CS 635, Lecture 7, Part 1

Asynchronous Transfer Mode

Basic Principles

• Future apps require higher bandwidth and generate heterogeneous mix of network traffic.
• Existing networks cannot provide the transport facilities to efficiently support a diversity of traffic with various service requirements
• ATM is potentially capable of support all classes of traffic (e.g, voice, video, data) in one transmission and switching fabric technology
• Based on the asynchronous mode technology used in broadband ISDN and other WANs
• ATM is also known as cell relay. Similar to packet switching using X.25 and to frame relay.
• Transfers data in discrete chunks
• ATM allows multiple logical connections to be multiplexed over a single physical interface
• Information is organized into fixed-size packets called cells, each comprising 5 bytes of header information and a 48-byte information field (payload). The reason for choosing a fixed-size packet is to ensure that the switching and multiplexing function could be carried out quickly and easily.
• ATM is connection-oriented technology in the sense that before two systems on the network can communicate, they should inform all intermediate switches about their service requirements and traffic parameters. This is similar to the telephone networks where a fixed path is set up from the calling party to the receiving party.
• In ATM networks, each connection is called a virtual circuit or virtual channel (VC), because it also allows the capacity of each link to be shred by the connections using that link on a demand basis rather than by fixed allocations.
• The connections allow the network to guarantee the quality of service (QoS) by limiting the number of VCs. Typically, a user declares key service requirements at the time of connection setup, declares the traffic parameters and may agree to control these parameters dynamically as demanded by the network.

ATM Protocol Reference Model

• The ATM protocol reference model is based on standards developed by the ITU. Communication from higher layers is adapted to the lower ATM defined layers, which in turn pass the information onto the physical layer for transmission over a selected physical medium. (see figure 10.1 page 318).
• The protocol reference model is divided into three layers. the ATM adaptation layer (AAL), the ATM layer, and the physical layer.

The ATM Adaptation Layer

• The ATM Adaptation Layer (AAL) interfaces the higher layer protocols to the ATM layer. It relays ATM cells both from the upper layers to the TM layer and vice versa.
• When relaying information received from the ATM layer to the higher layers, the AAL must take the cells and reassemble the payloads into a from the higher layers can understand. This is called Segmentation and Reassembly (SAR).
• Four types of AALs were proposed, each supporting a different type of traffic or service expected to be used on ATM networks. The service classes and the corresponding types of AALs were as follows:
• Class A - Constant Bit Rate (CBR) service: AAL1 supports a connection oriented service in which the bit rate is constant. Examples of this service include 64 Kbit/sec voice, fixed-rate uncompressed video and leased lines for private data networks
• Class B - Variable Bit Rate (VBR) service: AAL2 supports a connection-oriented service in which the bit rate is variable but requires a bounded delay for delivery. Examples of this service include compressed packetized voice or video. The requirement on bounded delay for delivery is necessary for the receiver to reconstruct the original uncompressed voice or video. AAL2 has not been fully developed yet.
• Class C- Connection-oriented data service: Examples of this service include connection-oriented file transfer and in general, data network applications where a connection is set up before data is transferred. This service has variable bit rate and does not require bounded delay for delivery. The ITU originally recommended two types of AAL protocols to support this service class, but these two types have been merged into a single type, called AAL3/4.
• Class D - Connectionless data service: Examples of this service include datagram traffic and in general, data network applications where no connection is set up before data is transferred. Either AAL3/4 or AAL5 can be used to support this class of service.
• Although each AAL is optimized for a specific type of traffic, there is no stipulation in the standards that AALs designed for on class of traffic cannot be  used for another. In fact, many vendors of ATM equipment currently manufacture products that use AAL 5 to support all the above classes of traffic.

The ATM Layer

• The ATM layer provides an interface between the AAL and the physical layer. This layer is responsible for relaying cells from the AAL to the physical layer for transmission and from the physical layer to the AAL for use at the end systems.
• When it is inside an end system, the ATM layer receives a stream of cells from the physical layer and transmits either cells with new data or empty cells if there is no data to send.
• When it is inside a switch, the ATM layer determines where the incoming cells should be forwarded to, resets the corresponding connection identifiers and forwards the cells to the next link. Moreover, it buffers incoming and outgoing cells, and handles various traffic management functions such as cell loss priority marking, congestion indication, and generic flow control access. It also monitors the transmission rate and conformance to the service contract (traffic policing).
• The fields in the ATM header define the functionality of the ATM layer. The format of the header for ATM cells has two different forms, one for use at the user-to-network interface (UNI) and the other for use internal to the network, the network-to-node interface (NNI) (Figure 10.4 page 325).
• At the UNI, the header dedicates four bits to a function called generic flow control (CFG), which was originally designed to control the amount of traffic entering the network. This allows UNI to limit the amount of data entering the network during periods of congestion.
• At the NNI, these four bits are allocated to the virtual path identifier (VPI). (Picture)
• The VPI and the virtual channel identifier (VCI) together form the routing field, which associates each cell with a particular channel or circuit.
• The VCI is a single-channel identifier;
• The VPI allows grouping of VCs with different VCIs and allows the group to be switched together as an entity. However, the VPIs and VCIs have significance only on the local link; the contents of the routing field will generally change as the cell traverses from link to link.
• For the UNI, the routing filed contains 24 bits and thus the interface can support over 16 million sessions.
• At the NNI, the field contains 28 bits, allowing for over 268 million sessions to share a link within a subnet.
• The payload type indicator (PTI) field is used to distinguish between cells carrying user data and cells containing control information. This allows control and signaling data to be transmitted on a different subchannel from user data and hence separation of user and control data. A particular combination is used to indicate that the cell has experienced congestion.
• The cell loss priority (CLP) bit provides the network with a selective discard capability. This bit could be set by a user to indicate lower-priority cell that could be discarded by the network during periods of congestion. For example, whereas data applications generally cannot suffer any cell loss without the need for retransmission, voice and video traffic usually can tolerate minor cell loss. One would therefore assign a higher cell loss priority to the CLP but for voice or video traffic than data traffic. The CLP bit could also be used by the network to indicate cells that exceed the negotiated rate limit of a user.
• The header error check (HEC) field is used to reduce errors in the header that cause a misrouting of the cell for one user into another user's data stream. This field contains the result of an 8-bit CRC checking on the ATM header (but not on the data). When a switch or an end system terminates the header, multiple-bit errors will be detected with a high probability.
• A single bit error can be corrected. This is a desirable since ATM is inteded for use on fiber optios link, where the error rate is less than 10^(-9) with current modulation techniques. Therefore, single-bit error correction is quite effective in removing most header errors.

The Physical Layer

• The physical layer defines the bit timing and other characteristics for encoding and decoding the data into suitable electrical/optical waveforms for transmission and rec eption on the specific physcial media used.
• It also provides cell delineation function, header error check (HEC) generation and processing, performance monitoring, and payload rate matching of the different transport formats used at this layer.
• The Synchronous Optical Network (SONET), a synchronous transmission structure, is often used for framing and synchronization at the physical layer. In addition to the optical media and line rates defined for SOENT, the ATM Forum has proposed a variety of physical layer standards, such as ATM over twisted-pair wire.

Top Next