PRINCIPAL CHARACTERISTICS OF ATM
The principal characteristics of ATM are as follows:
The ATM standard defines a full suite of communication protocols, from an application-level API (Application Programmers' Interface) all the way down through the physical layer.
The ATM service models include Constant Bit Rate (CBR) service, Variable Bit Rate (VBR) service, Available Bit Rate (ABR) service and Unspecified Bit Rate (UBR) service.
Service model standards were developed by the ATM Forum in 1997. Constant Bit Rate (CBR) network service was the first ATM service model to be standardized, probably reflecting the fact that telephone companies were the early prime movers behind ATM, and CBR network service is ideally suited for carrying real-time, constant bit-rate audio (for example, a digitized telephone call) and video traffic. The goal of CBR service is conceptually simple: to make the network connection look like dedicated copper or fibre connection between the sender and the receiver. With CBR service, ATM packets (referred to as cells in ATM jargon) are carried across the network in such a way that the end-to-end delay experienced by a cell (the so-called cell-transfer delay, CTD), the variability in the end-end delay (often referred to as jitter or cell-delay variation, CDV) and the fraction of cell that are lost or delivered late (the so-called cell-loss rate, CLR) are guaranteed to be less than some specified values. Also, an allocated transmission rate (the peak cell rate, PCR) is defined for the connection and the sender is expected to offer data to the network at this rate. The values for the PCR, CTD, CDV and CLR are agreed upon by the sending host and the ATM network when the CBR connection is first established.
A second conceptually simple ATM service class is unspecified bit rate (UBR) network service. Unlike CBR service, which guarantees rate, delay, delay jitter and loss, UBR makes no guarantee at all other than in order delivery of cells (that is cells, that are fortunate enough to make it to the receiver). With the exception of in order delivery, UBR service is thus equivalent to the Internet best effort service model. As with the Internet best effort service model, UBR also provides no feedback to the sender on whether or not a cell is dropped within the network. For reliable transmission of data over a UBR network, higher-layer protocols are needed. UBR service might be well suited for non-interactive data transfer applications such as e-mail and newsgroups.
If UBR (Unspecified Bit Rate) can be thought of as a "best effort" service, then available bit rate (ABR) network service might best be characterized as a "better" best-effort service model. Two of the most important additional advantages ABR service has over UBR service are:
A minimum cell transmission rate (MCR) is guaranteed to a connection using ABR service. If, however, the network has enough free resources at a given time, a sender may actually be able to successfully send traffic at a higher rate than the MCR.
Congestion feedback from the network. ATM network can provide feedback to the sender (in terms of a congestion notification bit, or a lower rate at which to send) that controls how the sender should adjust its rate between the MCR and the peak cell rate (PCR). ABR senders control their transmission rates based on such feedback.
Available Bit Rate (ABR) provides a minimum bandwidth guarantee but, on the other hand, will attempt to transfer data as fast as possible (up to the limit imposed by PCR). As such, ABR is well suited for data transfer, where it is desirable to keep the transfer delays low (for example, in web browsing).
The final ATM service model is a variable bit rate (VBR) network service. VBR Service comes in two factors (perhaps indicating a service class with an identity crisis). In real-time VBR service, the acceptable cell-loss rate, delay and delay jitter are specified as in CBR service. However, the actual source rate is allowed to vary according to parameters specified by the user to the network. The declared variability in rate may be used by the network (internally) to more efficiently allocate resources to its connections, but in terms of the loss, delay and jitter seen by the sender, the service is essentially the same as CBR service. While early efforts in defining a VBR service model clearly targeted real time services (for example, as evidenced by the packet cell rate (PCR) and cell loss rate (CLR) parameters), a second flavour of VBR service targets non-real time services and provides a cell loss rate guarantee.
(3) ATM uses packet switching with fixed-length packets of 53 bytes. In ATM jargon, these packets are called cells. Each cell has 5 bytes of header and 48 bytes of "pay load". The fixed-length cells and simple header have facilitated high-speed switching.
(4)ATM uses virtual circuits. In ATM jargon, virtual circuits are called virtual channels. The ATM header includes a field for the virtual channel number, which is called the virtual channel identifier (VCI) in ATM jargon. Packet switches use the VCI to route cells towards their destination.
(5)ATM provides no retransmissions on a link-by-link basis. If a switch detects an error in an ATM cell header, it attempts to correct the error using error correcting codes. If it cannot correct the error, it drops the cell rather than request a retransmission from the preceding switch.
(6)ATM provides congestion control only within the ATM ABR service class.
(7)ATM can run over just about any physical layer. It often runs over fibre optics using the SONET standard at speeds of 155.52 Mbps, 622 Mbps and higher.
ATM protocol stack consists of three layers: The ATM physical layer, the ATM layer and the ATM adaptation layer (AAL). These layers can be compared with the three lower layers of open system integration (OSI). The lowest layer is the ATM physical layer, which is further subdivided into the sub-layers;transmission convergence (TC) layer and the physical medium dependent (PMD) layer. The responsibilities of the convergence layer include:
Regulate cell rate
• Establish the header error control (HEC) sequence and method
• Generate and recover frames (cells)
• Convert between bit stream rates to ATM cell rates • Generate HEC
• Cell rate decoupling
• Use the HEC-based cell delineation process to locate cells in a directly mapped time division multiplexed stream of data.
Cell rate decoupling deals with inserting idle or unassigned cells into the ATM stream when a cell is expected, but not yet present. This aids in maintaining a desired flow rate through the system. When a time division multiplexed slot is ready to be sent and there is no cell available, the transmission convergence function inserts an idle or unassigned cell to that slot. This cell is stripped at the received end, which also sends assigned cells to their proper destination.
The physical medium layer does not specify parameters directly, but relies on existing physical layer protocols. Its main responsibility is establishing the means of accessing the medium used and to set up any bit timing required to allow the physical medium to accept ATM cell transmission. OSI's data link layer is represented by ATM as the ATM cell layer whose duties include:
• Flow control Cell header generation and extraction Translation of virtual path identifier (VPI) and virtual channel identifier (VCI) addresses• Multiplexing and de-multiplexing of ATM cells• Validation of ATM header• Cell loss priority processing •Explicit forward congestion control.Cell loss priority and explicit forward congestion control are methods to reduce the traffic load when heavy traffic leads to an over load and causes congestion with each cells backing up and waiting to be transferred. A bit in the header identifies whatever a cell may be dropped from the transmission queue when congestion occurs. This bit assigns a lower priority to that cell and if the traffic on the system becomes congested. the cell is dropped and will have to be retransmitted at a later time. The layer work of the ATM protocol suite falls into the ATM Adaptation layer (AAL), which is similar in nature to the OSI network layer.
AAL has two sub-layers: the convergence layer and the segmentation and reassembly (SAR) sub-layer. The convergence layer is responsible for placing data into a common format and mapping cells onto the transmission medium. The segmentation and reassembly (SAR) layer is responsible for segmenting upper level protocol data units into ATM cells and reversing the process once the cells are delivered to the endpoint network. The convergence layer is also responsible for assuring necessary error control and the sequencing of cells. The Adaption layer is also responsible for timing and flow control and the detection and handling of out-ofsequence cells.
Reference model
Figure 8.6 illustrates the reference model. The reference model consists of three separate planes:
(1) User plane (2) Control plane (3) Management plane.
The user plane specifies the transport of data and related issues such as flow control and error detection and correction. The control plane performs call control and correction control functions. The management plane includes plane management, which includes management functions related to a system as a whole and providing coordination between all the planes, and layer management, which involves management functions relating to resources and parameters residing in its protocol entities.
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