Asynchronous Transfer Mode(ATM)

Mathews VergisTEL 660

Winter trimester 2005 - 2006

Topics to be discussed:

• Need for network convergence

• Introduction to ATM

• ATM Interfaces and Service categories

• Basic ATM Concepts and Operation

• ATM Cell Structure and Addressing

• ATM Layers

Popular misconceptions even among Engineers

• The phasor was invented by Captain Kirk of Star Trek

• Armature reaction is a chemical reaction

• ATM stands for Automated Teller Machines

Need for Network Convergence

• PSTN sometimes used as a data network backbone – but since it is circuit switched (voice optimized) not very WAN efficient

• Delay sensitive traffic such as voice not possible on data networks since there is no guarantee of QoS

Types of Traffic and demand on a communication channel

Voice• Its generation is asynchronous (a speaker may speak

anytime) • Its transmission must be synchronous (once the

message starts, it must flow continuously as it is spoken)

• The bandwidth required for a voice conversation in digital communication is relatively small and constant (64K)

• The signals may contain a high degree of error and the information can still be retrieved correctly

Types of Traffic and demand on a communication channel

Video • The generation is synchronous (continuous) • Its transmission is synchronous. The

bandwidth required is variable and it could range from under 64 Kbps to several Mbps in the same session.

• Error control should be tight - otherwise the wrong information on the monitor may trigger severe wrongful actions

Types of Traffic and demand on a communication channel

Data • Its generation could be either asynchronous (text) or

synchronous (telemetry) • Its transmission in general can be asynchronous (data

typically can wait patiently in buffers)• The information is extremely error-sensitive, so

extreme caution must be exercised in transmission and error control must be very tight.

How can we combine voice , data and video on the same link?

• Fixed and relatively short packets

• Delays associated with each packet are going to be short and fixed – predictable transmission

• If Voice and Video can be given priority handling – then mixing is possible without any diminishing in quality

Introduction

• ATM – Asynchronous Transfer Mode

• It is a high speed, connection – oriented switching and multiplexing technology capable of transmitting voice, video and data and interconnecting LAN’s

• ATM is asynchronous because information streams can be sent independently without the need of a common clock

History of ATM

• Developed in the early 80’s as a switching technology for Broadband Integrated Services Digital Network

• Anchorage Accord in 1996 declares availability of specs required to implement a multi-service ATM network

Market Segments of ATM

Features and Benefits of ATM

• Convergence of Voice , Video and Data on one network

• High speed switching at hardware level• Bandwidth on demand• Predefined and guaranteed QoS and CoS• Superior Management features• Scalability in network size and speed• Ease of integration with other technologies

ATM Applications

ATM Fast Packet Standards and Services

• Handles traffic through fast – packet switching technique

• Must be able to handle both circuit and packet switching

• Must also be able to accommodate the different bit rates – variable (packet switching) and constant (circuit switching)

• Uses Cell relay technology

Important terms relevant to ATM

• Quality of Service (QoS) :A broadly used term that refers to the performance attributes of an end-to-end connection. A QoS definition for data would address attributes such as error rates, lost packet rates, throughput, and delay

• Class of Service (CoS) :It is a way of managing traffic in a network by grouping similar types of traffic together and treating each type as a class with its own level of service priority

• Fast Packet Switching :A packet switching technique that increases the throughput by eliminating overhead. Overhead reduction is accomplished by allocating flow control and error correction functions to either the user applications or the network nodes that interface with the user. Cell relay is an implementation of this.

Cells and Cell relay

• A Cell is a formatted packet that uses a fixed length data unit

• Cell relay is the process of moving these cells through switching elements

• Fixed size cells can be switched at a very high speed and add predictability to data transmissions

• Variable length frames produce unpredictable patterns and performances as the buffer time cannot be determined

• Cell tax – overhead imposed by ATM cells which can cut into amount of data that can be transferred

ATM Interfaces

• ATM is a connection oriented technique designed to transport both connection and connection-less services

• Operations at the boundary of a network are connection oriented

• Within the network the operation is connectionless

User to Network Interface (UNI) : Connection existing between the user equipment and ATM equipment.

Network to Network Interface (NNI) : Connection via which traffic travels between ATM devices in the same network.

Intercarrier Interface (ICI) : Used to send traffic across intermediate networks

Data Exchange Interface (DXI) : Used to transmit packets rather than cells to the ATM interface when non – ATM equipment is used

ATM Service Categories

• Allow for traffic to be buffered and queued for later transmission

• Can permit loose timing and asynchronous operations between sender and receiver

Fundamental ATM Operations Concept

• A virtual or logical connection is established• ATM forms a packet of fixed length – 53 octets ( 5

octet header and 48 octet information field )• Cells are placed in a queue, on reaching ATM switch• Cells are then multiplexed asynchronously with other

cells for transmission• Switch adapts the incoming bit rate to match the

transmit channel bit rate• Switch inserts dummy cells to meet the aggregate bit

stream rate of 155.52 Mbps

ATM Virtual paths and Channels

ATM Virtual Circuit Terminology• Virtual Channel (VC) – provides

a fixed pathway or route between 2 points. Setup across an ATM network whenever data transfer begins.

• Virtual Path (VP) – groups of VC’s used to tell a switch how to forward an ATM cell through an ATM network

• Virtual Path Identifier (VPI) – in the ATM header used to identify route established in the ATM Switch

• Virtual Channel Identifier (VCI) - in the ATM header used to identify a channel within a VP

ATM Virtual paths (cont’d)• Transmission path – physical

media transporting the cells• Virtual Channel Connection

(VCC) – connection from source end user VCI to destination user VCI

• Virtual path Connection (VPC) – connection from source end user VPI to the destination end user VPI

• Operation : Cell is received across a link on a known VPI or VCI value - their values are remapped as necessary as all VCI’s and VPI’s are only significant to the local link

ATM VCI and VPI Swapping• Logical ID Swapping : This is the technique by which the Logical ID

of one link is changed to another one as the cell passes through it. ATM switch changes the cell header VPI/VCI fields to reflect a new VPI and VCI for the outgoing cell. This can be done in one of 2 ways : VP is predefined in the switch or VP is set up dynamically when cell reaches the switch.

• Forwarding process depends on 2 lookup tables within the switch: VP table – record of VP’s on each link

VC table – maintain the output VP and VC to send the cell• Connection Admission Control (CAC) - procedure used to decide if a

request for an ATM connection can be accepted based on the attributes of both the requested connection and the existing connections

VC and VP Swapping

ATM Cell Structure

ATM Cell Structure

ATM Addressing• Uses addressing similar to

numerical addressing for telephone numbers

• Uses E.164 addresses for public ATM (B- ISDN) networks

• Extended ATM addressing to include private networks – Overlay model – ATM layer maps network layer (IP) addresses to ATM addresses. Address format uses OSI network service access point (NSAP) addresses

• NSAP address – providing the logical point between the network and transport layers of the OSI model – the location of this point is identified by network service provider

ATM Address formats

• Currently 4 formats used for ATM networks

• The fields are divided into 2 sections: network and user

• Network prefixes : fields as needed by the network side of the UNI

• User prefixes : Fields as needed by the user side of the UNI

ATM address formats (cont’d)

ATM Routing Domains and Areas

ATM address – 20 byte string that has the following fields :

• Country code• Administrative authority• Routing domain• Area identifier• End system identifier• NSAP• International code• ISDN telephone number

ATM Routing Domains and Areas

• Authority and Format Identifier (AFI): This identifier is part of the network level address header. Value for 1st AFI field can be : DCC (hex 39),E.164 (hex 45), ICD (hex 47)

• Routing domains : used for traffic management and allocating bandwidth capacity. Defined in the lookup tables in the switch.

• End System Identifier (ESI) : identifies an end system ( computer or LAN) within an area

• Selector Field not used by the ATM network• Purpose of the ATM address format is identify ATM devices

in an ATM network

ATM Routing Domains and Areas

ATM Layers

• ATM architecture uses a logical reference model to describe its functions

• ATM functions correspond to physical layer and part of the Data Link layer of the OSI model

• On its own ATM has function at layers 1and 2 of the OSI model, but today TCP/IP is routed over ATM networks which means it can also function at layers 3 and 4 of the OSI model.

ATM reference model

• The ATM reference model constructed as 3 planes which span all the layers :

• Control – generates and manages the signaling requests• User – manages the transfer of data• Management – contains 2 parts:

– Layer management : manages layer specific functions– Plane management : manages and co – ordinates functions related to

the whole system

ATM and OSI model

ATM Adaptation layer

• Has 2 sublayers:– Convergence Sublayer ( CS )

• Determines the Class of service (CoS) for the incoming traffic

• Provides a specific AAL service at an AAL network service access point (NSAP)

– Segmentation and Reassembly Sublayer (SAR)• Segments higher – level user data into 48 – byte cells

plus necessary overhead at the sending node and reassembles cells at the receiving node

AAL Types and Class of Service (CoS)

• Depending on data type, the AAL protocol provides 5 AAL types to accommodate a particular service class

• AAL 5 is the most popular AAL type – For IP, LAN frames , signaling messages, frame relay , video

ATM Layer

• Performs the framing , multiplexing / demultiplexing of cells and also does the switching

• Generates cell headers on transmitting node – based on information from higher layers

• Generic flow control• VCI /VPI translation• Extracts cell headers on a receiving node and

passes cell payload to higher layers

Physical Layer

• Transports ATM cells on a communications channel and defines mechanical specifications ( connectors etc.)

• 2 sublayers:– Transmission Convergence sublayer

• Maps cells into the physical layer frame format on transmit and delineates ATM cells in the received bit stream

• Generates HEC on transmit• Generates idle cells for cell rate decoupling or speed matching

– Physical medium sublayer• Medium dependent function like bit transfer, bit alignment

References:

• http://www.techfest.com/networking/atm/atm.htm

• http://www.dit.upm.es/snh/arhelp/glossaries/atmf/gloss-a.html

• http://www.rhyshaden.com/atm.htm

• http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/atm.htm

• Trivedi, Carol, “Wide Area Networks”; EMCParadigm 2004