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- Wiley Encyclopedia of Computer Science and Engineering - 1st Edition (5 Volume Set) (2009).pdf
- Wiley Encyclopedia of Electrical and Electronics Engineering Reconfigurable Computing
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Wiley Encyclopedia of Computer Science and Engineering - 1st Edition (5 Volume Set) (2009).pdf
Since its birth in the mids, ATM has been fortified by a number of robust standards and realized by a significant number of network equipment manufacturers. International standards-making bodies such as the ITU and independent consortia like the ATM Forum have developed a significant body of standards and implementation agreements for ATM 1,4.
As networks and network services continue to evolve toward greater speeds and diversities, ATM will undoubtedly continue to proliferate. Asynchronous transfer mode, or ATM, is a network transfer technique capable of supporting a wide variety of multimedia applications with diverse service and performance requirements.
It supports traffic bandwidths ranging from a few kilobits per second e. ATM is a form of packet-switching technology. That is, ATM networks transmit their information in small, fixedlength packets called cells, each of which contains 48 octets or bytes of data and 5 octets of header information. The small, fixed cell size was chosen to facilitate the rapid processing of packets in hardware and to minimize the amount of time required to fill a single packet.
This is particularly important for real-time applications such as voice and video that require short packetization delays. ATM is also connection-oriented. In other words, a virtual circuit must be established before a call can take place, where a call is defined as the transfer of information between two or more endpoints.
The establishment of a virtual circuit entails the initiation of a signaling process, during which a route is selected according to the calls quality of service requirements, connection identifiers at each switch on the route are established, and network resources such as bandwidth and buffer space may be reserved for the connection.
Another important characteristic of ATM is that its network functions are typically implemented in hardware. With the introduction of high-speed fiber optic transmission lines, the communication bottleneck has shifted from the communication links to the processing at switching nodes and at terminal equipment. Hardware implementation is necessary to overcome this bottleneck because it minimizes the cell-processing overhead, thereby allowing the network to match link rates on the order of gigabits per second.
Finally, as its name indicates, ATM is asynchronous. Time is slotted into cell-sized intervals, and slots are assigned to calls in an asynchronous, demand-based manner. Because slots are allocated to calls on demand, ATM can easily accommodate traffic whose bit rate fluctuates over time.
Moreover, in ATM, no bandwidth is consumed unless information is actually transmitted. ATM also gains bandwidth efficiency by being able to multiplex bursty traffic sources statistically. Because bursty traffic does not require continuous allocation of the bandwidth at its. Other standards bodies and consortia e. This section presents an overview of the standards, with particular emphasis on the protocol reference model used by ATM 2.
The purpose of the protocol reference model is to clarify the functions that ATM networks perform by grouping them into a set of interrelated, function-specific layers and planes. The reference model consists of a user plane, a control plane, and a management plane. Within the user and control planes is a hierarchical set of layers. The user plane defines a set of functions for the transfer of user information between communication endpoints; the control plane defines control functions such as call establishment, call maintenance, and call release; and the management plane defines the operations necessary to control information flow between planes and layers and to maintain accurate and fault-tolerant network operation.
Figure 2 summarizes the functions of each layer 1. The physical layer performs primarily bitlevel functions, the ATM layer is primarily responsible for the switching of ATM cells, and the ATM adaptation layer is responsible for the conversion of higher-layer protocol frames into ATM cells. The functions that the physical, ATM, and adaptation layers perform are described in more detail next.
Physical Layer The physical layer is divided into two sublayers: the physical medium sublayer and the transmission convergence sublayer 1. Cell delineation is the detection of cell boundaries. Transmission frame adaptation is the encapsulation of departing cells into an appropriate framing structure either cellbased or SDH-based. ATM Layer. Physical Medium Sublayer. The physical medium PM sublayer performs medium-dependent functions. The PM sublayer is also responsible for bit timing i.
The PM sublayer currently supports two types of interface: optical and electrical. Transmission Convergence Sublayer. Above the physical medium sublayer is the transmission convergence TC sublayer, which is primarily responsible for the framing of data transported over the physical medium.
In the cell-based case, cells are transported continuously without any regular frame structure. Regardless of which transmission frame structure is used, the TC sublayer is responsible for the following four functions: cell rate decoupling, header error control, cell delineation, and transmission frame adaptation. Cell rate decoupling is the insertion of idle cells at the sending side to adapt the ATM cell streams rate to the rate of the transmission path.
The ATM layer lies atop the physical layer and specifies the functions required for the switching and flow control of ATM cells 1. Although a octet cell payload is used at both interfaces, the 5-octet cell header differs slightly at these interfaces. A virtual channel connects two ATM communication endpoints. A virtual path connects two ATM devices, which can be switches or endpoints, and several virtual channels may be multiplexed onto the same virtual path. The 2-bit PT field identifies whether the cell payload contains data or control information.
The CLP bit is used by the user for explicit indication of cell loss priority. If the value of the CLP is 1, then the cell is subject to discarding in case of congestion. The GFC field, which appears only at the UNI, is used to assist the customer premises network in controlling the traffic flow.
At the time of writing, the exact procedures for use of this field have not been agreed upon. The table used to perform this translation is initialized during the establishment of the call. Because VPI and VCI values do not represent a unique endto-end virtual connection, they can be reused at different switches through the network. This is important because the VPI and VCI fields are limited in length and would be quickly exhausted if they were used simply as destination addresses.
Switched virtual connections are established and torn down dynamically by an ATM signaling procedure. That is, they exist only for the duration of a single call. Permanent virtual connections, on the other hand, are established by network administrators and continue to exist as long as the administrator leaves them up, even if they are not used to transmit data. Other important functions of the ATM layer include cell multiplexing and demultiplexing, cell header creation and extraction, and generic flow control.
Cell multiplexing is the merging of cells from several calls onto a single transmission path, cell header creation is the attachment of a 5octet cell header to each octet block of user payload, and generic flow control is used at the UNI to prevent shortterm overload conditions from occurring within the network.
Figure 4 lists the ATM service categories, their quality of service QoS parameters, and the traffic descriptors required by the service category during call establishment 1,4.
The constant bit rate or deterministic bit rate service category provides a very strict QoS guarantee. It is targeted at real-time applications, such as voice and raw video, which mandate severe restrictions on delay, delay variance jitter , and cell loss rate. The only traffic descriptors required by the CBR service are the peak cell rate and the cell delay variation tolerance.
A fixed amount of bandwidth, determined primarily by the calls peak cell rate, is reserved for each CBR connection. The real-time variable bit rate or statistical bit rate service category is intended for real-time bursty applications e. The VBR-rt service requires the specification of the sustained or average cell rate and burst tolerance i.
The available bit rate service category is defined to exploit the networks unused bandwidth. It is intended for non-real-time data applications in which the source is amenable to enforced adjustment of its transmission rate.
A minimum cell rate is reserved for the ABR connection and therefore guaranteed by the network. When the network has unused bandwidth, ABR sources are allowed to increase their cell rates up to an allowed cell rate ACR , a value that is periodically updated by the ABR flow control mechanism to be described in the section entitled ATM Traffic Control.
The value of ACR always falls between the minimum and the peak cell rate for the connection and is determined by the network. The ITU-T also defines an additional service category for nonreal-time data applications. The ATM block transfer service category is intended for the transmission of short bursts, or blocks, of data. Before transmitting a block, the source requests a reservation of bandwidth from the network. If bandwidth is not available for transporting the block, then it is simply discarded, and the source must retransmit it.
In both cases, the network temporarily reserves bandwidth according to the peak cell rate for each block. Immediately after transporting the block, the network releases the reserved bandwidth. AAL service class A corresponds to constant bit rate services with a timing relation required between source and destination. The connection mode is connectionoriented. The CBR audio and video belong to this class. Class B corresponds to variable bit rate VBR services. This class also requires timing between source and destination, and its mode is connection-oriented.
The VBR audio and video are examples of class B services. Class C also corresponds to VBR connection-oriented services, but the timing between source and destination needs not be related. Class C includes connection-oriented data transfer such as X. Class D corresponds to connectionless services. AAL Type 1 supports constant bit rate services class A , and AAL Type 2 supports variable bit rate services with a timing relation between source and destination class B.
The adaptation layer is divided into two sublayers: the convergence sublayer CS , which performs error detection and handling, timing, and clock recovery; and the segmentation and reassembly SAR sublayer, which performs segmentation of convergence sublayer protocol data units PDUs into ATM cellsized SAR sublayer service data units SDUs and vice versa. Figure 5 depicts the four service classes defined in recommendation I. Note that even though these AAL service classes are similar in many ways to the ATM layer service categories defined in the previous section, they are not the same; each exists at a different layer of the protocol reference model, and each requires a different set of functions.
AAL Type 5. Currently, the most widely used adaptation layer is AAL Type 5. AAL Type 5 supports connectionoriented and connectionless services in which there is no timing relation between source and destination classes C and D. Its functionality was intentionally made simple in order to support high-speed data transfer.
AAL Type 5 assumes that the layers above the ATM adaptation layer can perform error recovery, retransmission, and sequence numbering when required, and thus, it does not provide these functions. Therefore, only nonassured operation is provided; lost or corrupted AAL Type 5 packets will not be corrected by retransmission. An AAL specific flag end-of-frame in the. It contains the user data payload, along with any necessary padding bits PAD and a CS-PDU trailer, which are added by the CS sublayer when it receives the user information from the higher layer.
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Wiley Encyclopedia of Computer Science and Engineering
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Since its birth in the mids, ATM has been fortified by a number of robust standards and realized by a significant number of network equipment manufacturers. International standards-making bodies such as the ITU and independent consortia like the ATM Forum have developed a significant body of standards and implementation agreements for ATM 1,4. As networks and network services continue to evolve toward greater speeds and diversities, ATM will undoubtedly continue to proliferate. Asynchronous transfer mode, or ATM, is a network transfer technique capable of supporting a wide variety of multimedia applications with diverse service and performance requirements. It supports traffic bandwidths ranging from a few kilobits per second e. ATM is a form of packet-switching technology. That is, ATM networks transmit their information in small, fixedlength packets called cells, each of which contains 48 octets or bytes of data and 5 octets of header information.