ASYNCHRONOUS TRANSFER MODE

Protocol Architecture

Similarities between ATM and packet switching Transfer of data in discrete chunks Multiple logical connections over single physical

interface

In ATM flow on each logical connection is in fixed sized packets called cells

Minimal error and flow control Reduced overhead

Data rates (physical layer) 25.6Mbps to 622.08Mbps

Protocol Architecture (diagram)

Reference Model Planes

User plane Provides for user information transfer

Control plane Call and connection control

Management plane Plane management

whole system functions Layer management

Resources and parameters in protocol entities

ATM Logical Connections

Virtual channel connections (VCC)Analogous to virtual circuit in X.25Basic unit of switchingBetween two end usersFull duplexFixed size cells

Frame relay : variable-length packets called frames ATM : fixed-length packets called cells Data, user-network exchange (control) and network-network

exchange (network management and routing)Virtual path connection (VPC)

Bundle of VCC with same end points

ATM Connection Relationships

VC (Virtual Channels) all cells belonging to a single message follow the

same VC and remain in their original order until they reach dest. (as lanes of a highway).

VP (Virtual Path) a combination of VCs that bundled together because

part of their paths are the same (as highway itself!)

Advantages of Virtual Paths

Simplified network architectureIncreased network performance and

reliabilityReduced processingShort connection setup timeEnhanced network services

VP/VC Characteristics

Quality of serviceSwitched and semi-permanent channel

connectionsCall sequence integrityTraffic parameter negotiation and usage

monitoring

VPC only Virtual channel identifier restriction within VPC

Control Signaling - VCC

Done on separate connectionSemi-permanent VCCMeta-signaling channel

Used as permanent control signal channel

User to network signaling virtual channel For control signaling Used to set up VCCs to carry user data

User to user signaling virtual channel Within pre-established VPC Used by two end users without network intervention to

establish and release user to user VCC

ATM Cells

Fixed size5 octet(bytes) header48 octet(bytes) information field

Total size of packet = 53 octet (bytes)

Small cells reduce queuing delay for high priority cells

Small cells can be switched more efficientlyEasier to implement switching of small cells

in hardware

ATM Cell Format

VPI

VCI

HEC

Header Format

Generic flow control Only at user to network interface Controls flow only at this point

Virtual path identifierVirtual channel identifierPayload type

e.g. user info or network management

Cell loss priorityHeader error control

Generic Flow Control (GFC)

Control traffic flow at user to network interface (UNI) to alleviate short term overload

Two sets of procedures Uncontrolled transmission Controlled transmission

Every connection either subject to flow control or not

Subject to flow control May be one group (A) default May be two groups (A and B)

Flow control is from subscriber to network Controlled by network side

Header Error Control

8 bit error control fieldCalculated on remaining 32 bits of headerAllows some error correction

HEC Operation at Receiver

Effect of Error in

Cell Header

Impact of Random Bit Errors on HEC Performance

Transmission of ATM Cells

622.08Mbps155.52Mbps51.84Mbps25.6MbpsCell Based physical layerSDH based physical layer

Cell Based Physical Layer

No framing imposedContinuous stream of 53 octet cellsCell delineation based on header error

control field

Cell Delineation State Diagram

SDH Based Physical Layer

Imposes structure on ATM streame.g. for 155.52MbpsUse STM-1 (STS-3) frameCan carry ATM and STM payloadsSpecific connections can be circuit switched

using SDH channelSDH multiplexing techniques can combine

several ATM streams

(Synch dig hierc)

ATM Service Categories

Real time Constant bit rate (CBR) Real time variable bit rate (rt-VBR)

Non-real time Non-real time variable bit rate (nrt-VBR) Available bit rate (ABR) Unspecified bit rate (UBR) Guaranteed frame rate (GFR)

CBR

Fixed data rate continuously availableTight upper bound on delayUncompressed audio and video

Video conferencing Interactive audio A/V distribution and retrieval

rt-VBR

Time sensitive application Tightly constrained delay and delay variation

rt-VBR applications transmit at a rate that varies with time

e.g. compressed video Produces varying sized image frames Original (uncompressed) frame rate constant So compressed data rate varies

Can statistically multiplex connections

nrt-VBR

May be able to characterize expected traffic flow

Improve QoS in loss and delayEnd system specifies:

Peak cell rate Sustainable or average rate Measure of how bursty traffic is

e.g. Airline reservations, banking transactions

UBR

May be additional capacity over and above that used by CBR and VBR traffic Not all resources dedicated Bursty nature of VBR

For application that can tolerate some cell loss or variable delays e.g. TCP based traffic

Cells forwarded on FIFO basisBest efforts service

ABR

Application specifies peak cell rate (PCR) and minimum cell rate (MCR)

Resources allocated to give at least MCRSpare capacity shared among all ARB sourcese.g. LAN interconnection

Guaranteed Frame Rate (GFR)

Designed to support IP backbone subnetworksBetter service than UBR for frame based traffic

Including IP and EthernetOptimize handling of frame based traffic passing

from LAN through router to ATM backbone Used by enterprise, carrier and ISP networks Consolidation and extension of IP over WAN

ABR difficult to implement between routers over ATM network

GFR better alternative for traffic originating on Ethernet Network aware of frame/packet boundaries When congested, all cells from frame discarded Guaranteed minimum capacity Additional frames carried of not congested

ATM Bit Rate Services

ATM standard defines 3 layers

Layers and functions:

1. AAL (Application adaptation layer) Accept transmissions from upper-layer

services and maps them into ATM cells.

2. ATM Layer Providing routing, traffic management,

switching, and multiplexing services.

3. Physical layer

Segmentation and Reassembly PDU

AAL Type 1

Supports app. that transfer info at constant bit rates ,eg. video and voice

If data is long divided into 47 bytes long data + 1 byte(8 bits) header

Passed to ATM layer = 48-byte data unit

Example of calculation

An AAL1 layer receives data at 2Mbps. How many cells are created per second by the ATM layer?

Solution: 2M / 8 bits = 250,000 bytes per second 250,000 byte / 47 bytes = 5319.14 = 5320 ATM cells

AAL Type 2

Support variable bit-rate app.

If data is long divided into 45 bytes long data + 1 byte(8 bits) header

+ 2 bytes trailer

Passed to ATM layer = 48-byte data unit

AAL Type 3/4

Support connection-oriented and connectionless data services

Limitation : receive data packet <= 65,535 bytes Data divided into 44-byte long data

+ 4-byte header (at beginning of message) + padding (if necessary )= 0-43 bytes+ 4-byte trailer (at end of message)

Passed to ATM layer 1. First, 4-byte(16 bits) header 2. In the middle = 44-byte data unit (until finished)3. Padding (if necessary )= 0-43 bytes4. Finally, 4-byte trailer

AAL Type 5

Support app. where transmissions are not route through multiple nodes @ multiplexed with other transmission Sequencing & error correction mechanisms are

unnecessary overhead (in front)

Limitation : receive data packet <= 65,535 bytes

Data divided into 48-byte long data + padding (if necessary )= 0-47 bytes+ 8-byte trailer (at end of message)

Passed to ATM layer 1. First, 44-byte data unit (until finished)2. Padding (if necessary )= 0-47 bytes3. Finally, 8-byte trailer