Local Area Networks (LANs)

Local Area Networks (LANs)
An Introduction

Prof. David Bernstein
James Madison University

Computer Science Department

bernstdh@jmu.edu

Introduction

Definition:
- A shared transmission medium owned by a single individual/organization and hardware/software for interfacing devices to the medium
Issues to Consider:
- Topolgies
- Access Methods

Bus Topology

Components:
- Single cable (called a trunk/backbone)
- Terminators at each end of the cable
- Each device connects to the trunk with a tap
Properties:
- Signals travel in both direction from the insertion point
- The signal is stopped by the terminators
- Devices are passive (i.e., do not re-transmit signals)

Ring Topology

Components:
- One cable
Properties:
- Signals are sent in one direction
- Devices are active (i.e., re-transmit signals)
- If one device fails, the network fails (unless a shunt is used)
- Modifications disrupt service

Star Topology

LAN Access Methods

The Issue:
- Only one device can access the LAN (e.g., trunk, hub) at a time
Gaining Intuition:
- What access methods are used in a classroom?
Approaches in LANs:
- Token Passing
- Contention Detection/Resolution (e.g., Carrier Sense Multiple Access/Collision Detection)

Token Passing

A special packet (called a token) is used to control access to the network
When a device wants to transmit data it "takes" the token (which usually means that it doesn't re-transmit it) and transmits the data packet
The data packet is forwarded until it reaches the intended recipient who reads it, marks it as read and re-transmits it (so that the sender can know it was received and generate a new token)

Contention Resolution (e.g., ALOHA, CSMA/CD)

Any device can send information if the network is clear
This means that collisions are possible; when a collision occurs, the devices wait a random amount of time and try again

Examples

Token Ring:
- Uses token passing on a ring network
- It is deterministic (i.e., the maximum delay is known) if there is a maximum token hold time
Ethernet (Metcalfe and Boogs, 1976):
- Used contention resolution on a bus network (to connect PCs, printers and shared disks)
Wi-Fi (IEEE802.11) Infrastructure:
- Uses a star network