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 x 16 — 2).7 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.8

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 highspeed backplane. As a result, the only place the entire aggregate LAN traffic appears is

7Two 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.

8ATM is covered in Chapter 18.

on that backplane. Traffic between ports on a single card does not even appear on the backplane (Refs. 9-11).

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