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MA28140

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4.4 SEGMENTATION LAYER

Overview of the Layer

The segmentation layer provides the means to distribute

several distinct streams of variable-length data units (e.g. the

TC packets) to different applications by providing a number of

service access points called the Multiple Access Points

(MAPs). The data flow on each stream can be controlled by the

receiving application using handshake control.

A TC segment consists of three distinct protocol data

elements:

- an 8-bit segment header, the purpose of which is to

identify the MAP connection and flag the sequential

position of the segment relative to the complete TC

Packet,

- a segment data field, of maximum length 248 octets,

which contains all or a portion of a TC Packet,

- the 9-octet Segment Trailer specific to authenticated

segments is removed by the authentication layer.

Standard Data Structures Within the Layer

The structure of the TC segment is given below:

Segment Header

The Segment Header is the first octet (octet 0) of the TC

segment structure. The Segment Header is divided into two

major fields as follows:

- Sequence Flags (bits 0 & 1): this field is used by the

segmentation protocol to indicate the sequential position

of the segment relative to the complete data unit (e.g. the

TC Packet). The flags are interpreted as follows:

When the flags are set to 11 this means that the TC

Segment Data Field contains an entire TC Packet. Except for

the CPDU described in section 4.5, these flags are ignored by

the PTD.

- Multiplexed Access Point (MAP) Identifier: this 6-bit field

enables up to 64 MAP connection addresses to be associated

with a single Virtual Channel. The PTD supports MAP 1 to 63

as externally available MAPs. MAP 0 is dedicated to the

CPDU. MAP 63, when AU is enabled, is reserved for AU

commands; when the AU is disabled, MAP 63 is processed by

the segment layer like a standard MAP (see section 4.3).

Bit 0 (MSB)

Bit 1

Interpretation

First segment

Continuation segment

Last segment

Unsegmented

Segment Data Field

The segment data field may vary from 0 to 248 octets

maximum. When the optional Segment Trailer is used, the

maximum length of the segment data field will be reduced by 9

octets.

Standard Procedures Within the Layer

The following segmentation layer functions are

implemented in the PTD:

- the back-end buffer for the accepted TC segment. The

back-end buffer is shared between the Transfer Layer

and the Segmentation Layer.

- the MAP interface.

Upon reception of a new segment the Segment Layer

performs the following operations:

- Checks whether the segment is authenticated or not.

- Starts the AU process if the segment is authenticated

and if the AU is not disabled. The Segment Layer waits

for the completion of the AU process (internal or external).

A security mechanism is implemented, in case of AU

locking mechanism the user can stop the AU process by

activating the AU disable signal. In this case, the segment

layer stops waiting for the AU completion process and the

content of the back end buffer is lost.

- Checks if the frame is a CPDU command (MAP 0). In

this case, the CPDU layer is activated and no data is

output on the MAP interface.

- Checks if the frame is an AU command (MAP 63) and

the AU is not disabled. In this case no data is output on

the MAP interface.

- For a MAP 1 to 62 and for MAP 63 if the AU is disabled,

the data is provided in serial or in parallel via the MAP

interface. The MAP output frequency for serial MAP is

selectable by reading a value associated with each MAP

in the external ROM (see section 5.2).

SEGMENT HEADER

SEGMENT DATA

FIELD

Sequence

Flags

2 bits

MAP

Identifier

6 bits

variable

<--------------- 1 octet ------------><- from 0 to 248 octets ->

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