IST 126 Computer Networks Day 2

Server Security Only one password needed to access network Associated with the account is permission to access certain files and not others The access level you have for each file can be specified This security applies even if you have access to the server machine itself

Types of Servers File Server – Allows network users to share files Print Server – Manages printing in a network Application Server – Holds application software and is available to clients. The way this works differs considerably Message Server – Provides and work group applications. Example – Coordination of approval of a requisition from all necessary individuals and in the correct order Database Server – Provides powerful database capabilities to network

Classification of LANs LANs are classified into broad categories according to the shape of the network cables The primary topologies used in LANs are: bus, ring, star. Each topology has advantages and disadvantages

Physical Network Topologies for LANs Bus topology Star Topology Ring Topology

Bus Topology Consists of a single cable with devices connected to it along its length When one computer sends a signal, all computers on the cable receive the information but only one accepts it Only one computer at a time can transmit The end of the bus must be terminated so that the signal does not bounce back and travel back up the wire Ethernet 10Base2 (thinnet Ethernet) is a network based on a physical bus topology

Advantages of Bus Topology Simple and easy to use Requires the least amount of cable Easy to extend a bus with an appropriate connector

Disadvantages of a Bus Topology Heavy network traffic can cause a bus to slow considerably Each connector weakens the signal Difficult to troubleshoot a bus A cable break or a loose connector will cause reflections and bring down the whole network

Star Topology All cables run from the computers to a central location where a hub is located The hub either resends the message to all the computers (broadcast star network) or only to the destination computer (switched star network) We use a star topology here at Mercer in our labs

Advantages of Star Topology Easy to modify and add new computers Single computer failure does not bring down whole network Can use several cable types Is the most flexible and the easiest to diagnose when there is a problem

Disadvantages of the Star Topology If the central hub fails, the whole network fails It cost more to cable a star network

Ring Topology Each computer is connected to the next computer The last computer is connected to the first The IEEE standard defines the token ring network

Advantages of Ring No one computer can monopolize the network if you are using a token ring The network degrades gracefully as the load increases The network will continue to function in a useful manner rather than fail all at one time

Disadvantages of Ring Failure of one computer on the ring can affect the whole network Difficult to troublshoot a ring Adding or removing computers disrupts the network

Ethernet Most popular LAN technology Uses bus and star topologies Several versions of Ethernet have been defined – thick Ethernet uses thick coaxial cable –Thin Ethernet uses thin coaxial cable –Gigabit Ethernet uses CAT 5 twisted pair

Ethernet uses Manchester Encoding All data is encoded using binary Manchester encoding –0 – Represented by a transition in the middle of the bit time from high-to-low voltage –1- Represented by a transition in the middle of the bit time from low-to-high voltage

Ethernet Sharing Ethernet requires multiple computers to share access to a single medium A sender transmits a signal which propagates over the whole LAN Only one frame of information can be sent at a time – other computers have to wait to send After one frame is transmitted, the shared cable becomes available for another computer to use

How are Transmissions Coordinated on Ethernet LAN? There is not centralized controller telling each computer when it is their turn to trnamit All computers use a scheme called Carrier Sense Multiple Access (CSMA) Explain how CSMA works

CSMA Operation When no computer is sending a frame the network does not contain an electrical signal (called a carrier signal) A computer wishing to transmit can check cable for a carrier signal If no carrier is detected, the computer is allowed to transmit If carrier is present, the computer must wait until transmission is completed.

Collisions in Ethernet Two computers can check for a carrier at the same time Finding none they both can begin to transmit at the same time This causes a collision between the two signals This produces a garbled transmission

Collision Detection The two computers can detect the collision Each sending station immediately stops transmitting The computers must wait for the cable to become idle again before transmitting Ethernet requires that the computers wait a random amount of time before trying to transmit again.