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Asynchronous Transfer Mode

In electronic digital data transmission systems, the network protocol Asynchronous Transfer Mode (ATM) encodes data traffic into small fixed-sized cells. The standards for ATM were first developed in the mid 1980s. The goal was to design a single networking strategy that could transport real-time video and audio as well as image files, text and email. Two groups, the International Telecommunications Union and the ATM Forum were involved in the creation of the standards.

ATM, as a connection-oriented technology, establishes a virtual circuit between the two endpoints before the actual data exchange begins. ATM is a cell relay, packet switching protocol which provides data link layer services that run over Layer 1 links. This differs from other technologies based on packet-switched networks (such as the Internet Protocol or Ethernet), in which variable sized packets (known as frames when referencing Layer 2) are used. ATM exposes properties from both circuit- and packet switched networking, making it suitable for wide area data networking as well as real-time media transport. It is a core protocol used in the SONET/SDH backbone of the public switched telephone network.

ATM addressing

A Virtual Channel (VC) provides the transport of ATM cells which have the same unique identifier, called the Virtual Channel Identifier (VCI). This identifier is encoded in the cell header. A virtual channel represents the basic means of communication between two end-points, and is analogous to an X.25 virtual circuit.

A Virtual Path (VP) transports ATM cells belonging to virtual channels which share a common identifier, called the Virtual Path Identifier (VPI), which is also encoded in the cell header. A virtual path, in other words, is a grouping of virtual channels which connect the same end-points, and which share a traffic allocation. This two layer approach can be used to separate the management of routers and bandwidth from the setup of individual connections.

Successes and failures of ATM technology

ATM has proven very successful in the WAN scenario and numerous telecommunication providers have implemented ATM in their wide-area network cores. Many ADSL implementations also use ATM. However, ATM has failed to gain wide use as a LAN technology, and its complexity has held back its full deployment as the single integrating network technology in the way that its inventors originally intended. Since there will always be both brand-new and obsolescent link-layer technologies, particularly in the LAN area, not all of them will fit neatly into the synchronous optical networking model for which ATM was designed. Therefore, a protocol is needed to provide a unifying layer over both ATM and non-ATM link layers, as ATM itself cannot fill that role. IP already does that; therefore, there is often no point in implementing ATM at the network layer.

In addition, the need for cells to reduce jitter has declined as transport speeds increased (see below), and improvements in Voice over IP (VoIP) have made the integration of speech and data possible at the IP layer, again removing the incentive for ubiquitous deployment of ATM. Most Telcos now plan to integrate their voice network activities into their IP networks, rather than their IP networks into the voice infrastructure.

MPLS, a generic Layer 2 packet-switching protocol, adopted many technically sound ideas from ATM. ATM remains widely deployed, and is used as a multiplexing service in DSL networks, where its compromises fit DSL's low-data-rate needs well. In turn, DSL networks support IP (and IP services such as VoIP) via PPP over ATM and Ethernet over ATM (RFC 2684).

ATM will remain deployed for some time in higher-speed interconnects where oocommon carrier | carrier]]s have already committed themselves to existing ATM deployments; ATM is used here as a way of unifying PDH/SDH traffic and packet-switched traffic under a single infrastructure. However, ATM is increasingly challenged by speed and traffic shaping requirements of converged networks. In particular, the complexity of SAR imposes a performance bottleneck, as the fastest SARs known run at 10 Gbit/s and have limited traffic shaping capabilities. Currently it seems likely that gigabit Ethernet implementations (10Gbit-Ethernet, Metro Ethernet) will replace ATM as a technology of choice in new WAN implementions.

Recent developments

Interest in using native ATM for carrying live video and audio has increased recently. In these environments, low latency and very high quality of service are required to handle linear audio and video streams. Towards this goal standards are being developed such as AES47 (IEC 62365), which provides a standard for professional uncompressed audio transport over ATM. This is worth comparing with professional video over IP.

ATM concepts

Why cells?

The designers of ATM utilized small data cells in order to reduce jitter (delay variance, in this case) in the multiplexing of data streams. Reduction of jitter (and also end-to-end round-trip delays) is particularly important when carrying voice traffic, because the conversion of digitized voice into an analog audio signal is an inherently real-time process, and to do a good job, the codec that does this needs an evenly spaced (in time) stream of data items. If the next data item is not available when it is needed, the codec has no choice but to produce silence or guess - and if the data is late, it is useless, because the time period when it should have been converted to a signal has already passed.

Now consider a speech signal reduced to packets, and forced to share a link with bursty data traffic (traffic with some large data packets). No matter how small the speech packets could be made, they would always encounter full-size data packets, and under normal queuing conditions, might experience maximum queuing delays.

At the time of the design of ATM, 155 Mbit/s SDH (135 Mbit/s payload) was considered a fast optical network link, and many PDH links in the digital network were considerably slower, ranging from 1.544 to 45 Mbit/s in the USA (2 to 34 Mbit/s in Europe).

At this rate, a typical full-length 1500 byte (12000-bit) data packet would take 77.42 µs to transmit. In a lower-speed link, such as a 1.544 Mbit/s T1 link, a 1500 byte packet would take up to 7.8 milliseconds.

A queuing delay induced by several such data packets might exceed the figure of 7.8 ms several times over, in addition to any packet generation delay in the shorter speech packet. This was clearly unacceptable for speech traffic, which needs to have low jitter in the data stream being fed into the codec if it is to produce good-quality sound. A packet voice system can produce this in a number of ways:

  • Have a playback buffer between the network and the codec, one large enough to tide the codec over almost all the jitter in the data. This allows smoothing out the jitter, but the delay introduced by passage through the buffer would require echo cancellers even in local networks; this was considered too expensive at the time. Also, it would have increased the delay across the channel, and conversation is difficult over high-delay channels.
  • Build a system which can inherently provide low jitter (and minimal overall delay) to traffic which needs it.
  • Operate on a 1:1 user basis (i.e., a dedicated pipe).

The design of ATM aimed for a low-jitter network interface. However, to be able to provide short queueing delays, but also be able to carry large datagrams, it had to have cells. ATM broke up all packets, data, and voice streams into 48-byte chunks, adding a 5-byte routing header to each one so that they could be reassembled later. The choice of 48 bytes was political rather than technical. When the CCITT was standardizing ATM, parties from the United States wanted a 64-byte payload because this was felt to be a good compromise between larger payloads optimized for data transmission and shorter payloads optimized for real-time applications like voice; parties from Europe wanted 32-byte payloads because the small size (and therefore short transmission times) simplify voice applications with respect to echo cancellation. Most of the European parties eventually came around to the arguments made by the Americans, but France and a few others held out for a shorter cell length. With 32 bytes, France would have been able to implement an ATM-based voice network with calls from one end of France to the other requiring no echo cancellation. 48 bytes (plus 5 header bytes = 53) was chosen as a compromise between the two sides, but it was ideal for neither and everybody has had to live with it ever since. 5-byte headers were chosen because it was thought that 10% of the payload was the maximum price to pay for routing information. ATM multiplexed these 53-byte cells instead of packets. Doing so reduced the worst-case queuing jitter by a factor of almost 30, removing the need for echo cancellers.

Cells in practice

ATM supports different types of services via ATM Adaptation Layers (AAL). Standardized AALs include AAL1, AAL2, and AAL5, and the rarely used AAL3 and AAL4. AAL1 is used for constant bit rate (CBR) services and circuit emulation. AAL2 through AAL4 are used for variable bit rate (VBR) services, and AAL5 for data. Which AAL is in use for a given cell is not encoded in the cell. Instead, it is negotiated by or configured at the endpoints on a per-virtual-connection basis.

Following the initial design of ATM, networks have become much faster. A 1500 byte (12000-bit) full-size Ethernet packet takes only 1.2 µs to transmit on a 10 Gbit/s optical network, removing the need for small cells to reduce jitter. Some consider that this removes the need for ATM in the network backbone. Additionally, the hardware for implementing the service adaptation for IP packets is expensive at very high speeds. Specifically, at speeds of OC-3 and above, the cost of segmentation and reassembly (SAR) hardware makes ATM less competitive for IP than Packet Over SONET (POS). SAR performance limits mean that the fastest IP router ATM interfaces are OC12 - OC48 (STM4 - STM16), while (as of 2004) POS can operate at OC-192 (STM64) with higher speeds expected in the future.

On slow links (2 Mbit/s and below), ATM still makes sense, and for this reason many ADSL systems use ATM as an intermediate layer between the physical link layer and a Layer 2 protocol like PPP or Ethernet.

At these lower speeds, ATM provides a useful ability to carry multiple logical circuits on a single physical or virtual medium, although other techniques exist, such as PPP and Ethernet VLANs, which are optional in VDSL implementations. DSL can be used as an access method for an ATM network, allowing a DSL termination point in a telephone central office to connect to many internet service providers across a wide-area ATM network. In the United States, at least, this has allowed DSL providers to provide DSL access to the customers of many internet service providers. Since one DSL termination point can support multiple ISPs, the economic feasibility of DSL is substantially improved.

Why virtual circuits?

ATM operates as a channel-based transport layer, using Virtual circuits (VCs). This is encompassed in the concept of the Virtual Paths (VP) and Virtual Channels. Every ATM cell has an 8- or 12-bit Virtual Path Identifier (VPI) and 16-bit Virtual Channel Identifier (VCI) pair defined in its header. Together, these identify the virtual circuit used by the connection. The length of the VPI varies according to whether the cell is sent on the user-network interface (on the edge of the network), or if it is sent on the network-network interface (inside the network).

As these cells traverse an ATM network, switching takes place by changing the VPI/VCI values. Although the VPI/VCI values are not necessarily consistent from one end of the connection to the other, the concept of a circuit is consistent (unlike IP, where any given packet could get to its destination by a different route than the others).

Another advantage of the use of virtual circuits comes with the ability to use them as a multiplexing layer, allowing different services (such as voice, Frame Relay, n* 64 channels , IP).

Using cells and virtual circuits for traffic engineering

Another key ATM concept involves the traffic contract. When an ATM circuit is set up each switch is informed of the traffic class of the connection.

ATM traffic contracts form part of the mechanism by which "Quality of Service" (QoS) is ensured. There are four basic types (and several variants) which each have a set of parameters describing the connection.

  1. CBR - Constant bit rate: a Peak Cell Rate (PCR) is specified, which is constant.
  2. VBR - Variable bit rate: an average cell rate is specified, which can peak at a certain level for a maximum interval before being problematic.
  3. ABR - Available bit rate: a minimum guaranteed rate is specified.
  4. UBR - Unspecified bit rate: traffic is allocated to all remaining transmission capacity.

VBR has real-time and non-real-time variants, and serves for "bursty" traffic. Non-real-time is usually abbreviated to vbr-nrt.

Most traffic classes also introduce the concept of Cell Delay Variation Tolerance (CDVT), which defines the "clumping" of cells in time.

To maintain traffic contracts, networks usually use "shaping", a combination of queuing and marking of cells. "Policing" generally enforces traffic contracts.

Traffic shaping

Traffic shaping usually takes place at the entry point to an ATM network and attempts to ensure that the cell flow will meet its traffic contract.

Traffic policing

To maintain network performance, networks may police virtual circuits against their traffic contracts. If a circuit is exceeding its traffic contract, the network can either drop the cells or mark the Cell Loss Priority (CLP) bit (to identify a cell as discardable farther down the line). Basic policing works on a cell by cell basis, but this is sub-optimal for encapsulated packet traffic (as discarding a single cell will invalidate the whole packet). As a result, schemes such as Partial Packet Discard (PPD) and Early Packet Discard (EPD) have been created that will discard a whole series of cells until the next frame starts. This reduces the number of redundant cells in the network, saving bandwidth for full frames. EPD and PPD work with AAL5 connections as they use the frame end bit to detect the end of packets.

Types of virtual circuits and paths

ATM can build virtual circuits and virtual paths either statically or dynamically. Static circuits (permanent virtual circuits or PVCs) or paths (permanent virtual paths or PVPs) require that the provisioner must build the circuit as a series of segments, one for each pair of interfaces through which it passes.

PVPs and PVCs, though conceptually simple, require significant effort in large networks. They also do not support the re-routing of service in the event of a failure. Dynamically built PVPs (soft PVPs or SPVPs) and PVCs (soft PVCs or SPVCs), in contrast, are built by specifying the characteristics of the circuit (the service "contract") and the two endpoints.

Finally, ATM networks build and tear down switched virtual circuits (SVCs) on demand when requested by an end piece of equipment. One application for SVCs is to carry individual telephone calls when a network of telephone switches are inter-connected by ATM. SVCs were also used in attempts to replace local area networks with ATM.

Virtual circuit routing

Most ATM networks supporting SPVPs, SPVCs, and SVCs use the Private Network Node Interface or Private Network-to-Network Interface (PNNI) protocol. PNNI uses the same shortest-path-first algorithm used by OSPF and IS-IS to route IP packets to share topology information between switches and select a route through a network. PNNI also includes a very powerful summarization mechanism to allow construction of very large networks, as well as a call admission control (CAC) algorithm that determines whether sufficient bandwidth is available on a proposed route through a network to satisfy the service requirements of a VC or VP.

Call admission and connection establishment

A network must establish a connection before two parties can send cells to each other. In ATM this is called a VC ("Virtual Connection"). It can be a PVC ("Permanent Virtual Connection"), which is created administratively, or an SVC("Switched Virtual Connection"), which is created as needed by the communicating parties. SVC creation is done by "signaling" in which the requesting party indicates the address of the receiving party, the type of service requested, and traffic parameters if applicable to the selected service. "Call admission" is then done by the network to confirm that the requested resources are available, and that a route exists for the connection.

Structure of an ATM cell

An ATM cell consists of a 5-byte header and a 48-byte payload. The payload size of 48 bytes was chosen as described above ("Why cells?").

ATM defines two different cell formats: NNI (Network-Network Interface) and UNI (User-Network Interface). Most ATM links use UNI cell format.

Diagram of the UNI ATM Cell

7

4
3


0
GFC VPI
VPI
VCI
VCI
VCI PT CLP
HEC




Payload (48 bytes)



Diagram of the NNI ATM Cell

7

4
3


0
VPI
VPI
VCI
VCI
VCI PT CLP
HEC




Payload (48 bytes)



GFC = Generic Flow Control (4 bits) (default: 4-zero bits)
VPI = Virtual Path Identifier (8 bits UNI) or (12 bits NNI)
VCI = Virtual channel identifier (16 bits)
PT = Payload Type (3 bits)
CLP = Cell Loss Priority (1-bit)
HEC = Header Error Control (8-bit CRC, polynomial = X8 + X2 + X + 1)

ATM uses the PT field to designate various special kinds of cells for operations, administration, and maintenance (OAM) purposes, and to delineate packet boundaries in some AALs.

Several of ATM's link protocols use the HEC field to drive a CRC-Based Framing algorithm, which allows locating the ATM cells with no overhead required beyond what is otherwise needed for header protection. The 8-bit CRC is used to correct single-bit header errors and detect multi-bit header errors. When multi-bit header errors are detected, the current and subsequent cells are dropped until a cell with no header errors is found.

A UNI cell reserves the GFC field for a local flow control/submultiplexing system between users. This was intended to allow several terminals to share a single network connection, in the same way that two ISDN phones can share a single basic rate ISDN connection. All four GFC bits must be zero by default.

The NNI cell format replicates the UNI format almost exactly, except that the 4-bit GFC field is re-allocated to the VPI field, extending the VPI to 12 bits. Thus, a single NNI ATM interconnection is capable of addressing almost 212 VPs of up to almost 216 VCs each (in practice some of the VP and VC numbers are reserved).

References

Further reading

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