© Copyright Brian Brown, 1996-1999. All rights reserved.
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ETHERNET SWITCHES
Ethernet switches increase network performance by decreasing the amount of extraneous traffic on individual network segments attached to the switch. They also filter packets a bit like a router does. In addition, Ethernet switches work and function like bridges at the MAC layer, but instead of reading the entire incoming Ethernet frame before forwarding it to the destination segment, usually only read the destination address in the frame before retransmitting it to the correct segment. In this way, switches forward frames faster than bridges, offering less delays through the network, hence better performance.

When a packet arrives, the header is checked to determine which segment the packet is destined for, and then its forwarded to that segment. If the packet is destined for the same segment that it arrives on, the packet is dropped and not retransmitted. This prevents the packet being "broadcasted" onto unnecessary segments, reducing the traffic.

Nodes which inter-communicate frequently should be placed on the same segment. Switches work at the MAC layer level.

Switches divide the network into smaller collision domains [a collison domain is a group of workstations that contend for the same bandwidth]. Each segment into the switch has its own collision domain (where the bandwidth is competed for by workstations in that segment). As packets arrive at the switch, it looks at the MAC address in the header, and decides which segment to forward the packet to. Higher protocols like IPX and TCP/IP are buried deep inside the packet, so are invisible to the switch. Once the destination segment has been determined, the packet is forwarded without delay.

Each segment attached to the switch is considered to be a separate collision domain. However, the segments are still part of the same broadcast domain [a broadcast domain is a group of workstations which share the same network subnet, in TCP/IP this is defined by the subnet mask]. Broadcast packets which originate on any segment will be forwarded to all other segments (unlike a router). On some switches, it is possible to disable this broadcast traffic.

Some vendors implement a broadcast throttle feature, whereby a limit is placed on the number of broadcasts forwarded by the switch over a certain time period. Once a threshold level has been reached, no additional broadcasts are forwarded till the time period has expired and a new time period begins.

Cut-Through Switches

• only the first few bytes of the packet are read to obtain the source and destination addresses
• the packets are then passed through to the destination segment without checking the rest of the packet for errors
• invalid packets can still be passed onto other segments
• there is little delay involved in packet throughput

• Cross-bar switches
  
  • read the destination address then immediately forward
  • acts as a simple repeater once the path is established
  • can introduce delay: If the destination port is busy, it may need to buffer the packet
• Cell-backplane switches
  
  • break the frame into small fixed cell lengths
  • each cell is labeled with special headers which contain the addresses of the destination port
  • the cells are buffered at the destination port
  • the cells are then reassembled and transmitted
  • the data rate on the backplane is significantly greater than the aggregate data rate of the ports
  • in heavily overloaded networks, cell-backplane switching offers better performance than cross-bar switching

Store-Forward Switches

• examine the entire packet
• each incoming packet is buffered, then examined
• filters out any bad packets it detects
• good packets are forwarded to the correct segment
• detect more errors than the cut-through variety
• impose a small delay in packet throughput

Back Pressure Switches
Switches often employ buffering of packets. This is done so when packets arrive for a busy port, the packet is temporarily stored till the port becomes free. When the buffer becomes fill, packets become lost.

Back pressure switches overcome this problem by sending the overflow packets back to the workstation. This effectively slows the workstation transmission rate, and hence slows the arrival of new packets at the port.

Ethernet Switching: Advantges

• existing cabling structure and network adapters is preserved
• switches can be used to segment overloaded networks
• switches can be used to create server farms or implement backbones
• technology is proven, Ethernet is a widely used standard
• improved efficiency and faster performance due to low latency switching times
• each port does not contend with other ports, each having their own full bandwidth (there is no contention like there is on Ethernet)

Any traffic generated by these workstations can be sent to any other workstation in that domain. Workstations outside that domain are unable to see any packets (including broadcasts) that belong to the secure domain. Obviously, this has enormous implications for developing secure networks. Multiple virtual workgroups can exist, like email and www server. Users can belong to more than one virtual domain, thereby administration is centralized and security is maintained. The use of switch technology makes this possible.