Ethernet - Computer Busses - page 436

and to interconnect multiple LAN technologies, such as FDDI, 100VG-AnyLAN,
100BASE-T, and ATM.
• Shared memory - these use a common memory area (several megabytes) in which data
is passed between the ports. It is very common in low-cost, small-scale switches and has
the advantage that it can cope with different types of network, which are operating at dif-
ferent speeds. The main types of memory allocation are:

• Pooled memory - memory is allocated as it is needed by the ports from a common
memory pool.
• Dedicated shared memory - memory is fixed and shared by a single pair of I/O ports.
• Distributed memory - memory is fixed and dedicated to each port.

26.11 Switches and switching hubs

A switch is a very fast, low-latency, multiport bridge that is used to segment LANs. They are
typically also used to increase communication rates between segments with multiple parallel
conversations and also communication between technologies (such as between FDDI and
100BASE-TX).
A 4-port switching hub is a repeater that contains four distinct network segments (as if
there were four hubs in one device). Through software, any of the ports on the hub can di-
rectly connect to any of the four segments at any time. This allows for a maximum capacity
of 40 Mbps in a single hub.
Ethernet switches overcome the contention problem on normal CSMA/CD networks.
They segment traffic by giving each connect a guaranteed bandwidth allocation. Figure
26 .14 and Figure 26 .15 show the two types of switches; their main features are:
•

Desktop switch (or workgroup switch) - These connect directly to nodes. They are eco-
nomical with fixed configurations for end-node connections and are designed for stand-
alone networks or distributed workgroups in a larger network.

• Segment switch - These connect both 10 Mbps workgroup switches and 100 Mbps inter-

connect (backbone) switches that are used to interconnect hubs and desktop switches.
They are modular, high-performance switches for interconnecting workgroups in mid- to
large-size networks.

26.11.1 Segment switch
A segment switch allows simultaneous communication between any client and any server. A
segment switch can simply replace existing Ethernet hubs. Figure 26 .15 shows a switch with
five ports each transmitting at 10 Mbps; this allows up to five simultaneous connections giv-
ing a maximum aggregated bandwidth of 50 Mbps. If the nodes support 100 Mbps communi-
cation then the maximum aggregated bandwidth will be 500 Mbps. To optimise the network,
nodes should be connected to the switch that connects to the server with which it most often
communicates. This allows for a direct connection with that server.