token-passing access method. However, we cannot
connect a CSMA/CD segment to a token-passing
segment.
Bridges
Bridges handle the first two layers of the OSI
model—the physical layer and the data link layer. Like
repeaters, bridges connect physically-isolated networks
to forma single logical network; however, a bridge has a
little more intelligence and can provide some
translation between dissimilar protocols. For example,
our token-passing segment wants to communicate with
our CSMA/CD segment. The bridge will “repackage”
the message from the token-passing segment into a
format that the CSMA/CD segment will understand.
Then, the bridge will act as a workstation on the
CSMA/CD segment and contend for access. The same
thing happens in reverse. A message is sent from the
CSMA/CD segment to the token-passing segment. The
bridge then “repackages” the message into a format the
token-passing segment is expecting and waits for the
token, just like any other workstation. An important
point to remember is that a bridge will pass on any
message it receives. Because the bridge is not smart
enough to know that unlike LANs do not understand
each other, it will go ahead and send the message.
Because the two LANs speak a different “language,”
the message will be ignored.
Routers
Routers only connect networks running similar
access methods. They work at the third layer of the OSI
model—the network layer. Like bridges and repeaters,
routers can connect networks over different wiring
media and topologies. However, unlike bridges, routers
can intelligently determine the most efficient path to
any destination, based on predetermined delimiters.
Routers are often a better choice for interconnecting
remote installations and congested networks requiring a
single protocol. Let’s look at this more closely.
Let’s say we have a LAN made up of three token-
passing segments, and each segment is connected via a
bridge. For a message to go from LAN A to LAN C, it
would have to travel through LAN A and LAN B before
it reaches its final destination, which is LAN C. See
figure 2-2, frame A. On a LAN that has large amounts
of message traffic, we can see how a bridge may slow
down the system. On the other hand, if the segments are
separated by routers, the router on LAN A would look at
the destination of the message and determine the direct
route to LAN C that would be shortest route, as
shown in figure 2-2, frame B.
Brouters
A brouter can work in either the second and third
layers of the OSI model—the data link layer or the
network layer. A brouter is a combination of a bridge
and router combined. If it can’t route a packet, it acts as a
bridge. Brouters are particularly useful if you have two
or more different networks. Working as a bridge, a
brouter is protocol independent and can be used to filter
local are a network traffic. Working as a router, a
brouter is capable of routing packets across networks.
Gateways
Gateways work at OSI model layer 7—the
application layer. A gateway functions to reconcile
differences between two dissimilar networks.
Messages are not only repackaged for transmission
between different networks (CSMA/CD to token-
passing), but the contents of the messages are converted
into a format the destination can use and understand.
Now our unlike LANs can talk to each other. Gateways
can also provide links between microcomputer
networks and mainframes.
A gateway is generally a dedicated computer with
an interface card and at least some type of software for
both of the environments being connected. The
gateway then runs special software that provides the
necessary conversion and translation services which, in
turn, allow the two environments to communicate.
Figure 2-2.—Interconnecting LANS using (A) bridges and (B)
routers.
2-2
