Part II Data Transmission

1

ATM Technologies
2
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

3
ATM Characteristics

• End-to-end (application to application)
• Connection-oriented interface
• Establish connection
• Send data
• Close connection
• Performance guarantees (statistical)
• Uses cell switching

4
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)

5
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

6
ATM Cell Header
7
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

8
ATM Switch

• Building block of ATM network
• Connections to
• Computer
• Other ATM switches
• Accepts and forwards cells

9
Cell Forwarding

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

10
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

11
Example Of VPI/VCI Rewriting

• Sender uses VPI/VCI 3
• Receiver uses VPI/VCI 6
• Intermediate VPI/VCIs are established within each
  switch

12
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

13
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

14
Example Of Switched Network

• Network makes connection on demand

15
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)

16
Illustration Of ATM VC

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

17
ATM Quality Of Service

• Specified when connection established
• Endpoint specifies
• Type of data transfer
• Throughput desired
• Maximum packet burst size
• Maximum delay tolerated

18
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)

19
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

20
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