Specifications

Ethernet Data Frames 4-9
Ethernet Data Frames
Ethernet SNAP Frame Type
After the 802.2 frame was defined, there was some concern that the one
byte DSAP and SSAP fields were not adequate for the number of protocols
that eventually needed to be identified. In response from Apple Computer
and the TCP/IP community, another frame standard was defined for both
Ethernet and Token Ring. It was called the Sub-Network Access Protocol,
shown in Table 4-4 below.
Table 4-4. Ethernet SNAP Frame Type
Field Name Field Size Field Definition
Preamble 7 bytes Signals beginning of the frame.
Start Frame
Delimiter
1 byte Signals start of data.
Destination
Address
6 bytes
Contains the address of the destination of
the frame.
Source Address 6 bytes
Contains the address of the frame’s
origin.
Length Field 2 bytes
Contains the length of the Data plus LLC
fields.
Destination
Service Access
Point (DSAP)
1 byte Set to AA (hex) and 10101010 (binary).
Source Service
Access Point
(SSAP)
1 byte Set to AA (hex) and 10101010 (binary).
Control Field 1 byte Set to 03 (hex) and 00000011 (binary).
Protocol Identifier 5 bytes Specifies the upper layer protocol.
Data
38– 1,492
bytes
Contains the data to be transferred.
Frame Check
Sequence
4 bytes Determines integrity of the frame.