ATM Protocol Stacks

Reference Model of ATM Stacks

Terminal devices can be manufactured by different vendors. To guarantee the intercommunication of these devices, the I.321 standard of ITU-T defines the reference model of the B-ISDN protocol. Any two systems can communicate with each other on the condition that they accord with this reference model or the related standards.

The reference model of the B-ISDN protocol has three planes and three layers. The planes are the user plane, control plane, and management plane. The layers are the physical layer, ATM layer, and ATM adaptation layer (AAL).

Model of ATM Stacks

The three planes function differently.

The user plane: It uses the hierarchical structure to transmit users' information and to control flow and errors.

The control plane: It uses the hierarchical structure to control calls and connections. This plane uses signaling to call and to set up or monitor or release connections. The control and the user planes differ only at the upper and the AAL layers; however, the ATM and the physical layers do not distinguish these two planes. So the ATM and the physical layers process the user plane the same as the control plane.

The management plane: It contains the layer-management and the plane-management. The layer-management uses the hierarchical structure to manage the entities and parameters at the protocol layer. Besides, the layer-management processes the OAM information that is related to each layer. Nevertheless, the plane-management does not have layers; it manages the whole system and harmonizes all the planes.

Functions of the ATM Layers

The ATM Layers

Each layer of the ATM cell functions as follows:

The physical layer provides transmission path for ATM cells. The ATM cells and their transmission overhead form a continuous bit stream at the physical layer. Meanwhile, the physical layer receives the continuous bit stream from the physical media, and picks out the valid cells to send to the ATM layer. From top to bottom, the physical layer is classified into the transmission convergence sublayer (TC) and the physical medium sublayer (PM).

The PM sublayer defines the physical medium interface, such as the 155M/622M based on the SDH, and so on.

The TC sublayer performs the following five functions: decouple the cell rate, form/check the HEC sequence, delimitate cells, adapt/form/restore transmission frames. Forming/restoring/adapting transmission frames are related to the transmission systems with frame structure, including the SDH/SONET or the PDH systems. When the ATM cells are transmitted in such systems, the cells must be encapsulated in the transmission frame.

The cell rate decoupling indicates to insert some idle cells into the ATM cells in order to adapt the cell rate at the ATM layer to the transmission line rate.

The ITU-T recommends delimitating the ATM cells in the HEC manner. That is, calculate every 32 bits by means of the CRC, and if the result equals to the following eight bits, then a cell header has been found out.

The header error control (HEC) is also carried out in the HEC manner. The HEC can correct one-bit error, detect multi-bit errors, and discard the cells with error header.

The ATM layer locates above the physical layer, and uses the service provided by the physical layer to communicate with the peer layer with the cell as the information unit. Meanwhile, it provides service for the AAL layer. The ATM layer is irrelevant to the type of physical medium and of the service transmitted by the physical layer. The ATM layer only identifies and processes the cell header.

The ATM layer has three functions: cell multiplex/demultiplex, header operations and generic flow control.

Multiplexing/demultiplexing cell is finished in the interface of the ATM layer and the TC sublayer. The ATM layer at the transmitting end multiplexes the cells of different VPI/VCI, and sends them to the physical layer. The ATM layer at the receiving end identifies the VPI/VCI of cells from the physical layer, and sends the cells to the different modules for processing. If the ATM layer identifies the signaling cell, it will send the cell to the control plane for processing; if the ATM layer identifies the management cell, such as the OAM cell, it will send the cell to the management plane for processing.

The ATM layer fills in the VPI/VCI and PT values at the user terminal, and interprets the VPI/VCI value at the network node. The VPI/VCI value of users' information can be set by the caller during setting up the connection. the network node is informed of this value through the SETUP message of the signaling, and then the network node recognizes the value, and the network devices assign the value finally.

The GFC bits in the cell header controls flow.

The ATM adaptation layer (AAL) locates above the ATM layer and is related to services. That is, the AAL layer adapts different services by different means. However, this layer has to segment the information from the upper layer into the 48-byte ATM data units; meanwhile, the AAL layer assembles and restores the ATM data units from the ATM layer, and transmits them to the upper layer. Information types are diversified at the upper layer. Therefore, the processing at the AAL layer is complicated, and the AAL layer is divided into two sublayers to process data packets, that is, the convergence sublayer (CS) and the segmentation and reassembly sub-layer (SAR).

AAL Types and B-ISDN Service

To improve the rate of the switching network, the ATM layer is simplified as much as possible. Nevertheless, the ATM layer cannot complete some functions that are closely related to the QoS, such as the cell loss, transmission error, delay, delay variation, and so on. All the mentioned functions are performed by the AAL layer. Different types of service are adapted differently. The ITU-T studies all services, and classifies the services into four types based on the source and destination timing, bit rate and connection mode. The four types are AAL1, AAL2, AAL3/4 and AAL5.

AAL Types and B-ISDN Service

The AAL1 protocol handles the constant-rate and connection-oriented service, it needs to transmit the timed messaged between the source and the destination. At present, the circuit switching service is a typical example of this kind of service, such as the voice service, the NISDN service, and so on.

The AAL2 is developed for the variable bit rate (VBR) service that is timed from end to end, such as the VBR stereo sets and televisions. This type of AAL has not been maturely defined yet. The ITU-T may reinforce the AAL1 function to realize the AAL2 type.

Because of using the X.25 or the frame relay technologies, the remote connection of the current LAN suffers the bottleneck to different extent. Therefore, the ATM technology is crucial to realize the remote connection of the LAN. That is an important way to use the ATM technology in its early phase. The data services can be classified into two kinds on the ATM network. That is, the connectionless data service for LAN connection and the connection-oriented data service. The AAL3/4 protocol is used to adapt these services.

The AAL5 supports the VBR service that requires no time synchronization between the receiving and transmitting ends. It also provides the service that is similar to that provided by the AAL3/4. The AAL5 is used to transmit the data from computers, UNI signaling information, and the frame relay on the ATM network. The AAL5 type is defined to reduce overhead. And the AAL5 service must be simple and valid.

Flow for Processing the ATM Cells

Flow Processing ATM Cells

The ATM cells are processed as follows: The voice, video, data, image from the upper layer are first sent to the ATM adaptation layer (AAL) and then adapted by using the AAL1 or AAL5 type. That is,   encapsulate the data from the upper layer in the AAL frame format first, and then segment the data into 48-byte ATM data units. These data units are sent to the ATM layer and get the 5-byte cell headers there. The VPI/VCI whose value is assigned during its connecting is identified in the cell header. The ATM layer multiplexes the cells with different VPI/VCI together and sends them to the physical layer. At the physical layer, the ATM cells are encapsulated into the transmission frame and then sent out through the physical interface.

Data Service on the ATM Network

The ATM network can bear the traditional data services at the data link layer or the network layer. These services must be properly encapsulated before being input into the ATM network. In this case, the services can be correctly identified and processed at the peer end. The RFC1483 protocol defines the encapsulation manners.

Data Service on the ATM Network

The TCP/IP network bears services that are accessed from the data link layer. Such an access is called bridging, that is, the RFC1483B access. The RFC1483B access indicates to use the RFC1483 protocol to encapsulate Ethernet frames at the data link layer and send the frames to the AAL layer for processing.

If ATM is used to directly bear the data packet at the network layer, the packets will be encapsulated by the RFC1483R protocol. This is what we called the IP over ATM (IPoA).

ATM Service Types

The ATM forum distinguishes the constant bit rate service (CBR) from the variable bit rate service (VBR) from the aspect of the flow control; furthermore, it divides the VBR service into the real time VBR (rt-VBR) and non-real time VBR (nrt-VBR) services. Besides, due to the increasing demand of data service, the network provides data service in the Best Effort manner, and further defines the available bit rate service (ABR) and the unspecified bit rate service (UBR). The ABR service guarantees a certain loss ratio, but the UBR service guarantees nothing.

ATM Service Types

We can tell differences of the above five service types from the following aspects:

The five types of service differ in traffic parameters that can be called source traffic parameters and define the flow properties of services. These parameters include the peak cell rate (PCR), sustainable cell rate (SCR), minimum cell rate (MCR), and maximum burst size (MBS).

The five types of service differ in the QoS at the ATM layer. We have some parameters to describe the QoS, they are the peak-to-peak cell delay variation (peak-to-peak CDV), the maximum cell transfer delay (max CTD), the cell loss ratio (CLR), the cell error ratio, the severely error cell block ratio (SECBR), and the cell mis-insertion rate (CMR).

Traffic Parameters

The traffic parameters contain the peak cell rate (PCR), sustainable cell rate (SCR), minimum cell rate (MCR) and maximum burst size (MBS). These parameters are also called the source traffic parameters since they describe the features of the service flow.

Traffic Parameters

PCR, SCR and MCR represent the maximum, average and minimum cell rate respectively.

Burst size (BS) is ratio of the service peak bit rate to the average bit rate. Hence, the bigger the burst size is, the more greatly the service speed changes.

CBR Service

The CBR service usually refers to the connection that needs continuous and static bandwidth in all its lifecycle. Quantity of the allocated bandwidth is quantized by the peak cell rate (PCR). Network can promise to reserve resources for the CBR service on the premise that users set up connections. In this case, the network can guarantee the QOS performance of the cells that comply with the consistency test. In the CBR service, the transmitting end can send cells at the PCR rate at any time, and last for any period of time.

CBR Service

The CBR is usually applied to the real time service that requires limitation on the delay variation strictly, for example, voice, video, and circuit emulation services. In the CBR service, the transmitting end can send cells in a certain period at the negotiated PCR rate or at the rate lower than the PCR rate, otherwiser the end will not send any cell at all.

VBR Service

The rt-VBR is applied to the real time service. It requires limitation on the delay and delay variation strictly. The rt-VBR is mainly applied to voice and video services. This kind of service can be described by the peak cell rate (PCR), sustainable cell rate (SCR), maximum burst size (MBS), and cell delay variation tolerance (CDVT). The transmitting end sends cells at a variable rate, so it is burst. The rt-VBR service can statistically multiplex the real time resource.

VBR Service

The nrt-VBR is applied to the burst non-real time service. The PCR, SCR and MBS can describe the nrt-VBR connection. As for the cells that follow the flow agreement, the nrt-VBR service can guarantee very low cell loss ratio, but it sets no limit on the delay. The nrt-VBR service can statistically multiplex the connections.

UBR Service

The unspecified bit rate (UBR) is applied to the non-real time service. That is, the UBR service does not require much of the delay and the delay variation, such as the traditional application of computer in communication as file transmission and email.

UBR Service

The UBR service does not guarantee the QoS, and the limit of the cell loss ratio and the cell transmission delay. The network can decide whether it needs to apply the PCR in the CAC and UPC. If the network does not require the PCR, the PCR value is meaningless. The congestion of the UBR connection can controlled at the upper layer or on the end-to-end basis.

ABR Service

In the ABR service, the transmission properties of the network that are set up during connection can be changed later. A certain flow control mechanism can feed back the rate of the transmitting end to control the rate for sending cells by this end. This feedback is realized by the resource management (RM) cell, a specified control cell. It can be expected that the cell loss ratio will be very low when the terminal system adjusts the flow based on the feedback, and later the system will get an impartial and available bandwidth. For a given connection, the ABR service sets no limit on the delay and delay variation. That is, the ABR does not provide the real time service.

ABR Service

When the connection is set up for the ABR service, the terminal system will specify the maximum bandwidth and the available minimum bandwidth that it needs. The maximum and minimum bandwidths are described by the peak cell rate (PCR) and minimum cell rate (MCR). The MCR value can equal to zero, and the bandwidth provided by network can change but cannot be less than MCR.

The nrt-VBR, ABR and UBR serve for the non-real time service. They differ mainly in the QoS guaranteed by the network and the realization methods of the network and terminal system. The application decides the service type.