Networking Devices and Interconnections

Interconnecting networks can occur at different layers, each defined by a specific device type:
- Physical layer: Repeater
- Data link layer: Bridge
- Network layer: Router
- Higher level functions: Gateway

Repeaters operate at the physical layer and are used to extend signal range.
They regenerate and retransmit signals before data integrity is lost.
Common in Ethernet LANs with bus topology to connect segments and overcome coaxial cable length restrictions.
In star topology, a repeater can function as a hub, serving multiple connections.
Repeaters lack filtering capabilities and do not check data-link addresses.

Bridges connect multiple LANs, facilitating resource sharing within organizations.
Operate at both physical and data link layers:
- Physically regenerate signals.
- Check MAC addresses in data frames.

Transparent Bridges:
- Defined by the IEEE 802.1d committee.
- Stations are unaware of transparent bridges, requiring no configuration.
- Functions include forwarding frames, learning station locations, and avoiding topology loops.
Source Routing Bridges:
- Primarily used in Token Ring and FDDI networks.

Bridges maintain a forwarding table to decide frame forwarding:
- Discard frame if source and destination are in the same LAN.
- Forward frame if they are in different LANs.
- Use flooding if the destination is unknown.
Implements backward learning to update the table with source addresses and handle topology changes.

To accommodate dynamic networks, bridges:
- Introduce timers to age forwarding table entries for periodic relearning.
- Update entries when the arriving frame ports differ from those in the table.

A two-layer switch functions at the physical and data link layers, providing enhanced forwarding speed.
Redundant bridges improve reliability but can create loops; spanning tree algorithm resolves this issue.

Routers are versatile three-layer devices operating at the physical, data link, and network layers.
They regenerate signals at the physical layer and check physical addresses at the data link layer.
At the network layer, routers manage IP addresses and connect different networks (LANs/WANs).
Major components include input ports, output ports, the routing processor, and the switching fabric.
Input ports handle physical and data link layer functions by converting received signals into bits.