Portal Devices are devices on networks which act as the go-betweens of local and long distance network segments of media. The common portal devices are

Repeaters

A network segment can transmit data over a fixed distance before the signal degrades too much. This distance depends on the media type, eg UTP media has a range of 100 m whereas thick coaxial cable has a range of 500m.

A repeater can extend the reach of a network segment. A repeater will recognize an attenuated (weakened) baseband signal, clean it up, strengthen it, and then send it along to another network segment. A repeater operates in the OSI Physical Layer.

As a rule of thumb, no more than three repeaters should be used to extend the distance between nodes in an Ethernet LAN. Otherwise the system may experience propagation delay because of the pauses at the repeaters. Note that a repeater is for digital baseband signals, whereas an amplifier is for analog broadband signals.

Hubs

Aka concentrator or multiport repeater. A hub connects multiple network segments in a star topology and usually operates in the OSI Physical Layer. Each network segment is connected to a port on the hub.

There are three kinds of hubs: passive, active, and intelligent.

Hubs are commonly used in Ethernet LANs. A hub usually has 3-24 ports. One of the ports may be an uplink to another hub. Up to 3 hubs can be daisy chained together when using UTPs. If more than 3 hubs are needed, then a hub/bridge with a thin coaxial cable port is needed.

A common non-Ethernet hub is a USB hub. A USB hub connects multiple USB devices to a computer.

Bridges

A bridge is like a repeater (which connects two network segments while boosting the signal), but a bridge also has two other main functions:

A bridge helps network traffic congestion but it doesn't reduce traffic because it doesn't get rid of any of the packets like a router might. However, since all packets get passed (whether to the local or remote segment) a bridge can pass on any network protocol, eg TCP/IP or IPX/SPX. This can also make a router faster than a bridge. Of course, since both bridges and routers examine packets, both are slower than a repeater.

For a bridge to distinguish local packets from remote packets, it creates a bridging table with all the different MAC addresses and lists whether the address is local or remote. A bridge that does this automatically is called a transparent or learning bridge. A bridge that makes the LAN manager enter the table values manually is called a static bridge.

In addition to filtering local from remote packets, a bridge may also do more customized filtering. EG: A bridge may stop a particular user from sending to and/or receiving from outside of its local LAN.

The bridges so far performed transparent routing, i.e. the bridge is not aware of what route the packet will take to get to its destination, it just passes packets around. Some bridges, especially from IBM, may do source routing. In source routing the packet knows what path it needs to take to get to its destination and not merely its destination address. This means that a sending station would have do some route discovery first to acquire the route. Once the route is known, the packets it sends to the same address will go more quickly since the bridge will not have to examine each packet, instead the packet will tell the bridge where it needs to go.

Routers

A router connects two or more network segments at the OSI Network Layer. A router is either:

Just as a bridge makes a bridging table out of MAC addresses in the Data Link Layer, a router makes a routing table out of network addresses in the OSI Network Layer. Because network addresses have more information than MAC addresses, a router is more powerful than a bridge.

EG: Not only does a router separate local traffic from trans-LAN traffic like a bridge, but it can also discard packets. If a packet has a usable route in the routing table, then it gets sent, otherwise the packet is discarded. This reduces traffic and traffic congestion. In comparison a bridge reduces traffic congestion but not traffic since it does not delete any packets.

A router is tied to routable protocols like TCP/IP and IPX/SPX. A router cannot use non-routable protocols like NetBEUI because they don't have network address information. Routers must connect LANs using the same routable protocol and cannot pass a mix of protocols along like a bridge can.

Routers are also more extensively used to do a variety of custom filtering than bridges are.

Just as bridges come as either static or learning bridges, routers come as either static or dynamic. Static routers require the network administrator to manually build and update routing tables. Dynamic routers constantly communicate with each other to maintain current information on routes, bottlenecks, and link outages.

Brouters

A brouter is a combination bridge and router. Like bridges and routers, a brouter connects two or more network segments together.

A brouter first checks incoming data packets to see if they are of a routable protocol. If so then the brouter acts as a router and sends or deletes the packet depending on whether a route for the packet is found. If the packet is a non-routable protocol, then the brouter acts as a bridge and either keeps the packet local or sends it off to a remote LAN.

A brouter maintains both a bridging and routing table. A brouter operates in both the OSI Data Link and Network Layers. 

Gateways

A gateway connects two or more network segments together, where the segments are of dissimilar systems (eg PC with Macintosh) or protocols (eg TCP/IP with IPX/SPX). Neither a bridge or a router can perform these functions.

A gateway is usually a computer with gateway software. A gateway is slower than either a bridge or router because of all the processing it performs.

A gateway usually operates at the OSI Transport Layer or higher.

Page Modified: (Hand noted: 2007-09-13 21:16:33Z) (Auto noted: 2007-11-17 06:38:48Z)