Network Connection Devices

A variety of devices are used to connect networks:  repeaters, hubs, bridges, switches, and routers.





How to connect multiple LANs?

Use a bridge.

FIGURE

A bridge is a computer connected to both LANs.  The LANs may be the same or use different standards.

The bridge is responsible for

1.  Translating frame structure

• It strips off one LAN's framing and adds another's.
• It does not look at the contents of the frame -- so it only works at the data link level.
• Problems:  frame size, speed.
  

2.  Routing

• Forwarding frames received from one LAN to destinations on another LAN.
• Each node has a unique global address, so that is not a problem.

How to do routing:?

1.  Transparent bridge

The existence of bridges is transparent to the hosts.

• Each bridge keeps a table mapping host addresses to LANs.
• Initially empty, it is updated each time a frame is received.  If a message with source address A is received on LAN X, then (A,X) is entered into the table.
• Outdated entries can be overwritten with new information.  Periodically, the entire table can be purged.

The bridge then operates as follows:

1. Read every frame on each LAN it is connected to.
2. Look up the destination address of the frame.
• If LAN(destination) is the same as the LAN from which the frame came, discard it.
• If LAN(destination) is not the same, forward it to the desired LAN.
• If LAN(destination) is not found in the table, forward it to all other LANs.  This is called "flooding".

If several bridges are used to connect the same set of LANs, for redundancy/reliability, there may be loops in paths.  So, construct a logical "spanning tree" of bridges which contains no loops.


2.  Source routing bridge

Each host is responsible for routing its messages through bridges.

Each sender on each LAN must know

• which LAN the destination is on, and
• what route to take (throught bridges) to get to the destination

If it doesn't know, it sends out a "discovery frame" with the given destination address.  Every bridge floods it onto every LAN.  When (if) it reaches the destination, the destination sends back an acknowledgement.  On its way back, each bridge appends its (LAN#,Bridge#) so when it gets back to the sender, the sender knows the correct route.

• Advantage:  Once the sender knows the path, routing is faster and can be handled in hardware.
• Disadvantage:  More work for hosts (that is, not transparent to hosts).
• The network administrator must design LAN#s and bridge#s, so there is a possibility of error.

Bridges vs. Repeaters

• A repeater transmits all bits from one side to the other.
• It operates at the physical layer.
• Result is the extension of a single LAN, rather than connecting two LANs.
• All hosts belong to the same collision domain.
• With a bridge, there are separate LANs with separate collision domains.  But it appears to a host as a single LAN.