Information Technology Reference
In-Depth Information
merely be less than the allowable maximum of 100 m (90 m for the horizontal cable,
and 10 m for the patch and user cords at either end). As an added benefit, the hub
can automatically isolate any port that misbehaves.
10/100BaseT Switches A variation of the 10/100BaseT hub device is called a
switched hub . Standard hubs send all packets received from any port to any other
port, as you can see from Fig. 2.3. In this way, they share the twisted-pair media
among all ports. Undesired collisions can sometimes occur, because there are so
many devices sharing the same collision domain . In order to reduce this problem, it
is possible to use a simple technique called Layer 2 switching.
A switched hub is a very significant improvement in network interconnection.
In most situations (nonbroadcast, nonmulticast), a network intends to send each
data packet to one other device on the network. The switch can identify these two
stations through their unique Layer 2 media access control (MAC) addresses.
Initially, the switch “learns” the MAC addresses of each connected device and notes
its associated port number in an internal table. From then on, the switch knows how
to send data packets only to the particular switch port that has that device attached.
This technique greatly minimizes traffic going to individual ports, allows two (or
more) transmissions to/from two (or more) stations on independent ports to occur
simultaneously, and can allow a high-speed uplink to a backbone server to handle
many simultaneous data exchanges with workstations. The basic operation of a
switched hub is shown in Fig. 2.3.
10/100BaseT Advantages. A real advantage to 10/100Base T networks is that
workstations may be connected to or disconnected from the hubs (or switches) with-
out interrupting other stations on the network. Hubs usually have status lights that
indicate proper connection on the 10BaseT ports and collisions or other error con-
ditions. Standard 10BaseT hubs will automatically isolate (or partition) a port for a
wiring reversal, short, or open connection (including no device attached). More
sophisticated hubs may also detect more subtle errors and allow for automatic or
manual port isolation. Hubs may also be connected to a variety of management
tools, from simple network management protocol (SNMP) to sophisticated network
hardware managers, such as Hewlett-Packard Openview™ and Sun NetManager™.
The basic interface for 10BaseT is the medium-dependent interface (MDI), a
special wiring configuration of the common 8-pin modular jack that is used by some
telephone equipment. Two pairs of wires are used: one for the transmit data and one
for the receive data. The connections are polarity sensitive. That means that the con-
nection will not operate if the two wires of either pair are reversed. The wiring of
the 10BaseT interface is shown in Fig. 2.4.
Search WWH ::




Custom Search