Specifications

7-5
ETHERNET FRAME TYPES
Ethernet SNAP Frame Type
The Ethernet Sub-Network Access Protocol (SNAP) frame type is an extension of
the Ethernet 802.2 frame structure. The Ethernet SNAP frame adds a 5 byte
Protocol Identification Field immediately following the LLC header. The Protocol
Identification Field is made up of a 3 byte Protocol ID or Organizational Code
Field followed by a 2 byte Type Field "Ethertype". In accordance with RFC 1042,
SNAP frames are transmitted with the DSAP and SSAP fields set to AA (hex) and
the Control Field set to 03 (hex). The SNAP frame uses the Protocol Identifier to
determine which upper layer protocol the frame is intended for. Table 7-4
describes each of the Ethernet SNAP frame fields.
The total length of the packet must be a minimum of 64 bytes in length, with a
maximum size limit of 1518 bytes not including the Preamble and SFD.
Table 7-4. Ethernet SNAP Frame Type
Field Name Field Size Field Definition
Preamble 7 bytes Signals beginning of the packet.
Start Frame Delimiter 1 byte Signals start of data.
Destination Address 6 bytes Address of the destination of the packet.
Source Address 6 bytes Address of the packets origin.
Length Field 2 bytes 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 03 (hex) and 00000011 (binary)
Control Field 1 byte Indicates the type of LLC frame.
Protocol Identifier 5 bytes Specifies the upper layer protocol.
Data 43 - 1497
bytes
Contains the data to be transferred.
Frame Check
Sequence
4 bytes Determines integrity of the packet.