A D V E R T I S E M E N T
As signals travel along a network cable (or any other medium of transmission), they degrade and become distorted in a process that is called attenuation. If a cable is long enough, the attenuation will finally make a signal unrecognizable by the receiver.
A Repeater enables signals to travel longer distances over a network. Repeaters work at the OSI's Physical layer. A repeater regenerates the received signals and then retransmits the regenerated (or conditioned) signals on other segments.
Like a repeater, a bridge can join segments or workgroup LANs. However, a bridge can also divide a network to isolate traffic or problems. For example, if the volume of traffic from one or two computers or a single department is flooding the network with data and slowing down entire operation, a bridge can isolate those computers or that department.
A bridge reads the outermost section of data on the data packet, to
tell where the message is going. It reduces the traffic on other network
segments, since it does not send all packets. Bridges can be programmed
to reject packets from particular networks. Bridging occurs at the data
link layer of the OSI model, which means the bridge cannot read IP
addresses, but only the outermost hardware address of the packet. In our
case the bridge can read the ethernet data which gives the hardware
address of the destination address, not the IP address. Bridges forward
all broadcast messages. Only a special bridge called a translation
bridge will allow two networks of different architectures to be
connected. Bridges do not normally allow connection of networks with
different architectures. The hardware address is also called the MAC
(media access control) address. To determine the network segment a MAC
address belongs to, bridges use one of:
- Transparent Bridging - They build a table of addresses (bridging
table) as they receive packets. If the address is not in the
bridging table, the packet is forwarded to all segments other than
the one it came from. This type of bridge is used on ethernet
- Source route bridging - The source computer provides path
information inside the packet. This is used on Token Ring networks.
A router is used to route data packets between two networks. It reads the
information in each packet to tell where it is going. If it is destined for an
immediate network it has access to, it will strip the outer packet, readdress
the packet to the proper ethernet address, and transmit it on that network. If
it is destined for another network and must be sent to another router, it will
re-package the outer packet to be received by the next router and send it to the
next router. The section on routing explains the theory behind this and how
routing tables are used to help determine packet destinations. Routing occurs at
the network layer of the OSI model. They can connect networks with different
architectures such as Token Ring and Ethernet. Although they can transform
information at the data link level, routers cannot transform information from
one data format such as TCP/IP to another such as IPX/SPX. Routers do not send
broadcast packets or corrupted packets. If the routing table does not indicate
the proper address of a packet, the packet is discarded.
There is a device called a brouter which will function similar to a bridge
for network transport protocols that are not routable, and will function as a
router for routable protocols. It functions at the network and data link layers
of the OSI network model.
A gateway can translate information between different network data formats or
network architectures. It can translate TCP/IP to AppleTalk so computers
supporting TCP/IP can communicate with Apple brand computers. Most gateways
operate at the application layer, but can operate at the network or session
layer of the OSI model. Gateways will start at the lower level and strip
information until it gets to the required level and repackage the information
and work its way back toward the hardware layer of the OSI model. To confuse
issues, when talking about a router that is used to interface to another
network, the word gateway is often used. This does not mean the routing machine
is a gateway as defined here, although it could be.