Asynchronous Transfer Mode

Zaid Mahmood FarhatBEE-FA06-099

Department of Electrical Engineering.

Agenda

• Introduction• Design Goals• Problems Solved• Structure• Cell• Devices• Virtual Connections• Identification of

Path• Services

• Switching• Physical

Connections• Layers (Reference

Model)• Addressing• LAN• Advantages• Disadvantages• References

Intro…

ATM is• Connection oriented switching

technology• Handles digital data format• Consist upon 53-byte cells (Individually,

a cell is processed asynchronously relative to other related cells and is queued before being multiplexed over the transmission path.)

• Prespecified bit rates are either 155.520 Mbps or 622.080 Mbps.

• Speeds on ATM networks can reach 10 Gbps.

Intro…

• Basically Asynchronous Transfer Mode (ATM) is the cell relay protocol designed by the ATM Forum and adopted by the ITU-T.

• In fact, ATM can be thought of as the "highway" of the information superhighway.

Design Goals

• Want to be– Higher data rate or bandwidth– Wide area connectivity without

lowering the effectiveness– Implemented Inexpensively– Compatible to most of telecom

hierarchies– Connection Oriented– Converting from soft to hardware.

Problem Solved practically

• Frame Networks• Mixed Network traffic• Cell Networks• Asynchronous TDM

Structure or Architecture…

• ATM is a cell-switched network. • The user access devices, called the

endpoints, are connected through a user-to-network interface (UNI) to the switches inside the network.

• The switches are connected through network-to-network interfaces (NNIs).

ATM Cell

• ATM transfers information in fixed-size units called cells.

• Each cell consists of 53 octets, or bytes. The first 5 bytes contain cell-header information (contain the

information about the ATM network), and other 48 contain the payload (user

information).

Cell Header Format

• Generic Flow Control (GFC)

• Payload Type (PT)

• Cell Loss Priority (CLP)

• Header Error Control (HEC)

ATM Devices

• ATM Switches: An ATM switch is responsible for cell transit through an ATM network. It can reads and updates the cell header information and quickly switches the cell to an output interface toward its destination.

• ATM Endpoints: An ATM endpoint (or end system) contains an ATM network interface adapter.

Virtual Connection

• Connection between two endpoints is accomplished through transmission paths (TPs), virtual paths (VPs), and virtual circuits (VCs).

• A transmission path (TP) Physical connection (wire, cable, satellite,

and so on) between an endpoint and a switch or between two switches.

Virtual Connection

• A virtual path (VP) provides a connection or a set of connections between two switches.

• Cell networks are based on virtual circuits (VCs). All cells belonging to a single message follow the same virtual circuit and remain in their original order until they reach their destination.

Simple relationship of TP, VP and VC.

Practical relationship of VP and VC.

Identifications for paths

• The designers of ATM created a hierarchical identifier with two levels: a virtual path identifier (VPI) “For specific VP” and a virtual-circuit identifier (VCI) “For particular VC in side the

VP”.

Define the identifiers in pairs as one is VPI and other is VCI.

Identification (Contd…)

• For UNI and NNI we use identifiers of different length as shown in figure.

ATM services

• Two types of services are 1) PVC and 2) SVC

• PVC: Permanent Virtual Circuits allows direct connectivity between sites. In this way, a PVC is similar to a leased line.

• SVC: Switched Virtual Circuits is created and released dynamically and remains in use only as long as data is being transferred. In this sense, it is similar to a telephone call.

ATM Switching

• ATM uses switches to route the cell from a source endpoint to the destination endpoint.

• A switch routes the cell using both the VPIs and the VCIs.

ATM Layers (Reference Model)

ATM consist upon three basic layer format as

»Physical Layer»ATM Layer»AAL (ATM Application Adapter Layer)

ATM Reference Model• The ATM architecture uses a logical model to describe

the functionality that it supports.• ATM functionality corresponds to the physical layer

and data link layer of the OSI reference model.• The ATM reference model is composed of the

following planes, which span all layers:• Control—This plane is responsible for generating and

managing signaling requests.• User—This plane is responsible for managing the

transfer of data.• Management—This plane contains two components:

– Layer management manages layer-specific functions, such as the detection of failures and protocol problems.

– Plane management manages and coordinates functions related to the complete system.

ATM Reference Model• Physical layer—Analogous to the physical layer of the

OSI reference model, the ATM physical layer manages the medium-dependent transmission.

• ATM layer—Combined with the ATM adaptation layer, the ATM layer is roughly analogous to the data link layer of the OSI reference model. The ATM layer is responsible for– the simultaneous sharing of virtual circuits over a physical link

(cell multiplexing) and passing cells through the ATM network (cell relay).

– To do this, it uses the VPI and VCI information in the header of each ATM cell.

– Also use to interface between physical and adaption layer.• ATM adaptation layer (AAL)—Combined with the ATM

layer, the AAL is roughly analogous to the data link layer of the OSI model. The AAL is responsible for– isolating higher-layer protocols from the details of the ATM

processes. – The adaptation layer prepares user data for conversion into cells

and segments the data into 48-byte cell payloads.

ATM reference model

ATM Application Adaption Layer (AAL)

• AAL has four subparts as– AAL1– AAL2– AAL3/4– AAL5

AAL1

• AAL1, a connection-oriented service, is suitable for handling constant bit rate sources (CBR), such as voice and videoconferencing.

• AAL1 requires timing synchronization between the source and the destination.

• Most important task is to synchronize the bytes un the ATM cell.

AAL2

• Traffic type has timing requirements like CBR but tends to be bursty in nature. This is called variable bit rate (VBR) traffic.

• The AAL2 process uses 44 bytes of the cell payload for user data and reserves 4 bytes of the payload to support the AAL2 processes.

AAL3/4

• AAL3/4 supports both connection-oriented and connectionless data.

• An AAL 3/4 PDU (Protocol Data Unit) header consists of Type, Sequence Number, and Multiplexing Identifier fields.

AAL5

• AAL5 is the primary AAL for data and supports both connection-oriented and connectionless data.

• AAL5 also is known as the simple and efficient adaptation layer (SEAL)

• For all cells except the last, a bit in• The Payload Type (PT) field is set to 0 to

indicate that the cell is not the last cell in a series that represents a single frame.

• For the last cell, the bit in the PT field is set to 1.

ATM Physical Connections

• ATM supports two types of connections: point-to-point and point-to-multipoint.

• Point-to-point connects two ATM end systems and can be unidirectional (one-way communication) or bidirectional (two-way communication).

• Point-to-multipoint connects a single-source end system (known as the root node) to multiple destination end systems (known as leaves). Such connections are unidirectional only.

ATM LAN

• ATM can be implemented in the LAN as follows

• Pure ATM Architecture: Here an ATM switch is used to connect the stations in a LAN, in exactly the same way stations are connected to an Ethernet switch.

• Legacy ATM Architecture: Here the stations on the same LAN can exchange data at the rate and format of traditional LANs (Ethernet, Token Ring, etc.) and ATM act as the backbone.

Pure ATM Architecture

Legacy ATM Architecture

Advantages

• ATM supports voice, video and data allowing multimedia and mixed services over a single network.

• High evolution potential, works with existing technologies

• Provides the best multiple service support • Supports delay close to that of dedicated services • Able to use all common physical transmission

paths like SONET. • Ability to connect LAN to WAN

• High speed Mbps and possibly Gbps

Disadvantages

• Flexible to efficiency’s expense, at present, for any one application it is usually possible to find a more optimized technology

• Cost, although it will decrease with time • New customer premises hardware and

software are required • Competition from other technologies -100

Mbps FDDI, 100 Mbps Ethernet and fast Ethernet.

References• Behroz A. Frouzen, “Data Networking and

Communication” edition 4th

• Cisco Systems, “Internetworking Technologies Handbook” edition 4th

• Roger L. Freeman, “Fundamentals of Telecommunications” 1999 ISBN: 0-471-22416-2

• Microsoft ScinTech Library• IEEE Explore• Gilbert Held , “Understanding Data Communications” 6th

edition, published by New Riders Publishing • http://searchnetworking.techtarget.com/sDefinition/

0,,sid7_gci213790,00.html• http://www.tech-faq.com/atm.shtml• http://en.wikipedia.org

for obsession