ist 126 computer networks day 2. server security only one password needed to access network...
TRANSCRIPT
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 Topologiesfor 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 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.