Networking: A Beginner's Guide
Hubs and Concentrators
Intelligent LAN concentrators--usually just called concentrators or, even more simply,
hubs--are used to connect network nodes to network backbones. Nodes are connected
to hubs in a physical star fashion (cables fan out from the hub to each node), whether
they are used for a star topology or a ring topology network (these topologies are
discussed in Chapter 4). A simple network might consist of just a hub or two; smaller
networks usually don't require a network backbone.
Hubs are available for virtually any network media type, with the higher-end units
using replaceable modules to support multiple media types. For example, you can
purchase a high-end hub chassis that can house both Ethernet and Token Ring modules.
You can purchase hubs in a variety of sizes, ranging from those that support only
2 workstations to those that support more than 100 workstations. Many network
designers use stackable hubs, which usually support 24 node connections each. These
hubs are often used in concert with switches, which are discussed in the next section.
Hubs have two important properties:
Hubs echo all data from each port to all the other ports on the hub. Although
hubs are wired in a star fashion, they actually perform electrically (logically)
more like a bus topology segment in this respect. Because of this echoing, no
filtering or logic occurs to prevent collisions between packets being transmitted
by any of the connected nodes.
Hubs can automatically partition (in this context, cut off) a problematic node
from the other nodes--in effect, shutting down that node. Such partitioning
occurs if a cable short is detected, if the hub port is receiving excessive packets
that are flooding the network, or if some other serious problem is detected
for a given port on the hub. Automatic partitioning keeps one malfunctioning
connection from causing problems for all of the other connections.
Hubs are becoming much more sophisticated. They often have a number of
advanced built-in features, including the following:
Built-in management, where the hub can be centrally managed over the
network, using SNMP or other network management protocols and software.
Autosensing of different connection speeds. For example, Ethernet hubs that
can automatically detect and run each node at either 10 Mbps (10Base-T) or
100 Mbps (100Base-T) are common.
High-speed uplinks that connect the hub to a backbone. These usually operate
at ten times the basic speed of the hub. (For example, for a 100 Mbps hub, the
uplink ports might run at 1 Gbps.)
Built-in bridging and routing functions, which make it unnecessary to use
separate devices to perform bridging and routing.
Built-in switching, where nodes on the hub can be switched instead of shared.