LAN INTERWORKING VIA SPANNING DEVICES
address. Whereas a bridge worked on the data-link layer, a router operates at the network-
layer level. Routers commonly connect disparate LANs such as CSMA/CD to token ring
and FDDI to CSMA/CD.
Routers are addressable nodes in a network. They carry their own MAC address(es)
as well as a network address for each protocol handled. Because routers are addressable,
a station desiring the facility of a router must direct its packets/frames to the router in
question so that the traffic can be forwarded to the appropriate network. As one would
expect, networking software at each station is more complex with a network using routers
than one using bridges.
Routers handle only traffic addressed to them. They make decisions about forwarding
data packets/frames based on one or several criteria. The decisions may be based on the
cost of the link, the number of hops on each path, and the time-to-live.
Routers change packets/frames that pass through them such as MAC source and desti-
nation address; they may also modify the network protocol header of each frame (typically
decrementing the time-to-live in the case of IP and other protocol fields).
Because routers have more intelligence than bridges, routers will typically have better
network management agents installed. This enables them to be remotely configured, to
be programmed to pass or not to pass data for security purposes, and to be monitored for
performance, particularly error performance. Due to the additional processing performed
at routers, they tend to be slower than bridges. Reference 9 suggests that some protocols
do not lend themselves to routing, such as IBM's SNA and NetBios, among others.
Hubs and Switching Hubs
A hub is a multiport device that allows centralization. A hub is usually mounted in a
wiring closet or other central location. Signal leads are brought in from workstations/PCs
and other data devices, one for each hub port. Physical rings or buses are formed by
internally configuring the hub ports. A typical hub may have 8 or 16 ports. Suppose we
wished to incorporate 24 devices on our LAN using the hub. We can stack two hubs,
one on top of the other (stackables), using one of the hub ports on each interconnection.
In this case we would have a hub with a 30-port capacity
(2 × 16 - 2).
Hubs may also
have a certain amount of intelligence, such as the incorporation of a network management
capability. Also, each hub can include a repeater.
There are also hubs with higher levels of intelligence. These are typically modular,
multiprotocol, multimedia, multichannel, fault-tolerant, manageable devices where one
can concentrate all the LAN connections into a wiring closet or data center. Since these
types of hubs are modular (i.e., they have various numbers of slots to install LAN interface
boards), they can support CSMA/CD, token ring, FDDI, or ATM simultaneously as well
as various transmission media such as twisted pair, fiber cable, and others.
Switching hubs are high-speed interconnecting devices with still more intelligence
than the garden-variety hub or the intelligent hub. They typically interconnect entire LAN
segments and nodes. Full LAN data rate is provided at each port of a switching hub. They
are commonly used on CSMA/CD LANs, providing a node with the entire 10-Mbps data
rate. Because of a hub's low latency, high data rates and throughputs are achieved.
With a switching hub, nodes are interconnected within the hub itself using its high-
speed backplane. As a result, the only place the entire aggregate LAN traffic appears is
Two 16-port hubs are used for a total of 32 ports. However, two ports are required to connect one hub to the
other. This leaves just 30 ports for equipment connections.
ATM is covered in Chapter 18.