© Copyright Brian Brown, 1996-1999. All rights reserved.
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REPEATERS
Repeaters EXTEND network segments. They amplify the incoming signal received from one segment and send it on to all other attached segments. This allows the distance limitations of network cabling to be extended. There are limits on the number of repeaters which can be used. The repeater counts as a single node in the maximum node count associated with the Ethernet standard [30 for thin coax].

Repeaters also allow isolation of segments in the event of failures or fault conditions. Disconnecting one side of a repeater effectively isolates the associated segments from the network.

Using repeaters simply allows you to extend your network distance limitations. It does not give you any more bandwidth or allow you to transmit data faster.

It should be noted that in the above diagram, the network number assigned to the main network segment and the network number assigned to the other side of the repeater are the same. In addition, the traffic generated on one segment is propagated onto the other segment. This causes a rise in the total amount of traffic, so if the network segments are already heavily loaded, it's not a good idea to use a repeater.

A repeater works at the Physical Layer by simply repeating all data from one segment to another.

Summary of Repeater features

• increase traffic on segments
• have distance limitations
• limitations on the number that can be used
• propagate errors in the network
• cannot be administered or controlled via remote access
• cannot loop back to itself (must be unique single paths)
• no traffic isolation or filtering

During initialization, the bridge learns about the network and the routes. Packets are passed onto other network segments based on the MAC layer. Each time the bridge is presented with a frame, the source address is stored. The bridge builds up a table which identifies the segment to which the device is located on. This internal table is then used to determine which segment incoming frames should be forwarded to. The size of this table is important, especially if the network has a large number of workstations/servers.

The advantages of bridges are

• increase the number of attached workstations and network segments
• since bridges buffer frames, it is possible to interconnect different segments which use different MAC protocols
• since bridges work at the MAC layer, they are transparent to higher level protocols
• by subdividing the LAN into smaller segments, overall reliability is increased and the network becomes easier to maintain
• used for non routable protocols like NETBEUI which must be bridged [see also here]
• help localize network traffic by only forwarding data onto other segments as required (unlike repeaters)

• the buffering of frames introduces network delays
• bridges may overload during periods of high traffic
• bridges which combine different MAC protocols require the frames to be modified before transmission onto the new segment. This causes delays
• in complex networks, data may be sent over redundant paths, and the shortest path is not always taken
• bridges pass on broadcasts, giving rise to broadcast storms on the network

Transparent bridges (also known as spanning tree, IEEE 802.1 D) make all routing decisions. The bridge is said to be transparent (invisible) to the workstations. The bridge will automatically initialize itself and configure its own routing information after it has been enabled.

Bridges are ideally used in environments where there a number of well defined workgroups, each operating more or less independent of each other, with occasional access to servers outside of their localized workgroup or network segment. Bridges do not offer performance improvements when used in diverse or scattered workgroups, where the majority of access occurs outside of the local segment.

The diagram below shows two separate network segments connected via a bridge. Note that each segment must have a unique network address number in order for the bridge to be able to forward packets from one segment to the other.

Ideally, if workstations on network segment A needed access to a server, the best place to locate that server is on the same segment as the workstations, as this minimizes traffic on the other segment, and avoids the delay incurred by the bridge.

A bridge works at the MAC Layer by looking at the destination address and forwarding the frame to the appropriate segment upon which the destination computer resides.

Summary of Bridge features

• operate at the MAC layer (layer 2 of the OSI model)
• can reduce traffic on other segments
• broadcasts are forwarded to every segment
• most allow remote access and configuration
• often SNMP (Simple Network Management Protocol) enabled
• loops can be used (redundant paths) if using spanning tree algorithm
• small delays introduced
• fault tolerant by isolating fault segments and reconfiguring paths in the event of failure
• not efficient with complex networks
• redundant paths to other networks are not used (would be useful if the major path being used was overloaded)
• shortest path is not always chosen by spanning tree algorithm