Atm Module4

8/20/2019 Atm Module4

1/22

ATM Basics, Version 1.6 © T.O.P. BusinessInteractive GmbH Page 1 of 22

4 AAL functions


2/22


4.1 Overview .......................................................................................3 4.2 Quality of Service .........................................................................4 4.3.1 ATM Adaptation Layers (1/2) ....................................................5 4.3.1 ATM Adaptation Layers (2/2) ....................................................6 4.3.2 Service Classes .........................................................................7 4.3.3 AAL1 (1/5) ..................................................................................8 4.3.3 AAL1 (2/5) ..................................................................................9 4.3.3 AAL1 (3/5) ................................................................................10 4.3.3 AAL1 (4/5) ................................................................................11 4.3.3 AAL1 (5/5) ................................................................................12 4.3.4 AAL 2 (1/2)................................................................................13 4.3.4 AAL 2 (2/2)................................................................................14

4.3.5 AAL 3/4 (1/3)............................................................................15

4.3.5 AAL 3/4 (2/3)............................................................................16 4.3.5 AAL 3/4 (3/3)............................................................................17 4.3.6 AAL-Type 5 (1/3) ......................................................................18 4.3.6 AAL-Type 5 (2/3) ......................................................................19 4.3.6 AAL-Type 5 (3/3) ......................................................................20 4.4 Label Switching (1/2)..................................................................21 4.4 Label Switching (2/2)..................................................................22


3/22


4.1 Overview

In this module we will look at the functionality of the ATM Adaptation Layer (AAL) from the

point of view of service. We will discuss the following topics:

• Quality of service (QoS): the different types of user data have to be treateddifferently. Some data, for example, can be variable and connectionless, wheareasspeech is constant and connection-oriented. To be able to offer adequate services, ATM must set some standards as far as priority and the QoS are concerned.

• ATM Adaptation Layers (AALs): speech, video and data traffic are fed into thenetwork in different ways. Thus, each traffic type must be adapted to ATM differently.Therefore ATM has four adaptation layers.

ATM switching: each switch routes cells to different destination switches. It reads the

incoming cell header, forwards the cell to the exit port so it is forwarded in the correctdirection, and then changes the channel and path identifiers as far as necessary.


4/22


4.2 Quality of Service

To be able to guarantee a certain quality of service, or QoS, there are standardizedprocedures for dealing with irregularities in data transmission. The following aspects must beconsidered:

• Data types can have differing tolerance levels.• There is a mechanism to delete delayed cells.• Virtual channels can be handled as required.• Data types can have high or low priority.

The ITU-T defined a series of QoS parameters:• Latency indicates the nominal delay within a network.• Jitter indicates slight deviations in this delay.• Capacity indicates the average peak data throughput.• Errors tells the number of incomplete or damaged cells.• Losses indicates the number of lost cells.• Misinsertions indicates the number of misrouted cells.

A channel's QoS parameters can indicate a lower or, at most, an equal, but never a higherquality of service compared to the path. This means that a channel cannot stand out from itsgroup.


5/22


4.3.1 ATM Adaptation Layers (1/2)

The ITU defined different service classes for ATM. The classes differ in their relation to time,bit rates and connection type. An example for class A could be circuit-switched speechtransmission; or for example video applications for class B, or connection-oriented orconnectionless data transmission at a variable bit rate for class C and D. Each service typehas a specific AAL assigned to it. Let's look a bit closer at the ATM Adaptation Layerarchitecture.Data packets from higher protocol layers reach the AAL via the Service Access Point (SAP)and are first passed down to the appropriate Service Specific Convergence Sublayer(SSCS). The sublayer is optional. It can be used e.g. to set up or clear down a connection-oriented channel if several channels can be multiplexed onto the same cell stream.


6/22


4.3.1 ATM Adaptation Layers (2/2)

The Common Part Convergence Sublayer (CPCS) adds additional Protocol ControlInformation (PCI) as headers and/or trailers, thus forming a CPCS data packet. This is notrequired for AAL1. The Segmentation and Reassembly Sublayer (SAR) splits this CPCS datapacket into segments, with headers and/or trailers to form the 48 byte long payload sectionsof an ATM cell. These segments are reassembled at the destination's peer layer. Please notethat none of the AAL protocols retransmits missing cells, though they are able to identifythem. If any cells are missing, the complete CPCS data packet is deleted.


7/22


4.3.2 Service Classes

Service classes can cover a range of applications:

• Class A: A traffic is real-time traffic with a constant bit rate, such as uncompressedspeech and video data.

• Class B: B traffic is also real-time traffic, but with a variable bit rate across thenetwork and a constant one end-to-end, for example compressed voice and videodata.

• Class C: C traffic is connection-oriented, as we know from X.25 networks.• Class D: D traffic is connectionless, as in LANs and TCP/IP networks.• Class X: This class is not specified. It is used e.g. for signalling and control data.


8/22


4.3.3 AAL1 (1/5)

AAL1 handles real-time connection-oriented transmissions with constant bit rates. This

concerns uncompressed speech and video transmission. Let's look at the transmissionprocess at bit level. In the transmitting direction, a uniform bit stream containing dataarranged in a fixed sequence, is segmented into cells. These cells, however, have a variabledelay. To guarantee the service, however, the bit stream must be restored in the receivingdirection. Let's zoom in on the AAL Layer to understand the processes that are going on.


9/22


4.3.3 AAL1 (2/5)

Within the AAL, the Segmentation and Reassembly Sublayer (SAR) is of particular

importance. At the transmitting end, the SAR sublayer receives a 47 octet long data blockfrom the above-lying Convergence Sublayer (CS). It adds a 1 octet long SAR PDU header tothis data block. This 48 byte long data unit is called an SAR Packet Data Unit (SAR PDU).Let's look a bit closer at the header. It cosists of a 4 bit long Sequence Number field (SN),and an equally sized Sequence Number Protection (SNP).


10/22


4.3.3 AAL1 (3/5)

The SN field consists of the actual sequence number, which is 3 bits long, and of another bit,

the CS indicator bit. This depends on the payload type and indicates whether theConvergence SubLayer transmits synchronization information. The default value of this bit is0.The SNP field contains a 3 bit CRC control field, plus a parity bit to detect and correct errorsin the SAR PDU header. The SNP field protects the integrity of the sequence number andcontains a 3 bit CRC control field and a parity bit for error detection and correction of theSAR PDU-header. The receiver transmits the sequence number, the CSI bit and the CRC'serror status to the Convergence Sublayer, and can use this control information to detect andcorrect single bit errors in the SAR PDU header. A receiving switch can use the sequence number to detect missing cells and generate a nullbit stream. Eight consecutive lost cells are a problem, since they will not be detected from thesequence number.


11/22


4.3.3 AAL1 (4/5)

There are methods for clock recovery which allow AAL1 to keep up real-time conditions. One

way to do this is Synchronous Residual Time Stamp, or SRTS. Here the sender and thereceiver have a nominal clock in common. The difference in frequency between the actualand the nominal clock is measured on the sender's side with the help of the Residual TimeStamp (RTS) and then sent to the receiver, who then can restore the clock relevant for thereal-time transmission.


12/22


4.3.3 AAL1 (5/5)

Another method is Adaptive Clock. The information received is stored in a buffer and isfetched from there by the receiver. The buffer must be half-filled. If the information arrives ata higher speed than the receiver can fetch, the amount of information in the buffer increases.This means the clock at the receiving end is too slow. If the buffer exceeds two-thirds, thereceiver speeds up its clock. If the buffer falls below one-third, the receiver is instructed toreduce it. Control is based on continuously measuring the amount of information in thebuffer, which should be about half-full. This method allows at least an approximateregeneration of the original bit stream.


13/22


4.3.4 AAL 2 (1/2)

AAL type 2 handles real-time traffic with variable bit rate. The payload traffic is compressedon its way across the network and therefore will have a variable bit rate. This applies, forexample, to compressed video transmission. This data type is usually transmitted as packetsof variable length. The packet length of 45 bytes can be extended to 64 bytes.First, the variably sized data packets are provided with a 3 byte header. This contains an 8bit long Channel Identifier (CID), which allows the multiplexing of 248 channels onto one cellstream. Furthermore, it contains a 6 bit Length Indicator to indicate the size of the datapacket, a 5 bit User-to-User Indication (UUI), which is transmitted transparently end-to-end,and a 5 bit checksum for error detection in the header. This is called Header Error Control, orHEC checksum.


14/22


4.3.4 AAL 2 (2/2)

The resultant CPS data units, which are named after the Common Part Sublayer (CPS), varyin length, depending on their payload section. In the next step, the data units arestandardized in their length. The information is packaged into 47 byte cells and provided witha 1 byte AAL2 header. Any surplus data is packed into the next cell.The AAL2 header is also known as Start Field and contains a 6 bit Offset Field (OSF), whichmarks the beginning of the first packet in the cell. Furthermore, a 1 bit Sequence Number(SN) to indicate whether it is the first or the second part of a payload unit, as well as a paritybit to detect transmission errors in the AAL2 header.The 47 byte long payload field can contain one or more complete CPS data packets, or bitsof them. Empty payload fields are filled up with padding octets whose value is 0. The headerand the payload form the 48 byte payload section of the cells at the ATM Layer.


15/22


4.3.5 AAL 3/4 (1/3)

AAL type 3/4 handles non-real-time traffic of variable bit rate. It is a bundling of formerly twoseparate types, i.e. type 3 for connection-oriented, and type 4 for connectionless services.Both are supported by type 3/4. AAL type 3/4 has three sublayers:

• The Service Specific Convergence Sublayer : this is optional and provides specificfunctions for the service to be adapted.

• The Common Part Convergence Sublayer (CPCS): it provides functions shared by allthe type 3/4 services.

• The Segmentation And Reassembly Sublayer (SAR): this standardizes the packetsizes for further transmission and prepares the formatting of the packets before theyare passed on to the ATM Layer.


16/22


4.3.5 AAL 3/4 (2/3)

At the Convergence Sublayer, further information is added to the payload which has beenhanded down from higher layers. This is to allow the destination host to reassembletransmitted data. This additional information is composed of several parts, allocated to theheader and the trailer.

• The 1 byte Common Part Indicator (CPI) identifies the type of the following payloadunit CPCS PDU. That is the data packet from the Convergence Sublayer.

• The Beginning Tag (Btag) and the End Tag (Etag), which are the same size, i.e. 1byte, define the beginning and end of the CPCS PDUs.

• The 2 byte Buffer Allocation Size (BASize) field informs the receiver of the bufferspace required to assemble this message.

• The Padding field (PAD) expands the CPCS PDU payload by up to 3 empty octets to

form a whole number multiple of 4 octets. The Padding fields are used exclusively forpadding and don't contain any information. Their value is always set to 0 and isignored by the receiver.

• The 1 byte Alignment field (AL) expands the CPCS PDU trailer to 32 bits, i.e. 4octets. Like the padding field, this octet also doesn't contain any information, and itsvalue is set to 0.

• The header, trailer and CPCS payload field form the CPCS PDU.


17/22


4.3.5 AAL 3/4 (3/3)

The Segmentation and Reassembly Sublayer (SAR) organizes the CPCS PDUs into 44 bytepayload sections for further data processing at the ATM Layer. Of the remaining 4 bytes, twobytes are required for the SAR header and the other two for the SAR trailer. These provideindividual cell transport functions. Let's look at the header:

• The 2 bit Segment Type field (ST) indicates whether the CPCS PDU is to make upone or more SAR PDUs. There are several types of segments: Single SegmentMessage (SSM), Beginning of Message (BOM), Continuation of Message (COM), orEnd of Message (EOM).

• The 4 bit Sequence Number field (SN) identifies all SAR PDUs belonging to onespecific CPCS PDU. If the End SAR PDU, i.e. the EOM type, is smaller than 44

bytes, a padding bit is added.• The 9 byte long Multiplex Identifier is used to multiplex several SAR connections onto

one ATM Layer virtual circuit. It identifies all the SAR PDUs belonging to the sameCPCS PDU or SAR Service Data Unit (SAR SDU).

Now let's look at the trailer:• The 6 bit Length Indication field (LI) indicates the number of octets carrying PDU

information contained in the SAR PDU payload field. The value is usually 44, exceptin the EOM type.

• Finally, a 10 bit CRC field detects errors in the SAR PDU.


18/22


4.3.6 AAL-Type 5 (1/3)

Finally, let's examine AAL type 5, which, like type 3/4 is used for transmission at variable bitrates. AAL5 has the same sublayer structure as type 3/4. It consists of the optional SSCS,the CPCS and the SAR. Type 5 has considerably less network overheads: the SAR sublayerhas neither header nor trailer, and the CPCS PDU is only to be divided into 48 byte blocks.The cell may contain padding octets to guarantee that packet alignment is maintained. Let'stake a closer look.


19/22


4.3.6 AAL-Type 5 (2/3)

At the Convergence Layer, a trailer containing control information is attached to the datapacket, so that a CPCS PDU with one payload field is formed, which can be between 1 and65 535 octets long. The trailer contains the following elements:

• A padding field fills up between 0 and 47 octets as required, making the CPCS PDUdivisible by 48.

• In the User-to-User Indication field (UU), CPCS user-specific information istransmitted transparently between users.

• The Common Part Indicator field (CPI) expands the trailer to the 64 bit alignment andcan identify layer management information.

• The Length field is used to encode the CPCS PDU payload length.• Finally, a 32 bit CRC is used to detect bit errors in the CPCS PDU.


20/22


4.3.6 AAL-Type 5 (3/3)

So the SAR sublayer receives data packets of variable length, which is a whole numbermultiple of 48 octets, from the Convergence Layer. Then it segments them into SAR PDUsthat are 48 octets long. The SAR Layer adds no header or trailer. To indicate the last cell of adata unit, SAR uses a specific parameter of the ATM Layer. The last bit of the Payload TypeIdentifier (PTI), which is part of the ATM header, is set to 1 to mark the last cell of a data unit.Let's compare the AAL types 3/4 and 5. We will see that the structure of type 5 is muchsimplified, since multiplexing or sequencing are left to other layers. Therefore it is possible toimplement the CPCS functions of this type into the hardware. Thus, type 5 is the preferredlayer for high-performance data applications and is preferably used in LAN emulations.


21/22


4.4 Label Switching (1/2)

Label switching is a technique which enhances the routing performance, especially in thebackbone of IP networks. This technique is used in a variety of network systems, such asX.25 or Frame Relay, and has also been chosen for ATM. The basis of it is that each circuitis given an identifier which is unique to and valid for the particular link used.In a single switched network, the VPI/VCI values for one specific channel are different oneither side of the switch. Any cell enters a switch on a link with a specific VPI/VCI associatedto it, and leaves it on another link with another VPI/VCI value. The switch must thus swap theVPI/VCI values and calculate a new Header Error Check during the switching process.The switch does this by checking the incoming labels with the VPI/VCI values and bycomparing them with the entries in a table for incoming links. Let's take a closer look at this

table.


22/22

ATM Basics Version 1 6 © T O P B i I t ti G bH Page 22 of 22

4.4 Label Switching (2/2)

After checking the incoming link, the table entries specify the outgoing link and thus theappropriate label the header of the cell at the output is to be provided with. The tables can beimplemented via software or they can be directly configured on the switch's hardware. Theycan be configured by the network manager via a switch management system to set upPermanent Virtual Circuits (PVCs) or are generated dynamically at call set-up, as is the casewith Switched Virtual Circuits (SVCs).When using label switching, transmission is transferred to circuit level, compared to prefixsearch for addresses on the network level, and can be accelerated particulary with hardwaresupport.Routers between high-capacity lines are often overloaded, because the routing tables grow

longer and longer. Label switching offers a remedy to that. Data flows with similar QoSrequirements are bundled into larger units, for example to virtual paths, and are provided withtheir own labels, which are sufficient as routing information for transmission through thebackbone and ease the routing tables.

Documents

Atm Module4