ATM Technologies

Asynchronous Transfer Mode (ATM)

• Designed by phone companies

• Single technology meant to handle– Voice

– Video

– Data

• Intended as LAN or WAN

• Goal: replacement for Internet

ATM Characteristics

• End-to-end (application to application)

• Connection-oriented interface:– Establish “connection”

– Send data

– Close connection

• Performance guarantees (statistical)

• Uses cell switching

ATM Design Issues

• Different traffic has different demands • Variable packet size introduces more jitter

(variance in delivery time) • Even sending at a constant rate, contention

can result jitter • Small packets incur less jitter and delay, but

less efficient • Large packets more efficient, delay and jitter

is more serious (packet loss)

ATM Cell

Fixed size packet (for highest speed electronics) Size chosen as compromise between voice

(small) and data (large) 5 octet header

48 octet payload

Note: size not optimal for any application

ATM Cell Header

ATM Cell Format • Flow control is used for local flow control (user-to-

network only) • Flow control doesn't appear in network-to-network

interface • Example: it can be used to provide different priorities of

a cell • VPI/VCI together provides identification of the cell

connection (more later) • Payload type indicates the type of the cell (e.g. user data

cell, segment cell) • PRIO: one bit Cell Loss Priority whether or not the cell

can be dropped

ATM Switch

• Building block of ATM network

• Connections to – Computer

– Other ATM switches

• Accepts and forwards cells

Cell Forwarding

Performed directly by hardware Incoming cell sent to an outgoing interface Uses label in cell Motivation: highest speed

Label Switching

• ATM connection identified with 24-bit binary value

• Known as Virtual Path Identifier/Virtual Channel Identifier (VPI/VCI)– VPI -- 8 bit long, specifies the path the VC follows

through the network

– VCI – 16 bit long, specifies a single VC within a path

• VPI/VCI rewritten at each switch

Example Of VPI/VCI Rewriting• Sender uses VPI/VCI 3

• Receiver uses VPI/VCI 6

• Intermediate VPI/VCIs are established within each switch

How To Set Up VPI/VCI Entries

• Permanent Virtual Circuits (PVC) and Switched Virtual Circuits (SVC)

• PVC – Similar to leased line, an entry is permanently

set as long as fee is paid – A permanent path is established between two

points of connection – An organization may have multiple connection

(VPI/VCI) to the Internet

How To Set Up VPI/VCI Entries

• SVC – VCs are established and terminated as needed – To request a connection, the sender send a

message to the switch – The switch assigns an available slot for the

connection – Each pair of the switches negotiate in turn – The connection reaches the receiver – The connection is established

Example Of Switched Network

• Network makes connection on demand

Connection Multiplexing

• Typical computer has one physical connection to network

• All logical connections multiplexed over physical interconnection

• Data transferred must include connection identifier (VPI/VCI)

Illustration Of ATM VC

• Switch maps VPI/VCI– 17 to 12– 96 to 8

ATM Quality Of Service

• Specified when connection established

• Endpoint specifies– Type of data transfer

– Throughput desired

– Maximum packet burst size

– Maximum delay tolerated

Type Of Data Transfer

• Constant bit rate (CBR)– Example: audio

• Variable bit rate (VBR)– Example: video with adaptive encoding

• Available bit rate (ABR)– Example: data

• Unspecified bit rate (UBR)

• Each type has detailed parameters (e.g., mean, max, burst duration)

Sending Data Over ATM

• Uses ATM Adaptation Layer 5 (AAL5)

• Accepts and delivers large, variable-size packets

• AAL5 divides into cells for transmission

• Called segmentation and reassembly

Issues

• More expensive than traditional LAN technology • More connection setup time • Cell tax (header/data ~= 10%) • Need to specify service requirement at the

connection, some may not know which to specify • Lack of efficient broadcast • Complexity of QoS (Quality of Service): one can

specify the request, but hard to enforce it • Assumption of homogeneity