Communications Networks

Communications Networks
An Introduction

Prof. David Bernstein
James Madison University

Computer Science Department

bernstdh@jmu.edu

Different Levels of Abstraction

Low Level of Abstraction:
- Vertices are transmitters, receivers, and intermediaries
- Links/edges are "direct links" (either using a guided or unguided medium)
Higher Levels of Abstraction (Sometimes Called Overlay Networks):
- Vertices can be networks, cities, countries, etc...
- Links/edges can be routes/paths that use several different media

The Need for Intermediaries

Complete Network:
- Every transmitter/receiver pair is connected with a direct link
Other Networks:
- Data from some transmitters to some receivers must go through an intermediary

A Note About Edge Weights

In General:
- The edge weights will almost always be positive (which is important for some of the algorithms that we will consider)
In Some Situations:
- We may sometimes add "dummy" edges that have a weight of 0

Circuit Switching

Defined:
- Data from a transmitter to a receiver travels along a dedicated path (usually through one or more switches)
The Process:
1. Establish the path (reserve channel capacity)
2. Transfer data
3. Disconnect (release channel capacity)

Packet Switching

Defined:
- Data from a transmitter to a receiver are wrapped in a packet (that contains the receivers address) that is forwarded by intermediaries until it reaches the destination
Approaches:
- Datagram Approach - Each packet is routed independently
- Virtual Circuit Approach - All packets from transmitter to receiver follow the same path

Packet Switching (cont.)

Store-and-Forward:
- Most packet switches do not begin to transmit the packet until they have received the entire packet (i.e., they store the packet, however briefly, before forwarding it)
- This leads to a store-and-forward delay of a few microseconds or milliseconds
Queuing Delay:
- Most packet switches have an output buffer for each outbound link that holds packets in case that link is not available (e.g., is being used to transmit another, perhaps unrelated, packet)
- Typically a few microseconds or milliseconds
Processing Delay:
- The time required to examine/process the packet's header and perform error checking/correcting
- Typically a few microseconds

Examples

Circuit Switching:
- Leased line networks
- Integrated Service Digital Network (ISDN)
Packet Switching:
- X.25 (Tymnet)
- Telenet
- ARPAnet
- Frame Relay
- Asynchronous Transfer Mode (ATM)
- Internet Protocol (IP)

Comparison

Advantages of Circuit Switching:
- Almost no transmission delay
- Very little variation in delay (i.e., low jitter)
- No overhead (e.g., header bits) after the circuit is created
Advantages of Packet Switching:
- More efficient use of bandwidth (i.e., less idle time)
- Packets can be prioritized
- It is always possible to communicate (though, perhaps, with increased delivery delay)
- More robust (i.e., more paths between any two nodes)?

Network Topologies

Routing

The Issue:
- There might be multiple paths that can be used to transmit information from a sender to a receiver
Approaches:
- Pass it on and hope it gets there
- Use a path/route (e.g., determine the "best" path/route when needed; use a look-up table)