What is ATM?

#)ATM(Asynchronous Transfer Mode) is a transfer mode in which information is organized into cells.

#)It is asynchronous in the sense that the occurrence of a cell containing information from an individual user is not necessarily periodic.

#)ATM is a fusion between fast switching and synchronous multiplexing technique.

#)ATM is based on a 53-octet cell.

#)A typical cell has a 5-octet header and a 48-octet information field.

#)All traffic is transported in the information field of that cell, whether voice, data, or image

Why 53?

#)The cell header contains only 5 octets.

#)It was shortened as much as possible, designed to contain the minimum address and control functions for a working system.

#)It is obvious that the overhead is non-revenue-bearing. It is the information field that contains the revenue- bearing payload.

#)For efficiency, we’d like the payload to be as long as possible.

#)Yet the ATM designer team was driven to shorten the payload as much as possible.

#)The issue in this case was what is called packetization delay.

#)This is the amount of time required to fill a cell at a rate of 64 kbps—that is, the rate required to fill the cell with digitized (PCM) voice samples.

#)The design team was torn between efficiency and packetization delay.

#)One school of thought fought for a 64-octet cell, and another argued for a 32-octet cell size.

#)Thus the ITU-T organization opted for a fixed-length 53-octet compromise.

Benefits of ATM

#)ATM technology offers four principal benefits:

1)Bandwidth efficiency--By allowing any user access to the network whenever resources are available, ATM provides better use of bandwidth for bursty traffic. By breaking traffic into small, fixed-length cells, ATM prevents a user with a long message from blocking access to the network.

2)Scalable technology--ATM is flexible and accommodates a wide range of traffic rates and applications. ATM interface standards exist for data rates as low as 1.5 Mbps and as high as 1.2 Gbps.

3)Application transparency--The ATM cell size is a compromise between the long frames of data applications and the short, repetitive frames required in voice applications. Due to its asynchronous nature, ATM supports traffic at rates and degrees of burstiness compatible with the applications being run, not at rates convenient to the network.

4)Networking advantages--ATM is a simple, fast, switching and routing process based on the virtual channel identifier (VCI) in the cell address. Within the network, no processing occurs above the cell level, thus simplifying and increasing message-handling speed. ATM message- handling techniques foster the creation of efficient, self-routing switches that can grow in size and speed to meet the user's future communications requirements.

ATM's Position in the OSI Model

#)ATM standards define protocols that operate at Layer 2 (the data link layer) of the International Standards Organization (ISO) seven-layer Open Systems Interconnection (OSI) reference model.

#)The diagram below shows the layered architecture of the OSI model.

#)The data link layer is concerned with data transmission between two network switches.

#)This layer is not concerned with the transmission of an entire message between a source and a destination switch--this responsibility belongs to Layer 3 (the network layer).

#)Rather, the data link layer transports portions of messages (cells, in the case of ATM) between two points in the network.

#)These points may be the source and the destination of the message, or they may be only intermediate hops between the source and the destination.

#)The data link layer may divide higher level data into smaller units (cells, in this case), whose sizes are compatible with overall network requirements.

#)Layer 2 data units contain a cell header, an information field, and some method of checking for transmission errors.

Placing User Data into ATM Cells

#)Before frames can be transported across an ATM network, they must be divided into ATM cells.

#)The processes that divide the frames into cells occur at the data link layer (Layer 2).

#)Layer 2 is divided into two parts: the ATM adaptation layer (AAL) and the ATM layer.

#)Once frames are divided into ATM cells, the cells can be transferred to Layer 1, the physical layer

Broadband-ISDN Protocol Reference Model-ATM Functions and Layers

#)ATM is the underlying message format of B-ISDN. B-ISDN layering has no relationship whatsoever with the OSI reference model.

Physical Layer

#)The physical layer consists of two sublayers.

#)The physical medium (PM) sublayer includes only physical medium-dependent functions.

#)The transmission convergence (TC) sublayer performs all functions required to transform a flow of cells into a flow of data units (i.e., bits) which can be transmitted and received over a physical medium.

#)The service data unit (SDU) crossing the boundary between the ATM layer and the physical layer is a flow of valid cells.

#)The ATM layer is unique (independent of the underlying physical layer).

#)The data flow inserted in the transmission system payload is physical medium-independent and self-supported.

#)The physical layer merges the ATM cell flow with appropriate information for cell delineation, according to the cell delineation mechanism previously described, and carries the operations, administration, and maintenance (OAM) information relating to this cell flow.

#)The PM sublayer provides bit transmission capability including bit transfer and bit alignment, as well as line coding and electrical-optical transformation.

#)Of course, the principal function is the generation and reception of waveforms suitable for the medium, the insertion and extraction of bit timing information, and line coding where required.

#)The primitives identified at the border between the PM and TC sublayers are a con- tinuous flow of logical bits or symbols with associated timing information.

Transmission Convergence Sublayer

#)Among the important functions of this sublayer is the generation and recovery of transmission frame.

#)Another function is transmission frame adaptation, which includes the actions necessary to structure the cell flow according to the payload structure of the transmission frame (transmit direction), and to extract this cell flow out of the transmission frame (receive direction).

#)The transmission frame may be a cell equivalent (i.e., no external envelope is added to the cell flow), an SDH/SONET envelope, an E1/T1 envelope, and so on.

#)In the transmit direction, the HEC sequence is calculated and inserted in the header.

#)In the receive direction, we include cell header verification. Here cell headers are checked for errors and, if possible, header errors are corrected.

#)Cells are discarded where it is determined that headers are errored and are not correctable. Another transmission convergence function is cell rate decoupling.

#)This involves the insertion and removal of idle cells in order to adapt the rate of valid ATM cells to the payload capacity of the transmission system.

#)In other words, cells must be generated to exactly fill the payload of SDH/SONET, as an example, whether the cells are idle or busy.

ATM layer

#)The ATM layer is completely independent of the physical medium.

#)One important function of this layer is encapsulation.

#)This includes cell header generation and extraction.

#)In the transmit direction, the cell header generation function receives a cell information field from a higher layer and generates an appropriate ATM cell header except for the header error control (HEC) sequence.

#)This function can also include the translation from a service access point (SAP) identifier to a VP (virtual path) and VC (virtual circuit) identifier.

#)In the receive direction, the cell header extraction function removes the ATM cell header and passes the cell information field to a higher layer.

#)As in the transmit direction, this function can also include a translation of a VPI and VCI into an SAP identifier.

The ATM adaptation layer (AAL)

#)The basic purpose of the AAL is to isolate the higher layers from the specific characteristics of the ATM layer by mapping the higher-layer protocol data units (PDUs) into the payload of the ATM cell and vice versa.

#)To support services above the AAL, some independent functions are required of the AAL. These functions are organized in two logical sublayers:

#)The convergence sublayer (CS) and the segmentation and reassembly (SAR) sublayer. The primary functions of these layers are:

1)SAR—The segmentation of higher-layer information into a size suitable for the infor-mation field of an ATM cell. Reassembly of the contents of ATM cell information fields into higher-layer information.

2)CS—Here the prime function is to provide the AAL service at the AAL-SAP (service access point). This sublayer is service dependent.

Service Classification of the AAL

#)Service classification is based on the following parameters:

1)Timing relation between source and destination (this refers to urgency of traffic):required or not required.

2)Bit rate: constant or variable.

3)Connection mode: connection-oriented or connectionless.

#)When we combine these parameters, four service classes emerge.

i)Class A: constant bit rate such as uncompressed voice or video.

ii)Class B: variable bit rate video and audio, connection-oriented synchronous traffic.

iii)Class C: connection-oriented data transfer, variable bit rate, asynchronous traffic.

iv)Class D: connectionless data transfer, asynchronous traffic such as SMDS.

AAL Categories or Types

#)There are five different AAL categories.

1)AAL-0:

#)It just transmits cells down a pipe.

#)That pipe is commonly a fiber-optic link. Ideally, it would be attractive that the bit rate here be some multiple of 53 × 8 bits or 424 bits.

#)For example, 424 Mbps could handle 1 million cells per second.

2)AAL-1:

#)AAL-1 is used to provide transport for synchronous bit streams.

#)Its primary application is to adapt ATM cell transmission to typically E1/DS1 and SDH/SONET cir-cuits.

#)AAL-1 is specifically used for voice communications (POTS—plain old telephone service).

#)AAL-1 robs one octet from the payload and adds it to the header, leaving only a 47-octet payload.

#)This additional octet in the header contains two major fields: sequence number (SN) and sequence number protection (SNP).

#)The principal purpose of these two fields is to check that mis-sequencing of information does not occur by verifying a 3-bit sequence counter.

#)It also allows for the original clock timing of the data received at the far end of the link.

#)The SAR-PDU format of AAL-1 is shown

#)The 4-bit sequence number (SN) is broken down into a 1-bit CSI (convergence sublayer indicator) and a sequence count.

#)The SNP contains a 3-bit CRC and a parity bit. End-to-end synchronization is an important function for the type of traffic carried on AAL-1.

#)With one mode of operation, clock recovery is via a synchronous residual time stamp (SRTS) and common network clock by means of a 4-bit residual time stamp extracted from CSI from cells with odd sequence numbers.

#)The residual time stamp is transmitted over eight cells. It supports DS1, DS3, and E1 digital streams.

#)Another mode of operation is structured data transfer (SDT). SDT supports an octet-structured nXDS0 service.

3)AAL-2:

#)AAL-2 handles the variable bit rate (VBR) scenario such as MPEG (motion picture experts group) video.

#)It is still in the ITU-T organization definitive stages.

4)AAL-3/4:

#)Initially, in ITU-T, there were two separate AALs, one for connection-oriented variable bit rate data services (AAL-3) and one for connectionless service.

#)As the specifications evolved, the same procedures turned out to be necessary for both of these services, and the specifications were merged to become the AAL-3/4 standard. AAL-3/4 is used for ATM transport of SMDS, CBDS (connectionless broadband data services, an ETSI initiative), IP (Internet protocol) and frame relay.

#)AAL-3/4 has been designed to take variable-length frames/packets and segment them into cells.

#)The segmentation is done in a way that protects the transmitted data from corruption if cells are lost or mis-sequenced.

#)The cell format of an AAL-3/4 cell is shown below:

#)These types of cells have only a 44-octet payload, and additional overhead fields are added to the header and trailer.

#)These carry, for example, the BOM, COM, and EOM indicators (carried in segment type [ST]) as well as a MID (multiplexing identifier) so that the original message, as set up in the convergence sublayer PDU (CS PDU), can be delineated.

#)The header also includes a sequence number for protection against misordered delivery.

#)There is the MID (multiplexing identification) subfield which is used to identify the CPCS (common part convergence sublayer) connection on a single ATM layer connection.

#)This allows for more than one CPCS connection for a single ATM-layer connection.

#)The SAR sublayer, therefore, provides the means for the transfer of multiple, variable-length CS-PDUs concurrently over a single ATM layer connection between AAL entities.

#)The SAR PDU trailer contains a length indicator (LI) to identify how much of the cell payload is filled.

#)The CRC field is a 10-bit sequence used to detect errors across the whole SAR PDU.

#)A complete CS PDU message is broken down into one BOM cell, a number of COM cells and one EOM cell.

#)If an entire message can fit into one cell, it is called a single segment message (SSM), where the CS PDU is 44 or less octets long.

#)AAL-3/4 has several measures to ensure the integrity of the data which has been segmented and transmitted as cells.

#)The contents of the cell are protected by the CRC-10; sequence numbers protect against misordering.

#)Still another measure to ensure against corrupted PDUs being delivered is EOM/BOM protection.

#)If the EOM of one CPCS PDU and the BOM of the next are dropped for some reason, the resulting cell stream could be interpreted as a valid PDU.

#)To protect against these kinds of errors, the BEtag numeric values in the CPCS PDU headers and trailers are compared, to ensure that they match.

#)Two modes of service are defined for AAL-3/4:

1. Message Mode Service: This provides for the transport of one or more fixed-size AAL service data units in one or more CS-PDUs.

2. Streaming Mode Service:

#)Here the AAL service data unit is passed across the AAL interface in one or more AAL interface data units (IDUs).

#)The transfer of these AAL-IDUs across the AAL interface may occur separated in time, and this service provides the transport of variable-length AAL-SDUs.

#)The streaming mode service includes an abort service, by which the discarding of an AAL-SDU partially transferred across the AAL interface can be requested.

#)In other words, in the streaming mode, a single packet is passed to the AAL layer and transmitted in multiple CPCS- PDUs, when and as pieces of the packet are received.

#)Streaming mode may be used in intermediate switches or ATM-to-SMDS routers so they can begin retransmitting a packet being received before the entire packet has arrived.

#)This reduces the latency experienced by the entire packet.

5)AAL-5:

#)This type of AAL was designed specifically to carry data traffic typically found in today’s LANs.

#)AAL-5 evolved after AAL-3/4, which was found to be too complex and inefficient for LAN traffic.

#)Thus, AAL-5 got the name “SEAL” for simple and efficient AAL layer.

#)Only a small amount of overhead is added to the CPCS PDU.

#)There is no AAL level cell multiplexing. In AAL-5 all cells belonging to an AAL-5 CPCS PDU are sent sequentially.

#)To simplify still further, the CPCS PDUs are paddedto become integral multiples of 48 octets, ensuring that there never will be a need to send partially filled cells after segmentation.

User Plane:

#)The user plane provides for the transfer of user application information.

#)It contains physical layer, ATM layer, and multiple ATM adaptation layers required for different service users such as CBR and VBR service.

Control Plane:

#)The control plane protocols deal with call-establishment and call-release and other connection-control functions necessary for providing switched services.

#)The C-plane structure shares the physical and ATM layers with the U-plane.

#)It also includes ATM adaptation procedures and higher-layer signaling protocols.

Management plane:

#)The management plane provides management functions and the capability to exchange information between the U-plane and the C-plane.

#)The M-plane contains two sections: layer management and plant management.

#)The layer management performs layer-specific management functions, while the plane management performs management and coordination functions related to the complete system.