DP83901AV datasheet(8/62 Pages) NSC

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50 TransmitReceive Packet EncapsulationDecapsulation

A standard IEEE 8023 packet consists of the following

fields preamble Start of Frame Delimiter (SFD) destination

address source address length data and Frame Check

Sequence (FCS) The typical format is shown in

Figure 2

The packets are Manchester encoded and decoded by the

ENDEC module and transferred serially to the NIC module

using NRZ data with a clock All fields are of fixed length

except for the data field The SNIC generates and appends

the preamble SFD and FCS field during transmission The

Preamble and SFD fields are stripped during reception (The

CRC is passed through to buffer memory during reception)

PREAMBLE AND START OF FRAME DELIMITER (SFD)

The Manchester encoded alternating 10 preamble field is

used by the ENDEC to acquire bit synchronization with an

incoming packet When transmitted each packet contains

62 bits of alternating 10 preamble Some of this preamble

will be lost as the packet travels through the network The

preamble field is stripped by the NIC module Byte align-

ment is performed with the Start of Frame Delimiter (SFD)

pattern which consists of two consecutive 1’s The SNIC

does not treat the SFD pattern as a byte it detects only the

two bit pattern This allows any preceding preamble within

the SFD to be used for phase locking

DESTINATION ADDRESS

The destination address indicates the destination of the

packet on the network and is used to filter unwanted pack-

ets from reaching a node There are three types of address

formats supported by the SNIC physical multicast and

broadcast The physical address is a unique address that

corresponds only to a single node All physical addresses

have an MSB of ‘‘0’’ These addresses are compared to the

internally stored physical address registers Each bit in the

destination address must match in order for the SNIC to

accept the packet Multicast addresses begin with an MSB

of ‘‘1’’ The SNIC filters multicast addresses using a stan-

dard hashing algorithm that maps all multicast addresses

into a 6-bit value This 6-bit value indexes a 64-bit array that

filters the value If the address consists of all 1’s it is a

broadcast address indicating that the packet is intended for

all nodes A promiscuous mode allows reception of all pack-

ets the destination address is not required to match any

filters Physical broadcast multicast and promiscuous ad-

dress modes can be selected

SOURCE ADDRESS

The source address is the physical address of the node that

sent the packet Source addresses cannot be multicast or

broadcast addresses This field is simply passed to buffer

memory

LENGTH FIELD

The 2-byte length field indicates the number of bytes that

are contained in the data field of the packet This field is not

interpreted by the SNIC

DATA FIELD

The data field consists of anywhere from 46 to 1500 bytes

Messages longer than 1500 bytes need to be broken into

multiple packets Messages shorter than 46 bytes will re-

quire appending a pad to bring the data field to the minimum

length of 46 bytes If the data field is padded the number of

valid data bytes is indicated in the length field The SNIC

does not strip or append pad bytes for short packets

or check for oversize packets

FCS FIELD

The Frame Check Sequence (FCS) is a 32-bit CRC field

calculated and appended to a packet during transmission to

allow detection of errors when a packet is received During

reception error free packets result in a specific pattern in

the CRC generator Packets with improper CRC will be re-

jected The AUTODIN II (X32 a X26 a X23 a X22 a X16 a

X12 a X11 a X10 a X8 a X7 a X5 a X4 a X2 a X1 a 1)

polynomial is used for the CRC calculations

TLF10469 – 5

FIGURE 2

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