AsynchronousAsynchronous Transfer ModeTransfer Mode

(ATM)(ATM)

Issues Driving LAN ChangesIssues Driving LAN Changes

• Traffic Integration

– Voice, video and data traffic

– Multimedia became the ‘buzz word’

• One-way batch Web traffic

• Two-way batch voice messages

• One-way interactive Mbone broadcasts

• Two-way interactive video conferencing

• Quality of Service guarantees (e.g. limited jitter, non-blocking streams)

• LAN Interoperability

• Mobile and Wireless nodes

Stallings “High-Speed Networks”

Stallings “High-Speed Networks”

A/D

Voice

s1 , s2 …

Digital voice samples

A/D

Video

… Compression

compressed

frames picture frames

Data

Bursty variable-length

packets

cells

cells

cells

AAL

AAL

AAL

ATM Adaptation Layers

MUX

`

Wasted bandwidth

ATM

TDM

4 3 2 1 4 3 2 1 4 3 2 1

4 3 1 3 2 2 1

Voice

Data

packets

Images

Asynchronous Transfer Mode Asynchronous Transfer Mode (ATM)(ATM)

ATMATM • ATM standard (defined by CCITT) is widely

accepted by common carriers as mode of operation

for communication – particularly BISDN.

• ATM is a form of cell switching using small fixed-

sized packets.

Header Payload

5 Bytes 48 Bytes

Basic ATM Cell Format

ATM Conceptual ModelATM Conceptual Model Four Assumptions Four Assumptions

1. ATM network will be organized as a hierarchy.

User’s equipment connects to networks via a UNI (User-Network Interface).

Connections between provided networks are made through NNI (Network-Network Interface).

2. ATM will be connection-oriented.

A connection (an ATM channel) must be established before any cells are sent.

X

X

X

X

X

X

X

X

X

Private

UNI

Public

UNI

NNI

Private

NNI

Private ATM

network

Public

UNI B-ICI

Public ATM

network A

Public ATM

network B

Figure 9.5

ATM ConnectionsATM Connections

• two levels of ATM connections:

virtual path connections

virtual channel connections

• indicated by two fields in the cell header:

virtual path identifier VPI

virtual channel identifier VCI

Physical Link

Virtual Paths

Virtual Channels

ATM Virtual Connections ATM Virtual Connections

ATM Conceptual ModelATM Conceptual Model Assumptions Assumptions

3. Vast majority of ATM networks will run on

optical fiber networks with extremely low error rates.

4. ATM must support low cost attachments. • This decision lead to a significant decision – to

prohibit cell reordering in ATM networks.

ATM switch design is more difficult.

GFC (4 bits) VPI (4 bits)

VPI (4 bits) VCI (4 bits)

VCI (8 bits)

VCI (4 bits) PT (3 bits) CLP

(1 bit)

HEC (8 bits)

ATM cell

header

Payload

(48 bytes)

UNIUNI Cell FormatCell Format

2

3

N

1 Switch

N

1 …

5

6

video 25

video

voice

data

32

32 61

25

32

32

61

75

67

39

67

N

1

3

2

video 75

voice 67

data 39

video 67

… …

ATM Cell Switching ATM Cell Switching

c ATM

Sw

1

ATM

Sw

4

ATM

Sw

2

ATM

Sw

3

ATM

DCC

a b

d e

VP3 VP5

VP2

VP1

a

b c

d e Sw = switch

Digital Cross Connect

Only switches virtual paths

ATMATM Protocol ArchitectureProtocol Architecture

• ATM Adaptation Layer (AAL) – the protocol for packaging data into cells is collectively referred to as AAL.

• Must efficiently package higher level data such as voice samples, video frames and datagram packets into a series of cells.

Design Issue: How many adaptation layers should there be?

Plan

e man

agem

ent

Management plane

Control plane User plane

Physical layer

ATM layer

ATM adaptation layer

Higher layers Higher layers

Lay

er man

agem

ent

AAL

ATM

User

information

User

information

AAL

ATM

PHY PHY

ATM

PHY

ATM

PHY

…

End system End system Network

OriginalOriginal ATM Architecture ATM Architecture

• CCITT envisioned four classes of

applications (A-D) requiring four distinct

adaptation layers (1-4) which would be

optimized for an application class:

A. Constant bit-rate applications CBR

B. Variable bit-rate applications VBR

C. Connection-oriented data applications

D. Connectionless data application

ATM ArchitectureATM Architecture An AAL is further divided into:

The Convergence Sublayer (CS)

manages the flow of data to and from SAR sublayer.

The Segmentation and Reassembly Sublayer

(SAR)

breaks data into cells at the sender and reassembles

cells into larger data units at the receiver.

Original ATM Architecture

Transmission

convergence

sublayer

Physical medium

dependent sublayer

Physical

medium

ATM layer

Physical layer

• The AAL interface was initially defined as classes A-D

with SAP (service access points) for AAL1-4.

• AAL3 and AAL4 were so similar that they were

merged into AAL3/4.

• The data communications community concluded that

AAL3/4 was not suitable for data communications

applications. They pushed for standardization of AAL5

(also referred to as SEAL – the Simple and Efficient

Adaptation Layer).

• AAL2 was not initially deployed.

OriginalOriginal ATM ArchitectureATM Architecture

Revised ATM Architecture

RevisedRevised ATM Service CategoriesATM Service Categories

Class Description Example

CBR Constant Bit Rate T1 circuit

RT-VBR Real Time Variable Bit

Rate

Real-time

videoconferencing

NRT-VBR Non-real-time Variable Bit

Rate

Multimedia email

ABR Available Bit Rate Browsing the Web

UBR Unspecified Bit Rate Background file

transfer

QoSQoS, , PVCPVC, and , and SVCSVC • Quality of Service (QoS) requirements are

handled at connection time and viewed as part of signaling.

• ATM provides permanent virtual connections and switched virtual connections.

– Permanent Virtual Connections (PVC)

permanent connections set up manually by network manager.

– Switched Virtual Connections (SVC)

set up and released on demand by the end user via signaling procedures.

AAL ServicesAAL Services

Connectionles

s

Date Transfer

Bit Rate

Connection

Mode

Examples

of Services

Circuit

Emulation

Constant Bit

Rate

Video

and Audio

Constant Variable

Connection Oriented Connectionle

ss

Variable Bit

Rate Video and Audio

Connection-

oriented

Data

Transfer

AAL

TYPE AAL 1 AAL 2

AAL 3/4

AAL 5

Class A Class B Class C Class D

Related Not Related

Service

Timing

between

Source and

Destination

AAL3/4

AAL 5

AAL Type 1 AAL Type 1 ProtocolProtocol

• Supports Class A traffic, i.e., constant bit rate data with specific requirements for

delay, delay jitter, and timing, e.g., PCM voice, CBR video, and emulation of T-

carrier circuits (DS1, DS3)

• Receives constant bit rate stream with a well defined clock from source and

delivers the same to the destination

• Provides for timing recovery (using SRTS), synchronization, and indication of

lost information not recovered by AAL1

• Summary of AAL 1 functions

– Segmentation and reassembly of user information

– Handling of cell delay variation

– Handling of cell payload assembly delay

– Handling lost and misinserted cells

– Recovery of sending clock frequency at receiver

– Checking and handling AAL PCI (header) error

AAL Type 1 SARAAL Type 1 SAR--PDUPDU • Consists of 1 octet header (PCI) and 47 octets of payload

• Sequence Number (SN): A 1-bit Convergence Sublayer

Indication and 3-bit sequence count to detect deletion or

misinsertion of cells

• Sequence Number Protection (SNP): 3-bit CRC with even

parity for detecting and correcting SN error

SN SNP SAR-PDU Payload

4 bits 4 bits 47 Octets

AAL Type 1 Sequence Number and Sequence Number AAL Type 1 Sequence Number and Sequence Number ProtectionProtection

• The 4 bit RTS is transferred by the CSI bit in successive

SAR-PDU headers with an odd SN (SN = 1, 3, 5, 7)

• For P format operations, the CSI value in SAR-PDU headers

with an even SN (SN = 0, 2, 4, 6) is set to 1

Cyclic Redundancy Check Even Parity

1 bit 3 bit

1 bit 3 bits

Convergence Sublayer

Indication (CSI) Sequence Number (SN)

… Higher layer User data stream

Convergence

sublayer

SAR sublayer

ATM layer

CS PDUs

SAR PDUs

ATM Cells

47 47 47

1 47 1 47 1 47

H H H

5 48

H

5 48

H

5 48

H

b1 b2 b3

AAL 1

AAL Type 1 CS PDUAAL Type 1 CS PDU

• Two CS PDU formats

– Non-P format: No CS header (CS PCI), 47 octet user

information for transfer of unstructured data such as

circuit emulation of full DS1 or DS3

– P format: 1 octet header (Structure Pointer SP), and 46

octet user information for transfer of octet-aligned data

such as N x 64 kbps (e.g., fractional DS1) services

47 Octets AAL User Information

CS-PDU Non-P Format

CS-PDU P Format

46 Octets AAL User Information1-Octet SP

AAL Type 2 Protocol AAL Type 2 Protocol

• Designed to support Class B (VBR) traffic

• Supports variable bit rate data where a strong timing

relationship between source and destination is required, e.g.,

VBR audio and video

• Data passed to AAL2 from higher layers at the source at

fixed intervals and must be passed to the destination at the

same rate

• The amount of data passed to AAL2 may vary with each

transfer

• Supports voice compression and silence suppression

• Supports idle voice channel deletion

• Supports multiple user channels with varying bandwidth on a

single ATM connection

AAL Type 2 Voice Application ExamplesAAL Type 2 Voice Application Examples

• PBX-to-PBX trunking for compressed voice

• ATM trunking on public-switched telephone network

• ATM backbone for cellular systems and personal

communications services (PCS)

• ATM backbone connectivity to packet telephone

PBXPBX--toto--PBX Connectivity PBX Connectivity

Compression

Silence Suppression

Packetization

PBX

PCM Voice

Compression

Silence Suppression

Packetization

PBX

PCM Voice

AAL

ATM

PHY

ATM Network

AAL

ATM

PHY

AAL Type 2 PacketsAAL Type 2 Packets

• AAL2 provides bandwidth-efficient transmission of low-

rate, short, and variable-size packets for delay sensitive

applications

• AAL2 uses one ATM connection between two points to

carry packets from multiple native connections

• The ATM payloads from successive cells of the ATM

connection are used as a byte stream on which packets

from different native channels, called logical link channels

(LLCs), are packed without regard to the cell boundaries

• A channel identification (CID) field is used in the packet

header to identify the LLC to which a packet belongs

• A length indicator (LI) field is used to identify the

boundaries of variable-length LLC packets

AAL Type 2 Protocol AAL Type 2 Protocol SublayersSublayers

Service Specific Convergence Sublayer

(SSCS)

Common Part Sublayer (CPS)

AAL 2 Common Part AAL 2 Common Part SublayerSublayer

• Defines an end-to-end AAL connection as a concatenation of

AAL2 channels

• Each AAL2 channel is a bi-directional virtual channel, with the

same channel identifier value for both directions

• AAL2 channels are established over an ATM layer PVC, SPVC,

or SVC

• Provides basic structure for identifying the users of the AAL

• Assembling/disassembling the variable payload associated with

each individual user

• Error detection and correction

• Multiplexing multiple AAL channels (merging multiple streams of

CPS packets) onto a single ATM connection

• Provides QoS through the choice of AAL-SAP for data transfer

AAL 2 StructureAAL 2 Structure

SSCS-PDU Header

(if present)

SSCS-PDU Trailer

(if present) SSCS-PDU Payload

SSCS-PDU

Packet Header

(PH)

Packet Payload

(PP)

Packet

Start Field

(STF) CPS-PDU Payload

CPS-PDU

Cell Header Cell Payload

Service Specific

Convergence

Sublayer (SSCS)

Common Part

Sublayer (CPS)

ATM Layer

AAL-SAP

ATM-SAP

Format of AAL2 PacketFormat of AAL2 Packet

• Channel Identification (CID): Uniquely identifies the individual user

channel (LLC) within the AAL2, and allows up to 248 (8 - 255)

individual users within each AAL2 structure.

• Length Indicator (LI): Identifies the length of the LLC packet

associated with each individual user, and assures conveyance of

variable payload.

• User-to User Indication (UUI): Provides a link between CPS and an

appropriate SSCS that satisfies the higher layer application. Different

SSCS protocols may be defined to support specific AAL2 user

services, or groups of services. The SSCS may also be null.

CID

8 bits

Information

1 to 45/64 octets

LI

6 bits

UUI

5 bits

HEC

5 bits

Packet Header (PH) Packet Payload (PP)

Packet

Format of AAL2 CPSFormat of AAL2 CPS--PDUPDU

• Packets are combined into CPS-PDU payload

• The Offset Field (OSF) identifies the location of the start of the

remaining length of the packet that possibly started in the

preceding cell and is continuing in the current cell

• Data integrity is protected by the Sequence Number (SN)

• The Start Field is protected from error by a Parity bit (P)

• When it is necessary to transmit a partially filled cell to limit

packet emission delay, the remainder of the cell is padded with

all zero octets

OSF

6 bits CPS Information

SN

1 bit P

1 bit

PAD

0 to 47 octets

Start Field CPS-PDU Payload

CPS -PDU

Cell Header

5 octets

AAL 2 Service Specific Convergence AAL 2 Service Specific Convergence SublayerSublayer

• SSCS is the link between the AAL2 CPS and the

higher layer applications of the individual AAL2

users

• Standards for SSCS are being developed in ITU-T

and ATM Forum

• A null SSCS satisfies most mobile voice

applications

AAL Type 3/4 ProtocolAAL Type 3/4 Protocol

• Supports variable bit rate data where there is no timing relationship

between source and destination, e.g., X.25, frame relay, and

TCP/IP data

• Supports Class C (connection-oriented) and Class D

(connectionless) traffic

• Convergence sublayer divided into two parts:

– Common Part Convergence Sublayer (CPCS)

– Service Specific Convergence Sublayer (SSCS)

• SSCS layer may provide assured or non-assured services, or may

be null

– Assured service provides retransmission of missing or corrupted

SSCS-PDUs and flow control is mandatory

– AAL-SDUs may be lost or corrupted for non-assured service

and flow control is optional

• CPCS provides message mode or streaming mode services

AAL Type 3/4 Protocol AAL Type 3/4 Protocol SublayersSublayers

Service Specific Convergence Sublayer (SSCS)

Common Part Convergence Sublayer (CPCS)

Segmentation and Reassembly Sublayer (SAR)

AAL Type 3/4 CPCSAAL Type 3/4 CPCS--PDUPDU

4-octet header and 4-octet trailer

CPI Common Part Indicator

Btag Beginning Tag

BASize Buffer Allocation Size

PAD Padding

AL Alignment

Etag End Tag

Length Length of CPCS-PDU Payload

CPI Btag BASize CPCS-PDU Payload PAD AL Etag Length

0 - 3 1

1

2

1

1

2

AAL Type 3/4 SARAAL Type 3/4 SAR--PDUPDU

2-octet header, 44-octet payload, and 2-octet trailer

ST Segment Type

SN Sequence Number

MID Multiplexing Identifier

LI Length Indication

CRC-10 CRC Code

SN MID SAR-PDU Payload LI CRC-10 ST

2 bits 4 bits 10 bits 352 bits 10 bits 6 bits

AAL Type 5 ProtocolAAL Type 5 Protocol

• AAL5 is a simple and efficient AAL (SEAL) to perform a

subset of the functions of AAL3/4

• The CPCS-PDU payload length can be up to 65,535 octets

and must use PAD (0 to 47 octets) to align CPCS-PDU length

to a multiple of 48 octets

PAD Padding

CPCS-UU CPCS User-to-User Indicator

CPI Common Part Indicator

Length CPCS-PDU Payload Length

CRC-32 Cyclic Redundancy Chuck

CPCS-PDU Payload PAD CPI Length

1 2 40 - 47

CPCS

UU

1

CRC-32

AAL Type 5 SARAAL Type 5 SAR--PDUPDU

PAD CPCS-PDU

Trailer

CPCS-PDU

SAR-PDU

Payload

CPCS-SDU

SAR-PDU

Payload

SAR-PDU

Payload

SAR-PDU

Payload SAR-PDU

Payload

SAR-PDU SAR-PDU SAR-PDU SAR-PDU SAR-PDU

CPCS-PDU Payload

AAL Type 5 ProtocolAAL Type 5 Protocol

• The CPCS-PDU is divided into 48 octets SAR-PDUs

• Since CPCS-PDU is 48-octet aligned, there is no need for a

length field in the SAR-PDU

• The AAL5 SAR-PDU is 48 octets of data with no overhead of

SAR-PDU header or trailer

• The PTI field of the cell header identifies the beginning or end of

the CPCS-PDU

– PTI = 0X1: End-of-Message (EOM)

– PTI = 0X0: Beginning-of-Message (B0M), or Continuation-

of-Message (COM)

5050

ATM NETWORKSATM NETWORKS

• Public ATM Network:

– Provided by public telecommunications carriers (e.g., AT&T, MCI WorldCom, and Sprint)

– Interconnects private ATM networks

– Interconnects remote non-ATM LANs

– Interconnects individual users

• Private ATM Network:

– Owned by private organizations

– Interconnects low speed/shared medium LANs (e.g., Ethernet, Token Ring, FDDI) as a backbone network

– Interconnects individual users as the front-end LAN for high performance or multimedia applications

5151

Switches in

the middle

End systems

of ATM

5252

Public

ATM Network

Token Ring

Token Ring

FDDI

FDDI

Mainframe Computer

Video

Video

Video

Ethernet

Ethernet

Mainframe Computer

Edge Switch

Ethernet

File Server

Private ATM

Switch

Edge Switch

Edge Switch

Edge Switch

PBX

PBX

Voice

Voice

Private ATM

Network

FDDI

5353

ATM InterfacesATM Interfaces

•

Private UNI

Private

ATM LAN

Public UNI

Public

ATM Network

Public

ATM Network

B-ICI

Private

ATM WAN

P-NNI

5454

How ATM Works?How ATM Works?

• ATM is connection-oriented -- an end-to-end connection must be established and routing tables setup prior to cell transmission

• Once a connection is established, the ATM network will provide end-to-end Quality of Service (QoS) to the end users

• All traffic, whether voice, video, image, or data is divided into 53-byte cells and routed in sequence across the ATM network

• Routing information is carried in the header of each cell

• Routing decisions and switching are performed by hardware in ATM switches

• Cells are reassembled into voice, video, image, or data at the destination

5555

ATM Network

H

H

H H H

H

H

H

Voice Video Data Voice Video Data

BISDN Services

BISDN Services

Reassembly

User Applications User Applications

Workstation Workstation

Multiplexing Demultiplexing

H H H H H H

Segmentation