Principles and technologies of local area networks - Intelligent Buildings and Building Automation - page 60

1 0 0 1

Transmitted
data

Transmitted
signal

H
L
H
L
H
L

Tran smitted data
Trans mitted clock

Transmitted
clock
Transmitted
encoded data
Transmitted
signal

Medium
interface

Received
signal

Received data
Received clock

A B

Figure 4.15 Medium interface using Manchester encoding method and ASK analogue
signals: A. Input and output lines; B. Encoding of transmitted data.

sent. The self-clock of the data signal is achieved by assuring an H-to-L or
L-to-H transition in the middle of every bit time. These transitions are used
to resynchronize the receiver's clock. When encoding the data bits, a '0' bit
is represented by L followed by H . A '1' bit is represented by H followed by
L . If no information is sent, there are no transitions on the data line at all.
The coding and decoding processes are conducted by the medium interface
in both directions. Generally, BAS engineers and users will not need to have
details of the coding and decoding rules at this level.
An example of a medium interface using ASK analogue signalling and
Manchester encoding is shown in Figure 4.15. On the digital hardware (LAN
interface) side, pure digital signalling with TTL signal levels and an additional
clock-signal line for each data line is used.

4.3.4 Examples of medium access methods

On the Data Link Layer in the communication model, there are several ways
of organizing this medium access. CSMA/CD and token passing are the typi-
cal methods used.
CSMA/CD: CSMA/CD is a distributed principle which is used in Ethernet;
that is, in a bus/tree topology (Figure 4.16). CSMA/CD stands for carrier
sense multiple access/collision detect. 'Carrier sense' means that a device that
wishes to send on the medium senses whether there are other devices sending
a carrier at the same time. The CSMA/CD is a contentious method, in which
network nodes compete for the right to use the medium. If the medium is in
use, the station does not send, but waits until the medium is free.
When the medium is free, the device waits for a random time and if the
medium is still free, the device starts sending its frame onto the medium.
Usually, it works, but there is some chance of collisions. In the event of a
collision, the nodes involved stop sending and try again after a random wait.
In this way, many nodes can use the same medium.
It may seem strange that the nodes wait a while first before sending when
the medium becomes free. The reason is that when the medium is busy there