1. COMPUTER NETWORKS
CS610
Lecture-18
Hammad Khalid Khan

2. Review Lecture 17 Routing Table Computation
Shortest Path Computation in a Graph
Dijkstra’s Algorithm
Distributed Route Computation
Distance Vector Routing
Link State Routing
Example WAN Technologies

3. CONNECTION ORIENTED NETWORKING and ATM
Chapter 14
CONNECTION ORIENTED NETWORKING
and
ATM

4. Introduction
LANs and WANs can both connect multiple computers, but they have different base technologies and meet different goals
ATM is a single technology that is designed to meet the goals of both LANs and WANs
ATM uses the concept of Connection-Oriented Networking

5. Asynchronous Transfer Mode
Telephone companies (Telcos) introduced ATM to meet several goals:
Universal service – for all subscribers
Support for all uses – voice, video, and data
A single, unified infrastructure (no separate LANs & WANs)
Guaranteed service, when appropriate
Support for low-cost devices

6. Jitter Jitter is the term used for variance in transmission delays
Jitter is significant for voice, video, and data
Jitter in LANs can occur when a packet is delayed because the network is busy

7. Packet Sizes
Large packets result in less overhead because a smaller fraction of the packet is used for header information
Optimum networks use 4KB packets or larger
Large packets can’t easily be used for voice
8 bit samples (at 125 usec per sample) would require half a second to fill a 4KB packet.
Echo cancellation can only be used with low transmission delays

8. ATM Cells
To meet its goals, ATM uses small, fixed-sized packets called cells
Each cell has 53 octets
VCI/VPI fields identify the cell’s destination
PRIO tell if cell can be discarded

9. ATM Cells CRC checks the header’s bits only!
ATM header is about 10 percent of the cell
Ethernet can have overhead of only 1 %
Engineers sometime call ATM overhead the Cell Tax

10. ATM Cells

11. Connection-Oriented Service
The connection-oriented service paradigm for networking is similar to the manner in which telephones are used:
A caller dials the number of the destination
The telephone at the destination signals the arrival of a Connection Request
If the called person doesn’t answer, the caller gives up after waiting for a Timeout
If the called person does answer, then the connection is established, and data is exchanged

12. Connection-Oriented Service
In data communications, a binary connection identifier is given to each of the two parties to the connection to enable identification of the connection

13. Virtual Channels (or Circuits)
Connections in ATM are called virtual channels (VC), or virtual circuits (a term preferred by some)
These are called virtual, since connections are formed in ATM by storing values in memory locations (tables) in ATM switches, as opposed to making actual electrical connections

14. Virtual Channels (or Circuits)
The VC is identified by a 24-bit value formed from:
The VPI, or virtual path indicator (8 bits), which identifies a particular path through the network; &
The VCI, or virtual channel indicator (16 bits), which identifies the channel in the virtual path is being used by the connection
Most frequently, the 24-bit pair is treated as just a single connection identifier by computers

15. Summary Connection-Oriented Networking and ATM
Asynchronous Transfer Mode
ATM Design and Cells
Connection-Oriented Services
Virtual Channels