Ethernet - Computer Busses - page 424

When the protocol is IPX/SPX the frame type field contains the bit pattern 1000 0001
0011 0111, but when the protocol is TCP/IP the type field contains 0000 1000 0000 0000.

26.4 IEEE 802.2 and Ethernet SNAP

The LLC is embedded in the Ethernet frame and is defined by the IEEE 802.2 standard. Fig-
ure 26.5 illustrates how the LLC field is inserted into the IEEE 802.3 frame. The DSAP and
SSAP fields define the types of network protocol used. A SAP code of 1110 0000 identifies
the network operating system layer as NetWare, whereas 0000 0110 identifies the TCP/IP
protocol. These SAP numbers are issued by the IEEE. The control field is, among other
things, for the sequencing of frames.
In some cases, it was difficult to modify networks to be IEEE 802 compliant. Thus, an
alternative method was to identify the network protocol, known as Ethernet SNAP (subnet-
work access protocol). This was defined to ease the transition to the IEEE 802.2 standard and
is illustrated in Figure 26.6. It simply adds an extra two fields to the LLC field to define an
organisation ID and a network layer identifier. NetWare allows for either Ethernet SNAP or
Ethernet 802.2 (as Novell used Ethernet SNAP to translate to Ethernet 802.2).
Non-compliant protocols are identified with the DSAP and SSAP code of 1010 1010, and
a control code of 0000 0011. After these fields come

• Organization ID - which indicates where the company that developed the embedded
protocol belongs. If this field contains all zeros it indicates a non company-specific ge-
neric Ethernet frame.
• EtherType field - which defines the networking protocol. A TCP/IP protocol uses 0000
1000 0000 0000 for TCP/IP, whereas NetWare uses 1000 0001 0011 0111. NetWare
frames adhering to this specification are known as NetWare 802.2 SNAP.

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

Destination
service access
point (DSAP)

Source
service access
point (SSAP)

Control
field

DATA

1 byte 1 byte 1/2 bytes

Figure 26.5 Ethernet IEEE 802.3 frame with LLC