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 (older 2 Mbps) IEEE b (11 Mbps, 2.4 GHz, feet)  Also named Wi-Fi (Wireless Fidelity) IEEE g (54 Mbps, 2.4 GHz, in 2002) IEEE a (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 tools Groupware/collaboration tools  Content-management  Distributed file-sharingfile-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 Ethernet  Most common form of LAN today  Star-wired bus or bus as the topology  CSMA/CD as the medium access protocol  Ethernet comes in many forms depending upon medium used and transmission speed and technology.

Local Area Network Systems IBM Token Ring  Deterministic 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?