- Switches divide n/w into several dedicated channels parallel to each other.
- These multiple independent data paths increase throughput capacity of switch.
- If segment is attached to a part of switch then CSMA/CD is used for media access in that segment.
How Switch Works
The source and destination MAC address of incoming frame is looked up and if frames are to be forwarded, it is sent to destination port.
Distinguishing features of Switch:-
Full duplex
Full duplex mode of ethernet allows simultaneous flow of info from one station to the other without collision. So, ethernet in full duplex does not require CD, when one port station is attached to each port.
Effective Bandwidth= No of ports X bit rate of medium /2
24 X 100 mbps= 2400 mbps
Flow Control
Flow control is necessary when destination port is receving more traffic than it can handle. Frames can be dropped in this situation. This is a costly operation as delays of order of seconds are introduced for a dropped frame.
Traditional n/w do not have layer 2 flow control mechanism and rely on higher layer for the same. Switches come with various flow control strategies depending on the vendor. Some switches upon finding that destination port is overloaded will send a jam message to sender.
The decoding of MAC address is fast and S/w can respond in very little time with a jam message, collission or packet loss can be avoided.
To the sender a jam packet is like a virtual collision , so it will wait a random time before re-transmitting.
Switches-------------------------------------------------------------------
Static Dynamic
Hub (frame Xmit to all ports) Frame Xmit only to destination
Supports Learning and
Reconfiguration
Cut Through Vs Strore & Forward Switching
Cut Through
Marked by low latency, these switches begin transmission of the frame to the destinationport even before the whole frame is received.
Frame latency is about 1/20th of that of strore and forward switches.
Cut through Switch with runt (collission fragments) detection will store the frame in buffer and begin re-transmission as soon as possibility of runt is eliminated and it can grab the outgoing channel.
Filtering of runt is important as they seriously waste the bandwidth of network. The delay in thse switch is about 60 mu s. Compare this with store and forward switches where every frame is buffered (0.8 mu s/byte). The delay for 1500 byte is therefore, 1200 mu s.
No CRC verification is done in these switches.
Store and Forward Switches.
This type of switch receives the whole of frame before forwarding it. While the frame is being received, processing is done. Upon complete arrival of frame, CRC is verified and frame is directly forwarded to Output port. Store and Forward Switch is essential when:-
A slow port transmitting to a fast port.
High Traffic conditions when buffering may be required.
Address Resolution
To allow forward and filtering of packets at wire speed, LAN switches should be able to decode MAC address very quickly.
CPU based lookups are expensive, Hardware solutions may be used.
