Ethernet - Computer Busses - page 425

7 bytes 1 byte 6 bytes 6 bytes 2 bytes 46 to 1500 bytes 4 bytes 96 bytes

Start
delimiter

Destination
address

Source
address

Logical
link control

Preamble Length FCS Delay

10101011

10101...0101010 SNAP header

Organization
ID

DATA

1010 1010 1010 1010 0000 0011 EtherType

3 bytes 2 bytes

Figure 26.6 Ethernet IEEE 802.3 frame with LLC containing SNAP header

26.4.1 LLC protocol
The 802.3 frame provides some of the data link layer functions, such as node addressing
(source and destination MAC addresses), the addition of framing bits (the preamble) and
error control (the FCS). The rest of the functions of the data link layer are performed with the
control field of the LLC field; these functions are:

• Flow and error control - each data frame sent has a frame number. A control frame is
sent from the destination to a source node informing that it has or has not received the
frames correctly.
• Sequencing of data - large amounts of data are sliced and sent with frame numbers. The
spliced data is then reassembled at the destination node.

Figure 26.7 shows the basic format of the LLC frame. There are three principal types of
frame: information, supervisory and unnumbered. An information frame contains data, a
supervisory frame is used for acknowledgement and flow control, and an unnumbered frame
is used for control purposes. The first two bits of the control field determine which type of
frame it is. If they are 0X (where X is a don't care) then it is an information frame, 10 speci-
fies a supervisory frame and 11 specifies an unnumbered frame.

1 byte 1 byte 1/2 byte(s) ≥ 1 byte

DSAP SSAP Control Data

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Information
frame

Send seq. no
(0 - 127)

P 0 Receive seq. no
(0 - 127)

0

Supervisory
frame

P 0 Receive seq. no
(0 - 127)

1 0 S

RR, RNR
or REJ

Figure 26.7 LLC frame format