Transcript Day 2
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 e-mail 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 802.5 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 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.