ATM BASICS - Chapter 2 pot

9 Chapter 2 How Does ATM Work This chapter explains fundamental concepts that lay the basis for ATM technology. The reader is given the in-depth understanding of the terms such as ATM cells, statistical multiplexing, ATM switching and ATM layer processing. The layers of ATM reference model are discussed and explained. This in-depth view includes the Physical Layer, the ATM and the ATM Adaptation Layer. The basic understanding of these terms is recommended prior to reading through the following chapters. 2.1 ATM Protocol Reference Model As it was mentioned before, ATM can be viewed as a part of the B-ISDN con- cept. The development of B-ISDN protocols was facilitated by the definition of the B-ISDN Protocol Reference Model (PRM). The model was developed using the layered communication architecture based on the distinction between layer functions developed by the ISO (International Standards Organization). ATM plays a significant role in the B-ISDN PRM. The formal ATM PRM is a three-dimensional model but the relations between layers can be better viewed using one-dimensional layer model. It is important to understand that the layers in the ATM PRM (presented in the Fig.2-1) don’t have one-to-one mapping relationship with the seven lay- ers OSI protocol reference model. Some of the layers of ATM PRM provide the functionality of more than one OSI layer. For instance, the AAL (ATM Adaptation Layer) represent some of the features of OSI layer 4 (transport control), layer 5 (session control) and layer 7 (application control). Most of the ATM PRM layers can be further subdivided into a number of sublayers. 2.2 Physical Layer The physical layer (PHY) constitutes the lowest level of the ATM PRM. Its major task is to transmit ATM cells between ATM devices over the physical medium. ATM is designed to operate over potentially error free media. Therefore, successful transmission of ATM cells between ATM devices ATM Basics 10 Fig. 2-1, Simplified ATM Protocol Reference Model requires very low values of BER (BER = 10 -12 or better). There exist today a large variety of standards defining ATM physical interfaces. This situation is mainly caused by a number of underlying technologies that can be used by ATM. 2.2.1 Sub-layers A physical layer takes complete cells from the mid-layer and transmits them over the physical medium. The physical layer itself is subdivided into two sub-layers: •the Transmission Convergence (TC) sub-layer, •the Physical Medium Dependent (PMD) sub-layer. These two sub-layers work together to ensure that the physical interfaces receive and transmit cells efficiently, with the appropriate timing structure in place. Chapter 2 11 Fig. 2-2, Sublayers of the Physical Layer The Physical Medium Dependent is concerned with getting the bits on and off the wire. The PMD bit transmission includes bit transfer and bit align- ment. Technically, it covers bit timing, line coding, opto-electric conversion, modulation and demodulation functions necessary to transfer bits over a given medium. The physical connectors and signal characteristics differ from medium to medium. The Transmission Convergence sublayer is separated from details and char- acteristics of the physical medium being used. Due to the presence of the PMD sublayer the TC is specified independently of the underlying physical medium and operates over different media. In general, the purpose of the TC sublayer is to provide a uniform interface to the ATM layer in both direc- tions. The cells received from the ATM layer are encoded and pushed into the medium as a bit or a byte stream. The work of the TC sublayer can be characterized by the following functions: •Cell rate decoupling. This mechanism is used to insert idle cells in the transmit direction in order to compensate for the variable rate of the generation of ATM cells. At the receiving side all idle cells are identified and suppressed. •Header checksum generation and extraction. The TC sublayer can detect and if necessary correct errors affecting the contents of the ATM cell header. At the transmitting side the Header Error Check (HEC) field is gen- erated in hardware and inserted into the cell header. At the receiving side the HEC is recalculated and compared to the value that is extracted from the header of the received cell. The capabilities of the algorithm used to cal- culate the HEC allow for the detection and correction of single errors as well as detection of double errors. •Unpacking cells from the enclosing envelope. This function is also referred to as cell delineation. The receiver must be able to recover the cell boundaries. The TC sublayer must delineate the individual cells in the received bit stream, either directly from the TDM frame or with the help of the HEC field in ATM cells. This function can be complemented by the scrambling/descrambling operation. ATM Basics 12 •Frame generation. The cell flow must be adapted to the payload of the transmission system in the transmit direction. In the receive direction, the TC extracts the payload from the transmitted cell. The separation between the TC and the PMD sublayers is the key factor enabling flexibility in terms of variety of physical interfaces. Whereas the PMD sublayer is different for different carriers and cables, the TC just sends the cells as a string of bits to the PMD sublayer and converts the bit stream into a cell stream for the ATM layer. 2.2.2 Physical Interfaces ATM, while being an international transmission technology, has to be able to work with a variety of formats, speeds, transmission media and distances that may vary from operator to operator and from country to country. Single-mode fiber, multi-mode fiber, coaxial pairs of different categories, and shielded and unshielded twisted pairs are all standardized for the use in the ATM environment. ATM can be also run over Radio Frequency (RF)/satellite links. The ITU-T originally defined only two speeds, which should be supported by ATM: 155.52 Mbps and 622.08 Mbps. However, over time a number of additional speeds and interfaces have been defined, going as low as DS1/E1 and as high as the 2.5 and 10 Gbps. The standardization process of new physical interfaces is influenced by two factors. First of all, new interfaces are standardized following the development of transmission technologies (new types of o fibers and copper wires, e.g. UTP Cat. 6). However in recent years a few standards have been introduced, while direct- ly reflecting the market needs. Standards describing ATM over Fractional Links, Inverse Multiplexing for ATM (IMA), Frame-based ATM Transport over Ethernet (FATE) and Frame Based ATM over SONETS/SDH allow the operators to maximize the efficiency of their network infrastructure. Chapter 2 13 2.2 ATM Layer The ATM layer deals with moving cells from a source to a destination, which definitely implies the presence routing algorithms and protocols within the ATM switches. From the functional point of view, the ATM layer performs the work expected of the network layer in the OSI model. However, since ATM is quite frequently used to transport IP packets, the ATM layer is by many people characterized as a data link layer. This opinion is not precise due to the fact that the ATM layer has also some characteristics of a net- work layer: end-to-end virtual circuits, switching and routing. In result ATM is sometimes referred to as a layer 2 ½ solution. As it was mentioned earlier, the ATM layer is connection oriented, both in terms of the services it can provide and the way it is used by the operators. The basic concept present at the ATM layer is a virtual circuit (in official ATM terminology called a virtual connection). A virtual circuit should been seen as a connection from one source to one destination, although point-to- multipoint connections are also supported. It is important to note that vir- tual circuits are unidirectional but a pair of circuits is normally created at the same time. The same identifiers are used for both directions of the vir- tual circuit but the amount of reserved network resources can differ for both directions. The ATM layer is not guaranteed to be 100% reliable. The assumption has been made that it’s the task of the underlying physical layer to ensure the errorless transmission. Therefore, the ATM layer is unusual for a connec- tion-oriented protocol in that it operates without any acknowledgements. ATM Basics 14 2.2.1 ATM Cells The ATM cell is probably the most obvious term for anyone who has ever heard about ATM technology. The ATM cell, a very unique type of a packet, is comprised of a 5-byte header and 48-byte payload that typically contains user information. In total, ATM cells are 53-byte long (see in the Fig. 2-3). The final agreement on the cell size was influenced by the struggle between interests representing various interests. At the early stage of ATM stan- dardization process in the ITU-T two different values of the payload size were considered: 32-byte versus 64-byte value. The larger value was pro- Chapter 2 15 Fig. 2.3, The ATM cell posed by those who envisaged ATM as the technology satisfying data trans- mission needs in the first place. In large cells the relation between the pay- load and the total size is greater - the fixed size overhead represents the smaller percentage of transmitted data. Hence, the overall efficiency for data transmission is increased. The larger size of a cell, the greater amount of the delay is experienced by other sources generating data in the form of cells. This results in increased delays observed by real time applications such as voice and video transmission. Needless to say, 32-octet payload would be perfectly suited for the transmission of the E1 signal. The actual decision on the 48-bytes payload size was a compromise trying to minimize the disadvantages of two competitive proposals. It offered a tradeoff between the efficiency for data transmission and the delay requirements for data and video traffic. The size of ATM cells allowed operators to transmit voice over relatively long distances (round trips of 1000 km) whilst avoiding the need for expensive echo cancellers. The concept of fixed size cells is also present in work done in Australia under development of DQDB (Distributed Queue Dual Bus (DQDB), covered in IEEE 802.6. As the matter of fact, the smallest unit of traffic in DQDB is the slot, which is of 53 bytes length. 2.2.2 ATM Cell Header Fig. 2-4 shows the internal structure of an ATM cell header at UNI and NNI. In ATM the interface between the user equipment and the ATM switch is called User-to-Network Interface. All other interfaces, including those between ATM switches and between ATM networks as referred to as Node- to-Node Interfaces or Network-to-Node Interfaces. The key difference is the Generic Flow Control (GFC) field, which is not present in cell headers trans- mitted across the NNI. ATM Basics 16 GFC (Generic Fow Control) is the 4-bit field that is present only in cells transmitted between hosts and the network. Switches interfacing between the user’s equipment and ATM network overwrite GFC, and it is not deliv- ered to the destination. In early days of ATM it was intended to have some utility for flow and priority control between hosts and the networks, when multiple ATM devices were to be dropped on a single UNI. For any equip- ment using uncontrolled access, the GFC files shouldn’t be used and the bits must be always set to 0000 for the transmitted cells. VPI (Virtual Path Identifier) has 8 bits available at the UNI and 12 bits at the NNI, which gives either 256 or 4096 simultaneous virtual paths at maximum per a physical connection. Chapter 2 17 Fig. 2-4, The ATM cell header format VCI (Virtual Channel Identifier) is the 16-bit field that selects a partic- ular virtual channel within a given virtual path. It allows for up to 65,536 virtual channels per a virtual path. A number of combinations of VPI and VCI values are reserved for control functions, such as setting up and clear- ing virtual connections. PTI (Payload Type Identifier) is the 3-bit field, which is as part of cell control concept. The PTI defines the information carried in the cell payload. User data and network management cells are differentiated due to the value of the MSB bit (sometimes referred to as RM-celler). The second bit can be used to indicate congestion affecting data traffic that can occur in the net- work nodes. This bit is often called Explicit Forward Congestion Indicator (EFCI). The LSB bit is used to indicate the final cell in the cell stream, which has been filled with higher layer packet traffic. This bit is referred to as the Service DataUnit (SDU) bit and its application is mostly related to the presence of a certain adaptation layer type. ATM Basics 18 Table 2-1, PTI field values [...]... the rate of the underlying transmission technology For instance, an ATM source transmitting ATM cells over SONET would normally put out an OAM cell as every 27 cells In result, the data rate would slow the data rate down to 26 /27 of 150,336 (OC-3) and thus match SONET completely 20 Chapter 2 2 .2. 5 Cell Reception At the receiving end an ATM device has to take incoming bits first, locate the cell boundaries,... specific link connecting two ATM sites The values may change when routing through a switch In fact switching operations performed in an ATM switch are restricted to the cross-connection of a VPI/VCI combination on the input port to a VPI/VCI combination on the output port 25 ATM Basics Fig 2- 8 , ATM Switching Operation For any ATM cell that arrives to an ATM device, the ATM layer is first checked with... higher-layer stream of data Once so labeled, blocks of data are then transferred to the ATM cell payloads Part of this process is to add yet another header The principal use of this header is to identify the start of a flow if a payload of an AAL2 packet is split between two consecutive ATM cells The interaction between different layers in AAL2 is presented in Fig 2- 1 3 37 ATM Basics Fig 2- 1 3, AAL 2 model... format of the ATM layer would affect the TC sublayer 2. 2.6 Virtual Channels and Virtual Paths As it was stated earlier, the ATM layer provides connectivity by means of virtual circuits Given that within the ATM layer header there are two fields that carry identifiers, it can be easily noticed that a two-level connection hierarchy can be supported 22 Chapter 2 The physical link carrying ATM cells can... gain of statistical multiplexing in ATM is severely decreased at the physical links carrying AAL 1 traffic 36 Chapter 2 2.3 .2 AAL Type 2 AAL2 defines the transport of the traffic of variable requirements for throughput that is timing-sensitive, such as audio and video AAL2 is relatively new (standards released in the end of 90’s) As opposed to other AAL types, AAL 2 is inherently designed for the support... progress of any cells behind it, even if they could otherwise be switched This can lead to the situation, when a low priority cell may potentially block the high priority cell This effect is referred to as the head-of-line blocking Fig 2- 1 0, Head-of-line blocking 29 ATM Basics The problem can be overcome provided the use of the design that does the queuing on the output side In this model, when two cells... be set to 64 octets If the latter value is chosen, the payload will overlap between two ATM cells 38 Chapter 2 •The 5-bit UUI (User-to-User Indication), which can be used to establish communication between AAL2 instances on both sides of AAL 2 connection Given that the filed is 5-bit long, this allows for up to 32 different messages at maximum One current use for the UUI field is to negotiate a larger... highly probable that occasionally the AAL 2 packet payload is split between two adjacent ATM cells Therefore, it is needed to effectively point out the beginning of the next AAL 2 packet This can be done with the help of the secondary AAL 2 header Fig 2- 1 4, AAL 2 headers structure 39 ATM Basics The 1-byte secondary AAL 2 header includes Start Field, which identifies the location of the start of the next... services which are not available due to the nature of the ATM layer Theses services may cover error control, integrity and sequence check, reliable transmission, and connectionless transmission 32 Chapter 2 Fig 2- 1 1, AAL architecture The AAL functions are executed at the edges of an ATM connection, and not within the network Hence, AAL operates on end-to-end basis The layer itself is further divided into... 47-byte blocks that are then passed to the SAR sublayer At the receiving end the CS extracts the locks and reconstructs the original stream The CS does not introduce any header and trailer The SAR sublayer does not need to perform a segmentation or reassembly function since AAL 1 payload can be fitted to one cell directly The format of the SAR SDU is given in Fig 2- 1 2 Fig 2- 1 2, AAL 1 model 35 ATM Basics . considered: 3 2- byte versus 64-byte value. The larger value was pro- Chapter 2 15 Fig. 2. 3, The ATM cell posed by those who envisaged ATM as the technology satisfying data trans- mission needs. connec- tion-oriented protocol in that it operates without any acknowledgements. ATM Basics 14 2. 2.1 ATM Cells The ATM cell is probably the most obvious term for anyone who has ever heard about ATM. ATM is designed to operate over potentially error free media. Therefore, successful transmission of ATM cells between ATM devices ATM Basics 10 Fig. 2- 1 , Simplified ATM Protocol Reference Model