Principles and technologies of local area networks - Intelligent Buildings and Building Automation - page 54

CPU bus

LAN
Interface

Transmitted
Data & Clock

Received
Data & Clock

Medium
Interface

Transmitted
Signal

Received
Signal

Transceiver

Medium

Figure 4.9 Physical interface.

some form of signal encoding must be used. This function is performed
by the medium interface . Specific operations performed by the medium
interface unit depend on the signalling and synchronization methods used.
In general, it can be said that the medium interface provides bidirectional
signal conditioning. In the transmit direction, streams of transmitted data
bits are converted by a medium interface to electric signals appropriate for
medium use. In the receive direction, a signal arriving from the medium is
decoded, and the resulting stream of received data bits is produced. As shown
in Figure 4.9, there is one more element between the medium interface and
the medium itself, the transceiver . The role of the transceiver is to connect
two unidirectional signal lines into one bidirectional line, which then can be
tapped directly to the LAN cable bus.
As shown in Figure 4.9, the LAN interface operates between the CPU bus
and medium interface unit. All LAN interface inputs and outputs have the
form of pure digital signals with standard TTL levels. LAN interface typi-
cally implements a LAN coprocessor, which can operate on the same bus
as the host CPU (main processor) and can execute instructions from main
memory shared with the host CPU. The LAN interface functions include the
following:

￿ All Data Link Layer operations, e.g. node-level addressing, error detection

and medium-access control. In the token-bus protocol, medium-access
control consists of token passing, network reconfiguration and token