed in terms of cell transfer delay, delay jitter, cell loss ratio and burst cell loss etc.

2. Usage Parameter Control(UPC) and Network Parameter Control(NPC): UPC and NPC perform similar functions at User-to-Network Interface and Network-to- Node Interface respectively. They indicate the set of actions performed by the network to monitor and control the traffic on an ATM connection in terms of cell traffic volume and cell routing validity. This function is also known as “Police Function”. The main purpose of this function is to protect the network resources from malicious connection and to enforce the compliance of every ATM connection to its negotiated traffic contract. An ideal UPC/NPC algorithm meets the following features. 1. Capability to identify any illegal traffic situation. 2. Quick response time to parameter violations. 3. Less complexity and much simplicity of implementation.

3. Priority Control: CLP(Cell Loss priority) bit in the header of an ATM cell allows users to generate different priority traffic flows and the low priority cells are discarded to protect the network performance for high priority cells. The two priority classes are treated separately by the network Connection Admission Control and UPC/NPC functions to provide two requested QOS classes.

4. Network Resource Management: Virtual Paths can be employed as an important tool of traffic control and Network resource management in ATM networks. They are used to simplify Connection Admission Control(CAC) and Usage/ Network parameter control(UPC/NPC) that can be applied to the aggregate traffic of an entire virtual path. Priority control can also be implemented by segregating traffic types requiring different quality of service(QOS) through virtual paths. VPs can also be used to distribute messages efficiently for the operation of particular traffic control schemes like congestion notification. Virtual paths are also used in statistical multiplexing to seperate traffic to prevent statistically multiplexed traffic from being interfered with othe types of traffic, for example guaranteed bit rate traffic.

5.Traffic Shaping: Traffic shaping changes the traffic characteristics of a stream of cells on a VPC or VCC by properly spacing the cells of individual ATM connections to decrease the peak cell rate and also to reduce the cell delay variation. Traffic shaping must preserve the cell sequence integrity of an ATM connection. Traffic shaping is an optional function for both network operators and end users. It helps the network operator in dimensioning the network more cost-effectively and it is used to ensure conformance to the negotiated traffic contract across the user-to network interface in the customer premises network.

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Congestion Control in ATM

Congestion control plays an important role in the effective traffic management of ATM networks. Congestion is a state of network elements in which the network can not assure the negotiated quality of service to already existing connections and to new connection requests. Congestion may happen because of unpredictable statistical fluctuations of traffic flows or a network failure.

Congestion control is a network means of reducing congestion effects and preventing congestion from spreading. It can assign CAC or UPC/NPC procedures to avoid overload situations. To mention an example, congestion control can minimize the peak bit rate available to a user and monitor this. Congestion control can also be done using explicit forward congestion notification(EFCN) as is done in Frame Relay protocol. A node in the network in a congested state may set an EFCN bit in the cell header. At the receiving end, the network element may use this indication bit to implement protocols which will lower the cell rate of an ATM connection during congestion.

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Congestion Control in ATM

Congestion control plays an important role in the effective traffic management of ATM networks. Congestion is a state of network elements in which the network can not assure the negotiated quality of service to already existing connections and to new connection requests. Congestion may happen because of unpredictable statistical fluctuations of traffic flows or a network failure.

Congestion control is a network means of reducing congestion effects and preventing congestion from spreading. It can assign CAC or UPC/NPC procedures to avoid overload situations. To mention an example, congestion control can minimize the peak bit rate available to a user and monitor this. Congestion control can also be done using explicit forward congestion notification(EFCN) as is done in Frame Relay protocol. A node in the network in a congested state may set an EFCN bit in the cell header. At the receiving end, the network element may use this indication bit to implement protocols which will lower the cell rate of an ATM connection during congestion.

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ATM Applications

There are several practical applications using ATM Technology. ATM is going to be the Backbone Network for many broadband applications including Information SuperHighway. Some of the key applications can be mentioned as follows.

Video Conferencing

Desktop Conferencing

Multimedia Communications

ATM Over Satellite Communications

Mobile Computing over ATM for Wire-less Networks

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