MPC8540 datasheet(631/1302 Pages) FREESCALE

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Introduction

MPC8540 PowerQUICC III Integrated Host Processor Reference Manual, Rev. 1

Freescale Semiconductor

14-5

Figure 14-4. Ethernet/IEEE 802.3 Frame Structure With More Details

Relative to Figure 14-4, the IEEE 802.3 section 3.11 (MAC frame format) defines the frame

format such that the octets of a frame are transmitted from left to right (preamble first, the FCS

last), and the bits of each octet are transmitted least-significant bit (lsb) first. The destination

address example shown in Figure 14-4 (02608C:876543) which would normally be written as

0000 0010 0110 0000 1000 1100 1000 0111 0110 0101 0100 0011

is transmitted bit by bit as

0100 0000 0000 0110 0011 0001 1110 0001 1010 0110 1100 0010

which is an individual address (because the lsb is cleared) but locally-defined (because the second

least-significant bit is set).

When first originated, a type field was used for protocol identification. The IEEE 802.3

specification eliminated the type field, replacing it with the length field. The length field is used

to identify the length, in bytes, of the data field. The protocol type in 802.3 frames are held within

the data portion of the packet. The logical link control (LLC) is responsible for providing services

to the network layer regardless of media type, such as FDDI, Ethernet, token ring, and others. The

LLC layer makes use of LLC protocol data units (PDUs) in order to communicate between the

media access control (MAC) layer and the upper layers of the protocol stack. Three variables

determine access into the upper layers via the LLC-PDU.

The variables include the destination service access point (DSAP), the source service access point

(SSAP), and a control variable. The DSAP address specifies a unique identifier within the station

providing protocol information for the upper layer. The SSAP provides the same information for

the source address.

The LLC defines service access for protocols that conform to the open system interconnection

(OSI) model for network protocols. However, many protocols do not obey the rules for those

Start frame

delimiter

Preamble

Data

Destination

address

Source

address

Type/

Length

6 bytes

1 byte

7 bytes

Vendor

UserValue

2 bytes

Control

SSAP

DSAP

Information

6 bytes

46–1500 bytes

4 bytes

LLC frame

yzxx_xxxx

xxxx_xxxx

Bit 0

Bit 47

Destination address

U/L

I/G

Example: 02608C: 876543 (02608C = 3COM, 876543 = UserValue assigned by 3COM)

3 bytes

Frame check

sequence

Frame length is 64–1518 bytes

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