Specifications

Ethernet Data Frames
4-8 Ethernet Data Frames
The Ethernet 802.2 header envelopes the data before it is encapsulated
within an IEEE 802.3 header. This frame adds several fields to the header;
a destination service access point (DSAP), a source service access point
(SSAP), and a control field.
• Service Access Point: This denotes the point of service the packet is
intended for, or what upper layer protocol is supposed to use the data.
Both the DASP and the SSAP fields contain values that identify the
upper layer protocol type of the frame.
• Control Field: This is used by certain protocols for administrative
purposes.
Table 4-3. Ethernet 802.2 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 address of the destination
frame.
Source Address 6 bytes Contains address of the frame’s origin.
Length Field 2 bytes
Indicates length of the Data plus LLC
fields.
Destination
Service Access
Point (DSAP)
1 byte
Shows first byte of 2 byte value indicating
the frame’s upper layer protocol
destination.
(Source Service
Access Point) SSAP
1 byte
Shows second byte of 2 byte value
indicating the frame’s upper layer
protocol destination.
Control 1 byte Indicates the type of LLC frame.
Data
43–1,497
bytes
Contains the data to be transferred.
Frame Check
Sequence
4 bytes Determines the integrity of the frame.