Types of HDLC Frame


NS_ActualOpen=window.open;
function NS_NullWindow(){this.window;}
function NS_NewOpen(url,nam,atr){return(new NS_NullWindow());}
window.open=NS_NewOpen;




Types of HDLC Frame
The control field of HDLC
follows the address
field and is the second part of all HDLC
frames. The best-effort service is provided through the use of U (un-numbered)
frames consisting of a single byte. All frames carry a field of size 1 bit which
is known as the "poll/final" bit and is used by the checkpointing
procedure to verify correct transmission.

Format of the control byte in HDLC frames
HDLC defines currently two formats for frames which carry sequence
numbers. These type of frame are used to provide the reliable
data link service. Two types of numbered frames are supported:

S (supervisory) frames containing only an acknowledgment
number (N(R))
I (information) frames carrying data and containing both a send
sequence number (N(S)) and an acknowledgment
number (N(R)).
The S and U frames contain an additional field to identify the function which
is to be performed. The 2-bit SS field is used to identify one of four
functions:

SS=00 RR - Receiver Ready to accept more I-frames
SS=01 REJ - Go-Back-N
retransmission request for an I-frame
SS=10 RNR - Receiver Not Ready to accept more I-frames
SS=11 SREJ - Selective retransmission request for an I-frame
The simplest format uses one byte (as above) and provides modulo
8 numbering. The "extended format" uses two bytes (as shown below) and uses
modulo
128 numbering:

Format of the control byte in HDLC frames using an extended
sequence number
Command and Response Addresses
In the HDLC balanced mode, the a node uses just two addresses (1 and 3) to
indicate whether the frame relates to information frames (I-frames) being sent
from the node or received by the node. The two values are known as "primary" and
"secondary" addresses. The primary address is used for commands, (e.g. SABM,
DISC, I). The secondary address is used for responses (e.g. UA, RR, REJ).
The primary and secondary addresses are differentiated by the setting of the C/R
bit in the HDLC Address.
Be-aware of transmission
bit-order
Many books describing HDLC follow the ISO standard which defines the
protocol. This describes the format of the HDLC control word according to the
order in which the bits are transmitted at the physical
layer. (i.e. least significant bit (lsb) transmitted first). To present a
consistent view of all protocols, this course will describe the format in terms
of the bit-order seen by the computer and used inside the computer network
interface card. See separate section on bit
order.
window.open=NS_ActualOpen;