ATM-1Copyright © December 21, 2004 by Chaim Ziegler, Ph.D.

Asynchronous Transfer Mode (ATM) Networks

! All source media is first broken down into a stream of fixed sized unitsknown as cells.

! Cell streams relating to different media types are multiplexed togetheron a statistical basis for transmission and switching.

! The resulting networks are known as Cell-Based Networks orAsynchronous Transfer Mode (ATM) Networks.

! Transfer Mode:- Packet oriented transfer mode using ATDM- No link-by-link error control- No link-by-link flow control- End-to-End error control if needed- Use of internal virtual circuits- Switching based on table look-up- Fixed size blocks (cells)

ATM-2Copyright © December 21, 2004 by Chaim Ziegler, Ph.D.

ATM Protocol Architecture

! ATM Layer:- Common to all services and provides cell transfer capabilities.- Provides logical connections upon the physical layer:

! Virtual Channels! Virtual Paths

! ATM Adaptation Layer (AAL):- Provides a range of alternative service types known as service

classes.- Converts the source information into streams of 48-octet

segments.

ATM-3Copyright © December 21, 2004 by Chaim Ziegler, Ph.D.

ATM Connection Relationships

! Logical connections in ATM are referrred to as Virtual ChannelConnections (VCCs).

! VCCs are a concatenation of Virtual Channels (VCs).

! A Virtual Path is a group of VCs going the same way.

ATM-4Copyright © December 21, 2004 by Chaim Ziegler, Ph.D.

ATM Logical Connections

! Logical connections in ATM are referrred to as Virtual ChannelConnections (VCCs).

! VCCs are used for:- user-to-user exchange of variable rate, full duplex, fixed-size cells

carrying user data.- user-to-network exchange of control signaling information.- network-to-network exchange of management and routing

information.

! A Virtual Path Connection (VPC) is a bundle of VCCs that have thesame endpoints.

! All the cells flowing over all VCCs in a single VPC are switchedtogether.

! To set up a VCC, there must first be a VPC to the required destinationnode with sufficient available capacity to support the VCC with theappropriate QOS.

! The virtual path control mechanisms include:- calculating routes- allocating capacity- storing connection state information

ATM-5Copyright © December 21, 2004 by Chaim Ziegler, Ph.D.

Representation of VP and VC Switching Hierarchy

ATM-6Copyright © December 21, 2004 by Chaim Ziegler, Ph.D.

ATM Cell Format

! Cell Structure:Header (5 Octets) Information Field (48 Octets)

! User-Network Interface (UNI) Header Format:8 7 6 5 4 3 2 1

Generic Flow Control Virtual Path IdentifierVPI cont.

Virtual Channel IdentifierPayload Type CLP

Header Error Control (HEC)

Notes:GFC - Enables a local switch to regulate (flow control) the entry of

cells by a user into the network.CLP - Cell Loss Priority - provides guidance to the network in the

event of congestion:- 0 indicates cell of relatively high priority.- 1 indicates cell is subject to discard.

PT: 000 - user data, no congestion, SDU type 0001 - user data, no congestion, SDU type 1010 - user data, congestion, SDU type 0011 - user data, congestion, SDU type 1100 - OAM segment associated cell101 - OAM end-to-end associated cell110 - Resource management cell111 - Reserved for future function

ATM-7Copyright © December 21, 2004 by Chaim Ziegler, Ph.D.

ATM Cell Format cont.

! Network-Network Interface (NNI) Header Format:8 7 6 5 4 3 2 1

Virtual Path Identifier

Virtual Channel IdentifierPayload Type CLP

Header Error Control (HEC)

ATM-8Copyright © December 21, 2004 by Chaim Ziegler, Ph.D.

ATM Service Categories

! Real-Time Service:- Constant Bit Rate (CBR)- Real-Time Variable Bit Rate (rt-VBR)

! Non-Real-Time Service:- Non-Real-Time Variable Bit Rate (nrt-VBR)- Available Bit Rate (ABR)

Application specifies a peak cell rate (PCR) that it will use anda minimum cell rate (MCR) that it requires.

- Unspecified Bit Rate (UBR)This is a best-efforts service using capacity not allocated toany other service.

ATM-9Copyright © December 21, 2004 by Chaim Ziegler, Ph.D.

AAL Sublayers

! Convergence Sublayer (CS):Performs a convergence function between the service offered at thelayer interface and that provided by the underlying ATM layer.

! Segmentation and Reassembly (SAR) Sublayer:Provides cell segmentation and reassembly functions.

ATM-10Copyright © December 21, 2004 by Chaim Ziegler, Ph.D.

CS & SAR PDU Formats for AAL 5

AAL 5

! Known as the Simple and Efficient Adaptation Layer (SEAL).! AAL 5 is also used as the AAL in the C plane (Signaling AAL).

! CS PDU and SAR PDU Formats:

Notes:- There is no header in the CS PDU.- The CS PDU is padded to an integral multiple of 48 octets.- The UU fields enables the two correspondent user layers to relate

the AAL SDU to a particular SAP.- The use of the CPI field is yet to be defined.- The Length field indicates the number of octets in the data field.- There is no header or trailer in the SAR PDU.- The SAR protocol is said to be null.- Segments relating to the same CS PDU are identified using the

field in the header of the ATM cell that is used to transport thesegment.

- The SDU Type Bit in user data cells, is used to indicate BOM, COM(type 0) or EOM (type 1)

ATM-11Copyright © December 21, 2004 by Chaim Ziegler, Ph.D.

Example of AAL 5 Transmission