Chapter 7

Local Area Networks: The Basics

Topics

Definition of LAN Primary function, advantage and disadvantage Difference between Client/Server network and

peer-to-peer network Topologies of LANs MAC techniques Applications

Definition of LAN

A local area network is a communication network that interconnects a variety of data communicating devices within a small geographic area and broadcasts data at high data transfer rates with very low error rates.

Primary Function

To provide access to hardware and software resources that will allow users to perform one or more of the following activities:

Manufacturing Education (lab, office) Interconnectivity with other networks

Advantages

Ability to share hardware and software resources. Individual workstation might survive network failure. Component and system evolution are possible. Support for heterogeneous forms of hardware and

software. Access to other LANs and WANs (Figure 7-1). Private ownership. Secure transfers at high speeds with low error rates.

Disadvantages

Equipment and support can be costly.Level of maintenance continues to grow.Some types of hardware may not

interoperate.Just because a LAN can support two different

kinds of packages does not mean their data can interchange easily.

A LAN is only as strong as it weakest link

LAN Topologies

Four basic ones:

Bus/Tree Topology

The original topology. Workstation has a network

interface card (NIC) that attaches to the bus (a coaxial cable) via a tap.

Data can be transferred using either baseband digital signals or broadband analog signals.

Bus/Tree Topology

Baseband signals are bidirectional and more outward in both directions from the workstation transmitting.

Broadband signals are usually uni-directional and transmit in only one direction. Because of this, special wiring considerations are necessary.

Buses can be split and joined, creating trees.

Star-wired Bus Topology

Logically operates as a bus, but physically looks like a star.

Star design is based on hub. All workstations attach to hub.

Unshielded twisted pair usually used to connect workstation to hub.

Hub takes incoming signal and immediately broadcasts it out all connected links.

Hubs can be interconnected to extend size of network.

Single Hub

Multiple-Hub

Star-wired Bus Topology

Advantage:

Disadvantage

Star-wired Ring Topology

Logically operates as a ring but physically appears as a star

Star-wired ring topology is based on MAU (multi-station access unit) which functions similarly to a hub

Where a hub immediately broadcasts all incoming signals onto all connected links, the MAU passes the signal around in a ring fashion

Like hubs, MAUs can be interconnected to increase network size

Logical View

Physical View

Wireless LANs

Not really a specific topology since a workstation in a wireless LAN can be anywhere as long as it is within transmitting distance to an access point

Two basic components Wireless NIC card Access point (AP)

Acts as a bridge between the wired and wireless networks and can perform basic routing functions

Wireless Standards

IEEE 802.11 (older 2 Mbps) IEEE 802.11b (11 Mbps, 2.4 GHz, 200-300 feet)

Also named Wi-Fi (Wireless Fidelity) IEEE 802.11g (54 Mbps, 2.4 GHz, in 2002) IEEE 802.11a (54 Mbps, 5 GHz, in 2002, 90 feet) HiperLAN/2 (European standard, 54 Mbps in 5

GHz band, 90 feet)Distance is inversely proportional to transmission

speed - as speed goes up, distance goes down

Peer-to-Peer Networks

Different from client-server architecture May not possess servers Focus on applications

Groupware/collaboration toolsContent-managementDistributed file-sharing Distributed processing

Medium Access Control Protocols

How does a workstation get its data onto the LAN medium?

A medium access control protocol is the software that allows workstations to “take turns” at transmitting data.

Three basic categories:

Contention-Based Protocols

Essentially first come, first served Carrier sense multiple access with collision detection

(CSMA/CD) If no one is transmitting, a workstation can transmit If someone else is transmitting, the workstation “backs off” and

waits If two workstations transmit at the same time, a collision occurs When the two workstations hear the collision, they stop

transmitting immediately Each workstation backs off a random amount of time and tries

again CSMA/CD is an example of a

Round Robin Protocols

Each workstation takes a turn transmitting and the turn is passed around the network from workstation to workstation.

Most common example is token ring LAN in which a software token is passed from workstation to workstation.

Token ring is an example of a Token ring more complex than CSMA/CD. What

happens if token is lost? Duplicated? Hogged? Token ring LANs are losing the battle with CSMA/CD

LANs.

Reservation Protocols

Workstation places a reservation with central server. Workstation cannot transmit until reservation comes up. Under light loads, this acts similar to CSMA/CD. Under heavy loads, this acts similar to token ring. Powerful access method but again losing out to

CSMA/CD. Most common example of reservation protocol is

demand priority protocol.

Local Area Network Systems

EthernetMost common form of LAN todayStar-wired bus or bus as the topologyCSMA/CD as the medium access protocolEthernet comes in many forms depending

upon medium used and transmission speed and technology.

Local Area Network Systems

IBM Token RingDeterministic LAN offered at speeds of 4, 16

and 100 Mbps.Very good throughput under heavy loads.More expensive components than CSMA/CD.Losing ground quickly to CSMA/CD. May be

extinct soon.

Local Area Network Systems

FDDI (Fiber Data Distributed Interface) Based on the token ring design using 100 Mbps fiber

connections. Allows for two concentric rings - inner ring can

support data travel in opposite direction or work as backup.

Token is attached to the outgoing packet, rather than waiting for the outgoing packet to circle the entire ring.

Losing the battle to Ethernet?

Local Area Network Systems

100VG-AnyLAN Deterministic LAN based on demand priority

access method. Similar to hub topology (star design). Two levels of priority - normal and high. Supports a wide-variety of media types. Losing ground quickly to CSMA/CD. Will be

extinct soon

A Small Office Solution

What about home networking?