Handbook for Sound Engineers

Digital Audio Interfacing and Networking 1493

buffering the outputs, they break the network diameter
limitation.
Switches have another difference from repeater
hubs. Repeater hubs and stations connected to them
operate in half duplex mode. In other words a given
station can only receive or transmit at different times. If
a station that is transmitting in half duplex mode sees a
received signal, that tells it a collision has occurred.
Since switches store and buffer the packets, they can
operate in full duplex mode with other switches or with
stations which can operate full duplex. When a station is
connected to a switch in full duplex mode it can receive
at the same time as it transmits and know that a collision
can’t occur since it is talking to a full duplex device
which does not allow collisions to occur internally, Fig.
39-31.

Full duplex operation has the added benefit of
doubling the communications bandwidth over a half
duplex link. A half duplex fast Ethernet connection has
100 MBit/s of available bandwidth which must be split
and shared between the packets going each direction on
that link. This is because if packets were going both
directions at once, that by definition would be a colli-
sion. A full duplex link on the other hand has no
problem allowing packets to flow in both directions at
once, so a fast Ethernet link has 100 MBit/s capability
in each direction.
The internal packet routing function inside a switch
is called the switch fabric or switch cloud. Switches
which contain enough packet routing capability in their

cloud to never run out, even if all ports are receiving the

maximum possible amount of data, are known as

“nonblocking” switches.

Proper Ethernet network design includes ensuring

that no packet may go through more than 7 switches on

its way from the source to the destination.

When switches were first introduced their expense

limited their application to the few situations which

required their capabilities. Today the price of switches

has come down until they are hardly any more expen-

sive than repeater hubs. As a result the repeater hub is

becoming a vanishing part of Ethernet history, Fig.

39-32.

The device that provides this function between a pair

of collision domains is called a bridge. As the tech-

nology became cheaper multiport bridges started to

appear which were called switches. As switches became

popular and bridges faded from use, you will sometimes

see a bridge referred to as a two-port switch.

Ethernet switches receive a packet into memory.

They examine the destination address and decide which

of their ports has attached to it the station with the

address in question. Then if the destination is not on the

same port as the packet was received from, the switch

forwards the packet to only the correct port. Packets

where the destination address is on the same port the

packets were received from are discarded.

Switches determine which addresses are connected

to each port by recording the source address of every

packet received, and associating that address with the

port from which it was received. This information is

assembled in a look up table (LUT) in the switch. Then

as each packet is received, the switch checks to see if

the destination address is in the LUT. If it is, the switch

knows where to send that packet and only sends it out

the appropriate port. If a destination address is not in the

LUT, the switch sends that packet out every port but the

one from which it was received. Since most Ethernet

Figure 39-31. Two Ethernet switches showing full duplex
operation between them, and the isolation of two collision
domains.

node

node

node

hub node

node

node

node

hub node

switch

switch

Collision domain

Collision domain

Figure 39-32. Switch functional diagram.

LUT Switch Cloud

Tx Tx Tx

Rx Rx Rx

1 2 3