Telecommunication. AAL (ATM Adaptation Layer) Krzysztof Wajda. Department of Telecommunications, AGH-UST November, 2016

Transcription

1 Telecommunication Networks and Systems AAL (ATM Adaptation Layer) Krzysztof Wajda Department of Telecommunications, AGH-UST November, 2016

2 Outline Motivation to introduce so many AAL versions Description of AAL1-AAL5 Interworking between AAL and applications (also: higher layers)

3 Lecture scope Concepts: application requirements traffic classes low-level support for service differentiation

4 AAL Layer AAL Layer (ATM Adaptation layer)

5 AAL Layer This is an interim layer between higher layers (applications) and ATM layer, main functions: error detection and reaction, recognition of lost and wrong packets, flow control, bit timing.

6 Service classes class A class B class C class D Synchronization required Not required between terminals bitstream constant variable Connection mode Connection-oriented Conn.less AAL type 1 2 3/4 i 5 type 5

7 CS functions Matching the size and features of application PDUs with transport features of ATM networks, Error correction, Synchronization.

8 SAR functions Transformation of higher layers PDUs into SDU (and in opposite way) n+1 layer PDU n+1 n layer PCI n + SDU n PDU n n-1 layer SDU n-1

9 AAL1 Transfer of data in CBR (connectionoriented CBR service), Decreasing the effect of cell jitter, Reaction for lost or out-of-order cells, Clock frequency recovery inside receiver, Monitoring and error recovery service for control field of AAL called SNP.

10 SAR-PDU for AAL1 cell header SN SNP SAR-PDU payload SAR-PDU header SAR-PDU SN SNP = Sequence Number = Sequence Number Protection (4 bits) (4 bits)

11 SAR-PDU header The most significant bit of SN field is used for special purposes 3 other bits of SN field constitute cell counter modulo 8 Pole SNP field secures whole SAR- PDU header

12 SAR-PDU header for AAL1 SN-field SNP-field Special Purpose Indication Sequence Number Count CRC Parity Bit (Even)

13 AAL1 - summary Single bits, bytes Higher layers 47 bytes 48 bytes Convergence sublayer (CS) Segmentation and reassembly sublayer (SAR) 53 bytes ATM layer

14 AAL2 Transfer of data in VBR (connectionoriented real-time service), CS-PDUs with variable length, For long time without precise specification for AAL2, For voice services, mobile systems,

15 CS sublayer functions for AAL2 Error correction for audio and video services Synchronization of terminals by sending time stamps Service for lost and out-of-sequence cells

16 SAR-PDU structure for AAL2 cell header SN IT SAR-PDU payload LI CRC SAR-PDU header SAR-PDU trailer SAR-PDU SN IT LI CRC = Sequence number = Information Type = Length Indicator = Cyclic Redundancy Check

17 AAL3 Transfer of data in VBR (connectionoriented real-time service), Synchronization between sender and receiver is not necessary, Two modes of operation: framed mode, message mode, streaming mode, Monitoring and service of errors in AAL (CRC field in SAR-PDU).

18 AAL3/4 - summary

19 AAL1-4 -summary SN SNP SAR-PDU Informacja użyteczna nagłówek 1-bajtowy 47 bajtów a) Struktura danych podwarstwy SAR dla AAL typ 1 SAR-PDU ST IT LI CRC Informacja użyteczna nagłówek 1-bajtowy 45 bajtów b) Struktura danych podwarstwy SAR dla AAL typ 2 zakończenie 2-bajtowe ST SN Res SAR-PDU Informacja użyteczna LI CRC nagłówek 2-bajtowy c) Struktura danych podwarstwy SAR dla AAL typ 3 44 bajty zakończenie 2-bajtowe ST SN MID SAR-PDU Informacja użyteczna LI CRC nagłówek 1-bajtowy 44 bajty d) Struktura danych podwarstwy SAR dla AAL typ 4 zakończenie 2-bajtowe SN ( Sequence Number - 4 bity) - numer kolejny SNP ( Sequence Number Protection - 4 bity) - ochrona SN IT ( Information Type - 4 bity) - typ informacji LI ( Length Indicator - 6 bitów) - wskaźnik, długości CRC ( Cyclic-Redundancy-Check - 10 bitów) - ciąg kontrolny kodu cyklicznego ST ( Segment Type - 2 bity) typ segmentu Res ( Reserved - 10 bitów) - rezerwa MID ( Multiplexing IDentification - 10 bitów) -identyfikator multipleksowania

20 AAL5 - summary Universal version of AAL, covering functions of AAL3/4 and more. VBR mode, connection-oriented, without synchronization, Without numbering of subsequent cells, No retransmission, SEAL (Simple and Efficient Adaptation Layer),

21 SAR-PDU for AAL5 ATM cell header SAR-PDU payload SAR-PDU

22 Split of CS-PDU into cells CS-PDU CS-PDU trailer CS sublayer User data PAD CF LF CRC SAR sublayer PT ATM cell (AAL5) ATM cell (AAL5) ATM cell (AAL5) ATM cell (AAL5) PAD = Pad field (0 to 47 bytes) CF = Control Field (2 bytes) LF = Length Field (2 bytes) CRC= Cyclic Redundancy Check (4 bytes)

23 Summary Flexible but complicated concept of AAL, Extensive and explicit service specification, AAL5 mostly used.

24 Bibliography ATM Forum standards ITU-T Recommendations D. Ginsburg, ATM. Solutions for enterprise networking, Addison-Wesley, 1999 Lecture notes:

25 Thank you!