Course Code (MSc-TNS)

Asynchronous Transfer Mode (ATM) Switching Report

Assignment 1

Author:

Abid Afsar

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Abstract: Asynchronous Transfer Mode (ATM) switching is a switching technique use for

Broadband Integrated Services Digital Network (B-ISDN). In this paper we will investigate

the ATM switching architecture, operation, functional components and feature such as,

switch fabrics, switch buffering, and so on. We will explore the ATM switching designs

types, and the trade-offs it involve. ATM switch sends information in the form of cell relay or

cell and use virtual channel for internal routing. The core duties of ATM switching involve

routing, cell forwarding, and management. Moreover, the ATM switching techniques were

not a part of ATM standards.

Keywords: ATM, Cell Relay, B-ISDN, virtual channel, switch architecture, ATM standards.

1. Introduction

Asynchronous Transfer Mode (ATM) is a standard network switching method formed by

International Telecommunication union telecommunication standardisation Sector (ITU-T). It

was developed in the early 1980’s for the purpose to unify telecommunication and computer

networks, and to get together the requirements of Broadband Integrated Services Digital

Network. The need of ATM is vital for an increase integration of every sort of application

such as data and voice, audio and video, and so on. The internet is evolving at a very high

speed and this shift in network paradigm brings a unique amalgamation of multimedia, high

speed, and real time services.

The integration of communication needs powerful switches to process, and to perform

routing with a high accuracy. ATM is a connection oriented switching technique, it use

virtual channel to connect network nodes. The virtual channel (VC) has a unique

identification and is refereed as virtual channel identification (VCI). ATM switching takes

different approach in regards to packets propagation, and size constraints as compare to

packet switching, frame really and Ethernet. It use cells of equal size, and propagates it by

using time division multiplexing mechanism. ATM taking services of data link layer and

physical layer of Open System Interconnection (OSI) model. ATM inherits the features of

circuit switching because it based on cell switching and multiplexing technology. The

asynchronous nature of ATM makes it more efficient as compare to the synchronous

technique such as time-division multiplexing because it do not waste empty slots.

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2. Switch Fabric Architecture:

ATM switch architecture is available in several flavours. The most know architectures which

are widely used are single bus, multiple bus and self routing. Self routing can be further

divide into two forms blocking and non-blocking switch architecture. The core purpose of

the ATM switch fabric is to transfer cell from one point to another. The switching use the

combination of software and hardware to pass the incoming data from network node to

input ports and from output ports to other network node, the process is referred as switching

fabrics or switching matrices. The switch designs can be hybrids and can accommodate one

larger switch fabrics to connect other switches or combinations of smaller switch fabrics

make one larger switch fabrics. We will discuss the switch architecture in terms components,

and attributes such as, input module (IM), output module (OU), admission connection control

(CAC), switch management (SM), scalability, blocking level, maximum overall speed, and

support of multicast. Diagrammatically the switch architecture can be represented as

Figure 1: A generic view of ATM switch

2.1. Single Bus:

The simplest type of switch architecture is single bus. It consists on single bus which in

parallel connects multiple circuit board trace. The speed of single bus is between 1 and 10

Giga bit per second (Gbps). The complexity level involved with single bus is low. In terms of

scalability the single bus performance is poor because of the load on single bus. The blocking

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level at each port is low due to bus controller call to arbiter to decide that who will get the bus

first and so on. And multicasting is easy in this sort of bus for the reason that all output

listening to the same bus. Diagrammatically it is represented as,

Figure 2: Single Bus

2.1. Multi Bus:

Multi bus switching architecture works on the concept of broadcasts and do broadcast on

each input port. It eliminates the requirement of bus arbitrations unlike single bus. As the

architecture is changed it adds extra requirements to output port. In multiple buses each bus

shared with multiple circuit board trace. The maximum speed range is between 1 to 20 Gbps.

In relation to output port, if it receives numbers of cells simultaneously then the input

buffering technique is used for arbitration purpose. And bus control entails blocking to

mange output ports. This architecture of bus is also called knockout switch architecture

because every outputs inlet receives only limited number of inputs simultaneously which

makes it favourable choice for scalability. And multicasting is entailed in the nature of this

sort of bus. Diagrammatically it is represented as,

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Bus

Multi- Buses

Figure 3: Multiple Buses

2.3. Self-Routing Bus:

Self routing is a complex switching architecture and use amalgamated switching attributes

and components. It is widely in use in Batcher Banyan networks; it can easily scale large

size of switching components, and is suitable for Very Large Scale Integration (VLSI). It still

needs in-depth research due to its complex nature of operations. The speed of self routing and

inputs are same, therefore it has a high probability of blocking. As compare to the

counterparts of switching architecture it run with high speeds. Diagrammatically it is

represented as

Figure 4: Self Routing

3. Switching Operation:

ATM switch carry out a number of operations through input and output ports. Generally

switch involved a list of jobs such as routing, cell forwarding, and connection controls

managements with other switches and so on.

3.1. User platform: The job of switch is to route input cells to designated output ports. The

cells are encapsulated in a header with payload bytes. When these cells reach to input

ports a virtual connection is established which are uniquely identified by virtual path

identifier and virtual circuit identifier (VPI/VCI) and drive these cells to expected end

user node. In user plan the core functions are mainly performed by input module, switch

matrix, and output modules.5

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Self Routing (Non-Blocking)

Self Routing (Blocking)

3.2. Control Platform: The control platform is responsible for connection establishing, and

controls of virtual path and virtual circuit (VP/VC). The information in the payload in this

case in not visible to the network. In this platform the switch tagging signal and even

produce some signal by itself. The Connection Admission Control (CAC) doing the job

of signalling and which is indispensable. The switch fabric may or may not receives the

signalling or may be pass by Signalling System No. 7 (SS7) network signalling protocol

standard.

3.3. Management Platform: This platform is to control and maintain the operation of

network, and to guarantee that everything is working at optimum rate. The management

operations are further categorised as, traffic management operation, accounting

management operation, performance management operation, fault management operation,

security management operation and so on. These operations are accomplished by switch

management module; it is also responsible for ATM Operation and Management layer

(OAM) functions. Furthermore, it has to determine OAM signalling and may produce

OAM signal. The switch management module also reinforce UNI interface that is called

Interim Local Management Interface (ILMI).

3.4. Traffic Control Operations: The operations such as connection admission control

(CAC), congestion control, and network parameter UPC/NPC are also reinforcing by

switching technique. The operation to control congestion is the responsibility of switch

management module, and the operations to maintain network parameter UPC/NPC is the

responsibility of input module.

4. Switch Interfaces

4.1 Input Module: The function of input module (IM) is to manage table lookup and VCI/VPI

translation. When a cell arrives at input module in first attempt it is VPI value is extracted

in cell header because VPI value is used in the address table for link identification. The

VPI/VCI are not unique on every link and chances of duplication can occur, to resolve

this issue ATM switch use separate address table for each link and also add link identifier

with VPI/VCI. A number of methods can be used for table lookups such as hashing, 6

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search trees or content addressable memories and so on. It can be determine from the

table lookups that the particular VPI value may matches VPC or VCC. If the VPI value

corresponds to VCC then the concerned information can be fetched from translation

lookup table, such as new VPI value, outgoing interface identifier, cell priority, jitter, and

delay. In addition the input module convert SONET optical signal to electrical signal,

doing HEC error check for every incoming cell and discard faulty of damage cells, it

differentiate between OAM cells and signalling cells.

4.2 .Output Module: The function of the output module is the converse of input module. It

prepares the ATM cells for physical transmission medium. It converts the electrical

signals into optical signals, create new HEC field and encapsulate it in header,

multiplex user cells, OAM cells, and signalling cells; doing cell rate decoupling.

5. Cell Switch fabric:

The main purpose of the cell switch fabric module is to transfer packets from input ports

to out ports. A typical switch fabric is a combination of switch elements and transmission

links. Switch fabric is also called interconnection structure. The transmission link only

carries packets and has no power to make intelligent decisions. On the other hand switch

elements are performing intelligent decisions such as internal routing, management and so

on. In addition, it also accomplishing activities such as cell buffering, multitasking and

broadcasting, cell priority and delay scheduling, provide redundancy in relation to fault

tolerance, monitor congestion, traffic concentration and multiplexing.

5.1. Connection Admission Control (CAC): The function of CAC is to establish,

terminate, modifies VP/VC connections. The CAC core duties cover the followings issues

such as to work with higher layer signalling protocol, VPCs/VCCs switch resource

distribution and section of route, parameters generation of UPC/NPC, negotiations in

regards to user traffic contracts, and modification of VPCs/VCCs, provide signalling

reinforcement to ATM Adaptation layer (AAL) in terms of interception or generation of

signalling cells, and making decisions on VPCs/VCCs request acceptance or refusal. In 7

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case of centralised CAC the single processing unit receives the signalling cells from input

module. And further CAC performed interpretation of incoming signalling cells and

making all decisions in relation to admissions control, and switch resource allocation for

different connection.

5.2. Switched Management: The function of the switched management is to manage the

internal operations and components, elements of the switch which are listed as support

of network management, configuration management, customer network management,

switch and their security control, traffic management and performance management. In

relation to switch management area and research. The area is under development and

have no particular standards are established until today. The switch management is a wide

and complex area, entails a number of management operations and the level of

management can vary from nominal to complex. The initial jobs of switch management is

to gather information and their management, close contacts with network manger and

user, and supervisions and coordination of all operations performing in relation to switch.

A bottleneck situation arises when switch is overloaded in centralized switch

management. This can negatively affects the overall performance of the switch

management. Therefore, it is vital that a switched management administer and supervise

all incoming inputs to input module, and similarly for output module.

5.3. Concentration: In ATM switch the concentration work is to resolve all the traffic flow

at the input module before recuing the switch fibre module. Concentrator workout to

combine lower variable bit rate to higher variable bit rate for the sake to standardize the

flow or speed of traffic at input interface.

5.4. Routing and Buffering: Routing and buffering is a core function of switch fabric.

Although ATM switch is not oblige to use routing protocol. Two types of routing are

normally which are dynamic and static, if static routing protocol is in implementation

then dynamic routing is not required. In ATM switch the concept of virtual path was

purposely added to make things easier for routing. The incoming cell arrives at input

ports at switch fabric are simply route to the anticipated output ports. The role of the

virtual path is working as similar as pipes with a specified bandwidth assigned to it. To 8

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implement dynamic routing in ATM switch it use a link-state routing protocol is referred

as Public Network Network Interface (PNNI). Buffers are required when an input or

output module receives a number of cells simultaneous for same input or output.

5.5. Shared Memory Approach: in this approach the incoming cells are in first attempt

converted from parallel to serial structure and stored in a random access memory

sequentially. The controller job is to make decisions in regards memory reading, writing,

and sequence. It can also inspect cell header, and switch fabric internal routing tags.

6. Switched Designs Principal

6.1. Buffering Approaches: The use of buffer has a positive effect on switch performance

and efficiency. Three approaches of buffering are applied in switch which are input

buffering, internal buffering and out buffering. The input buffer is placed before the

input module to control the simultaneous traffic for the input ports but this is suffer from

head of line problem and it is still subject for further study. Out buffer is implementing at

the exit point of out module to manage the flow of cells to the next network node and

improve the level of traffic which are passing through. It improves the overall delay time

and throughput but it entails speed factor which negatively impact the level of scalability

for large networks. Internal Buffering

6.2. Internal Blocking: The internal blocking is a one of the operating mode of ATM

switches. It arises when N numbers of cells are addressed to N number of multiple

outputs which result conflict or collide with each other at output ports level. In general

unblocking switches are preferable to blocking switches but un-blocking fabric switch do

not use buffer which is known cause of congestion and collision. It negatively affects the

throughput, performance of the fabric module. It also affects the scalability of the fabric

switch.

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7. Conclusion

In this brief report our central focus was on ATM switching. ATM switching was

elaborated in a detail which involves switching architecture approaches that entails single

bus, multi bus, and self-routing. In this paper ATM switching system is consist on

interfaces which is composed of input and output module, the switched management

performing duties such as connection, OAM layer functions, and cell fabric switch

module were briefly studied. Furthermore, the cell switch fabric is also refereed as switch

matrix that constitutes concentration, buffer management, and shared memory were

discussed.

8. Glossary

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ATM - Asynchronous Transfer Mode

B-ISDN - broadband integrated services digital network.

CAC - connection admission control

NNI - network to node interface

PNNI - private network to network interface

PVC – permanent virtual circuit

VC - virtual circuit

VCI - virtual circuit Identifier

VCC - virtual channel connection

HEC- header error check

Bibliography

[1]. McDysan, David E., and Darren L. Spohn. Hands-on ATM. New York: McGraw-Hill,

1998. Print

[2]. Kasera, Sumit. . Atm Networks Concepts And Protocols. Second ed. New Delhi: Tata

McGraw-Hill Professional, 2008. Networking Ser. Google. Tata McGraw-Hill Publishing

Company Limited, 01 Jan. 2008. Web. 12 Nov. 2011. <http://books.google.ie>.

[3]. Perros, Harry G. "An Introduction to ATM Networks - Harry G. Perros." Google Books.

N.p., n.d. Web. 14 Nov. 2011. <http://books.google.ie/books?id=ghy9BOw6svMC>.

[4]. "Asynchronous Transfer Mode (ATM) Switching." CCS Voice, Data, Video, Fiber Optic

Cabling Los Angeles, California. N.p., n.d. Web. 14 Nov. 2011. <http://www.ccs-

cabling.com/>.

Reading List

[1]. "ATLAS I Overview Transparencies (ICS-FORTH, ATM Switch)." CARV-ICS-

FORTH, Heraklion, Crete, Greece. N.p., n.d. Web. 14 Nov. 2011.

<http://archvlsi.ics.forth.gr/atlasI/gen_talk1.html>.

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