ISDN - Computer Busses - page 540

The non-automatic assignment involves the user setting the address of each of the devices
connected to the network. When a device transmits data it inserts its own TEI address and
only receives data which has its TEI address. In most cases devices should not have the same
TEI address, as this would cause all devices with the same TEI address, and the SAPI, to re-
ceive the same data (although, in some cases, this may be a requirement).
The network allocates addresses to devices requiring automatic assignment before they
can communicate with any other devices. The global TEI address is used to broadcast mes-
sages to all connected devices. A typical example is when a telephone call is incoming to a
group on a shared line where all the telephones would ring until one was answered.
The C / R bit is the command/response bit and EA0 / EA1 are extended address field bits.
G.4.2 Bit stuffing
With zero bit stuffing the transmitter inserts a zero into the bitstream when transmitting five
consecutive 1s. When the receiver receives five consecutive 1s it deletes the next bit if it is a
zero. This stops the unique 01111110 sequence occurring within the frame. For example if
the bits to be transmitted are
101000101011111100001010001010000111110101010
then with the start and end delimiter this would be
01111110 101000101 01111110 0001010001010000111110101010 01111110
It can be seen from this bitstream that the stream to be transmitted contains the delimiter
within the frame. This zero bit insertion is applied to give
01111110 101000101 011111 0 10 000101000101000011111 0 0101010 01111110
Notice that the transmitter has inserted a zero when five consecutive 1s occur. Thus the bit
pattern 01111110 cannot occur anywhere in the bitstream. When the receiver receives five
consecutive 1s it deletes the next bit if it is a zero. If it is a 1 then it is a valid delimiter. In the
example the received stream will be
01111110 101000101011111100001010001010000111110101010 01111110
G.4.3 Control field
ISDN uses a 16-bit control field for information and supervisory frames and an 8-bit field for
unnumbered frames, as illustrated in Figure G.8. Information frames contain sequenced data.
The format is 0SSSSSSSXRRRRRRR , where SSSSSSS is the send sequence number and
RRRRRRR is the frame sequence number that the sender expects to receive next ( X is the
poll/final bit). As the extended mode uses a 7-bit sequence field then information frames are
numbered from 0 to 127.
Supervisory frames contain flow control data. Table G.3 lists the supervisory frame types
and the control field bit settings. The RRRRRRR value represent the 7-bit receive sequence
number.