16-02-2013, 11:13 AM
More on LANS
More on LANS.ppt (Size: 1.14 MB / Downloads: 37)
LAN Wiring, Interface
Mostly covered this material already
NIC = Network Interface Card
Separate processor, buffers incoming/outgoing data
CPU might not be able to keep up network speeds!
Distributes processing to avoid overburdening CPU
Typically has DMA access
Wiring
ThickNet – 10Base5 ; Thick coax cable
ThinNet – 10Base2 ; “Thin” coax cable
Use BNC connectors, “T” connectors for bus
10BaseT, 100BaseT ; Twisted Pair
Use central hub
Comparison of Wiring Schemes
10Base2
Separate transceiver allows computer to be powered off or disconnected from network without disrupting other communication
Transceiver may be located in an inconvenient place
Finding malfunctioning transceiver can be hard
Thin coax takes minimum of cable
Disconnecting one computer (or one loose connection) can disrupt entire network
10BaseT
Hub wiring centralizes electronics and connections, making management easier
Easier to pull cable
Bottom line - 10Base-T most popular because of cost
Extending LANs
LANs were designed with constraints of speed, distance, cost, and usage
Typical distance = 100 to 500 meters
CSMA/CD optimal with bursty traffic
Works best to connect computers in a single floor or building
Possible to improve on LAN performance
Scale up to larger amounts of traffic
Extend to connect computers across longer distances
LAN Distance
Length of medium affects strength of electrical signals and noise immunity
LANs use shared medium - Ethernet, token ring
Length of medium affects consistency and fair, shared access to medium
Token passing - circulation time for token increases with long cable
CSMA/CD –with minimum frame size, if propagation delay too long we won’t detect collision while transmitting
Standards set a maximum distance for LANs
100 meters for UTP
200 to 500 meters for coax
Can extend distances with fiber optic connections
Install a fiber modem from computer to transceiver, perhaps in a different building
Repeaters
May want to extend LAN medium
Ethernet - timing constraints allow longer medium
Signal strength constraints limit length
Repeater – simplest form is a bidirectional, analog amplifier that retransmits analog signals
Simply copy signals between segments; includes noise/collision
Do not understand frame formats or addresses
Not the case with digital repeaters
Hub acts as a repeater
One repeater can effectively double the length of an LAN segment, e.g. 100m to 200m
Limits on Repeaters
Can't extend Ethernet with repeaters indefinitely
CSMA/CD requires low delay; if medium is too long, CSMA/CD won't work
Run into the problem of not being able to detect collisions while transmitting the frame
Ethernet standard includes limit of 4 repeaters between any two Ethernet stations
Reasons for limitations
Ethernet was designed to connect a floor of a building, or a couple floors, but not across wider distances
Will need to go to routers to cross larger distances (connect two separate LANs)
Aside from hubs, bridges are used today in favor of repeaters
Switches
Like bridges, switches operate at the data link layer (generally). Switches connect two or more computers or network segments that use the same data link and network protocol. They may connect the same or different types of cable.
Incoming frame switches to appropriate outgoing line
Most switches enable all ports to be in use simultaneously, making them faster than bridges.
Unused lines can also be used to switch other traffic
Ex: with two pairs of lines in use, overall capacity is now 20Mbps instead of 10Mbps