WRET2104

Protokol ATM

Protokol ATM

ATM Adaptation Layer

ATM Adaptation Layer

ATM Adaptation-Layer Protocol ialah untuk membawa protokol data units (PDUs) daripada lapisan pertama ( lapisan aplikasi) dan menukarkan kepada format yang sesuai untuk transmisi dalam sel ATM. menyediakan antaramuka di antara lapisan

aplikasi dan lapisan ATM low delay and fast transport

ATM Adaptation Layer

Fungsi asas AAL adalah untuk melakukan segmentasi dan penyusunan semula upper layer SDU 48 oktet

Fungsi lain pada lapisan ini termasuk pemulihan kesalahan , mengendalikan pelbagai kelewatan sel (cell delay), pemulihan sumber frekuensi masa pada penghujung penerimaan dan lain-lain

ATM Adaptation Layer

Lapisan penyesuaian telah dibahagikan kepada 2 sublapisan iaitu: CS menunjukkan fungsi service-dependent

SSCS yang ditakrifkan dengan perkhidmatan khas

CPCS yang biasa digunakan dalam semua perkhidmatan.

Sublapisan SAR menunjukkan segmentasi dan penyusuran semula.

ATM Adaptation Layer: Service TypesService Class Types that Determine the functions of the CS

Class Atributes A Connection-oriented, constant-bit-rate, and needs to transmit timing information over ATM cells. An example is the circuit emulation of a T1/PRI interface B Connection-oriented, variable-bit-rate, and needs to transmit timing information over the ATM cells. An example is a multimedia service with variable –bit-rate Video and audio. C Connection-oriented/connectionless, variable -bit- rate, but does not need to transmit timing information over the ATM cells. D Connectionless, variable –bit-rate and does not need to tansmit timing information over the ATM cells

ATM Adaptation Layer: AAL Type 1 ATM Adaptation Layer Type 1: AAL functions in support

of constant bit rate, time-dependent traffic such as voice and video.

AAL-1 requires timing synchronization between the source and destination. For this reason, AAL-1 depends on a medium, such as SONET, that supports clocking.

First, synchronous samples (for example, 1 byte of data at a sampling rate of 125 microseconds) are inserted into the Payload field.

Second, Sequence Number (SN) and Sequence Number Protection (SNP) fields are added to provide information that the receiving AAL-1 uses to verify that it has received cells in the correct order.

Third, the remainder of the Payload field is filled with enough single bytes to equal 48 bytes.

ATM Adaptation Layer: AAL Type 1

SN - Sequence number.Numbers the stream of SAR PDUs of a CPCS PDU (modulo 16).

CSI - Convergence sublayer indicator. Used for residual time stamp for clocking

SC - Sequence court.

SNP - Sequence number protection.

CRC - Cyclic redundancy check calculated over the SAR header.

Parity - Parity calculated over the CRC.

SAR PDU - payload 47-byte user information field.

ATM Adaptation Layer: AAL Type 2 ATM Adaptation Layer Type 2: This

AAL is still undefined by the International Standards bodies. It is a placeholder for variable bit rate video transmission.

ATM Adaptation Layer: AAL Type 3/4 AAL-3/4 supports both connection-

oriented and connectionless data. It was designed for network service providers and is closely aligned with Switched Multimegabit Data Service (SMDS). AAL-3/4 will be used to transmit SMDS packets over an ATM network.

ATM Adaptation Layer: AAL Type 3/41. The convergence sublayer (CS) creates a protocol

data unit (PDU) by prepending a beginning/end tag header to the frame and appending a length field as a trailer.

2. The segmentation and reassembly (SAR) sublayer fragments the PDU and prepends a header to it.

3. The SAR sublayer appends a CRC-10 trailer to each PDU fragment for error control.

4. The completed SAR PDU becomes the Payload field of an ATM cell to which the ATM layer prepends the standard ATM header.

ATM Adaptation Layer: AAL Type 3/4 An AAL-3/4 SAR PDU header consists of type,

sequence number, and multiplexing identifier fields. Type fields identify whether a cell is the beginning, continuation, or end of a message. Sequence number fields identify the order in which cells should be reassembled. The multiplexing identifier determines which cells from different traffic sources are interleaved on the same VCC so that the correct cells are reassembled at the destination.

ATM Adaptation Layer: AAL Type 3/4

ATM Adaptation Layer: AAL Type 3/4 (CS PDU) CPI - Common part indicator. Represents the units of the BAsize and length

field. A value of only zero indicates bytes

Btag - Beginning tag. This field must have the same value as Etag for same CPCS-PDUs and a different value as the Btag/Etag preceding and successive CPCS-PDUs.

BAsize - Buffer allocation size. In message mode this is equal to the length field. In streaming mode this is equal to or greater than the length field.

PAD - Up to 3 bytes of padding to achieve 32-bit alignment in the information field

AL - Alignment. A filling byte coded with zero.

ETAG - End tag. Refer to Btag.

Length - Length of the Information field. This value is used to indicate information loss or gain.

ATM Adaptation Layer: AAL Type 3/4 (SAR PDU) ST - Segment type. Values 10 = Beginning of message; 00 =

Continuation of message; 01 = End of message; 11 = Single segment message

SN - Sequence number. Numbers the stream of SAR PDUs of a CPCS PDU (modulo 16).

MID Multiplexing identification. This is used for multiplexing several AAL-3/4 connections over one ATM link.

LI - Length indication. Contains the length of the SAR SDU in bytes

CRC - Cyclic redundancy check calculated over the SAR header.

ATM Adaptation Layer: AAL Type 5 AAL-5 is the primary AAL for data and

supports both connection-oriented and connectionless data. It is used to transfer most non-SMDS data, such as classical IP, over ATM. AAL-5 also is known as the simple and efficient adaptation layer (SEAL) because the SAR sublayer simply accepts the CS-PDU and segments it into 48-octet SAR-PDUs without adding any additional fields

ATM Adaptation Layer: AAL Type 51. The CS sublayer appends a variable-length pad and an 8-byte

trailer to a frame. The pad ensures that the resulting PDU falls on the 48-byte boundary of an ATM cell. The trailer includes the length of the frame and a 32-bit cyclic redundancy check (CRC) computed across the entire PDU. This allows the AAL-5 receiving process to detect bit errors, lost cells, or cells that are out of sequence.

2. The SAR sublayer segments the CS PDU into 48-byte blocks. A header and trailer are not added (as is in AAL-3/4), so messages cannot be interleaved.

3. The ATM layer places each block into the Payload field of an ATM cell. For all cells except the last, a bit in the Payload Type (PT) field is set to zero to indicate that the cell is not the last cell in a series that represents a single frame. For the last cell, the bit in the PT field is set to one

AAL-5 is used to carry computer data such as TCP/IP. It is the most popular AAL and is sometimes referred to as SEAL (simple and easy adaptation layer

ATM Adaptation Layer: AAL Type 5

ATM Adaptation Layer: AAL Type 5 (CS PDU) PAD - Padding used to cell align the trailer which may be

between 0 and 47 bytes long

UU - CPCS user-to-user indication to transfer one byte of user information.

CPI - Common part indicator is a filling byte (of value 0). This field is to be used in the future for layer management message indication.

Length - Length of the Information field.

CRC-32 - Cyclic redundancy check computed from the Information field, PAD, UU, CPI and Length fields. It is a 32-generator polynomial.

ATM Adaptation Layer: AAL Type 5 (CS PDU) The structure of the AAL-5 SAR PDU

consists of a 48 byte payload which is just a standard ATM cell.

ATM Adaptation Layer: Summary The ATM Adaptation Layer has several

ways to segment and reassemble packets.

The type of AAL used depends on the type of information needing to be sent.

The AAL used also depends on the type of physical layer transport.

Physical Layer

Protokol lapisan fizikal yang biasa digunakan dalam rangkaian ATM ialah SONET atas fiber

protokol lapisan fizikal dibahagikan kepada dua sublapisan: Physical Medium-Dependent(PMD) sublapisan

Sublapisan PMD termasuk fungsi penyiaran sebagai pemindahan bit dan susunan bit

Transmission Convergence(TC) sublapisan. Sublapisan TC, membekalkan fungsi yang tidak

bergantung pada fizikal medium dan termasuk juga menggariskan sel dan juga ATM sel header error checking

Physical Layer