The Asynchronous Transfer Mode has been chosen as the standard system concept for integrated broadband communication networks by the ITU?T. The system is predicted to grow rapidly as soon as it becomes widely accepted by network operators and users. Why has communications evolved in history towards the ATM concept and why has it been chosen as the broadband solution?
In the late 1800’s public telephone networks capable of transmitting analogue voice signals were established. The users were connected together via switches across the network to form a circuit. This was the first transfer mode used in telephone networks, and it is known as circuit switching. The invention of the vacuum tube led to the introduction of frequency division multiplexing (FDM) in 1925, and therefore the ability to make multiple connections on a single line. The public switched telephone networks (PSTNs) expanded, but with the circuit switched transfer mode remaining in use. This is because it provided an obvious way of keeping the constant connection necessary for voice traffic.
The invention of the transistor and the concept of pulse code modulation (PCM) allowed digital communications to be developed in the late 1960’s. The interconnection of computer systems over telecommunication networks soon became a requirement. Modems were used at first to generate analogue signals compatible with the PSTNs from the digital computer data to allow such interconnections. The already wide spread use of the PSTNs was an advantage of this scheme, however it was soon recognized as not being an optimum solution for data transfer.
The analogue PSTNs were unsuitable in terms of switching, capacity (bandwidth) and channel noise. In data communication applications, data tends to be transferred in bursts, separated by silence. The constant connection provided by circuit switching therefore does not provide optimum usage of the network resources. The PSTNs were ordinally dimensioned to provide capacity to transport voice signals, which provides very limited bandwidth and therefore a low maximum rate of data transfer. The noise present in the PSTNs due to poor channel quality lowers information transfer rates, since redundancy in the data is required to perform error checking.
Solutions for data communications were developed, including the packet switching transfer mode. With this system, the transmitter splits the data to be transferred into discrete units and sends them individually across a network, where at the other end, the data is reconstructed by the receiver. Packets need only be sent when data is available which therefore provides a more optimum use of network resources.
Computer technology advances created the ability for faster information processing and therefore the need for faster communication systems. Specific data networks were introduced such as packet switched data networks (PSDNs), to meet the new service requirements. In the early 1970’s, high bit rate digital time division multiplexing (TDM) systems were realized, allowing multiple high speed digital connections on a single line. The requirement for the integration of voice and data signals on a single network emerged.
In the early 1980’s, the design for an integrated systems digital network (ISDN) was proposed. The design described a digital packet switched network capable of providing telephone services and other data services. Optical fibre technology emerged providing a high speed transmission media with a low susceptibility to noise. These advancements coupled with the ISDN concept have led to the current information age: the notion of wideband networks capable of supporting high speed data communications, video, multi?media, etc. is emerging.
The broadband?ISDN (B?ISDN) concept is the proposed realization of an integrated broadband communications network. The transfer mode for implementing the B?ISDN defined and accepted by the ITU?T is the asynchronous transfer mode (ATM). Is has been widely accepted that a single network capable of supporting all required communications services is at the core of the current movement of communications evolution.
The ATM design has been influenced by the performance requirements of the B?ISDN, new wideband teleservice requirements and networking technology advancements. By considering all these factors, it can be shown why the ITU?T accepts ATM as the ultimate solution for B?ISDN.
ATM has grown out of the need for a worldwide standard to allow inter-operability of information, regardless of the “end?system” or type of information. With ATM, the goal is one international standard. There is an unprecedented level of acceptance throughout the industry of both the technology and the standardization process. With ATM, we are seeing an “emerging technology” being driven by international consensus, not by a single vendor’ s view or strategy.
Historically, there have been separate methods used for the transmission of information among users on a Local Area Network (LAN), versus “users” on the Wide Area Network (WAN). This situation has added to the complexity of networking as user’ s needs for connectivity expand from the LAN to metropolitan, national, and finally world wide connectivity. ATM is a method of communication which can be used as the basis for both LAN and WAN technologies. Over time, as ATM continues to be deployed, the line between local and wide networks will blur to form a seamless network based on one standard?ATM.
Today, in most instances, separate networks are used to carry voice, data and video information?mostly because these traffic types have different characteristics. For instance, data traffic tends to be “bursty”?not needing to communicate for an extended period of time and then needing to communicate large quantities of information as fast as possible. Voice and video, on the other hand, tend to be more even in the amount of information required?but are very sensitive to when and in what order the information arrives. With ATM, separate networks will not be required. ATM is the only standards based technology which has been designed from the beginning to accommodate the simultaneous transmission of data, voice and video. ATM is the emerging standard for communications. This is possible because ATM is available at various speeds from Megabits to Gigabit speeds. When information needs to be communicated, the sender negotiates a “requested path” with the network for a connection to the destination. When setting up this connection, the sender specifies the type, speed and other attributes of the call, which determine the end?to?end quality of service. An analogy for this negotiation of qualities would be similar to determining a method of delivery using US mail. One can choose to send 1st class, overnight, 2 day delivery, etc. and can ask for certified mail.
Another key concept is that ATM is a switched based technology. By providing connectivity through a switch (instead of a shared bus) several benefits are provided:
Dedicated bandwidth per connection
Higher aggregate bandwidth
Well defined connection procedures
Flexible access speeds
The basic idea of ATM is to segment data in small cells and then transfer them by the use of cell?switching. Such cells have a uniform layout and a fixed size of 53 bytes, which greatly simplifies switching. Being more complex, packet?switching is not nearly fast enough to be of use for isochronous data (i.e. realtime video and sound). Cell?switching gives maximum utilization of the physical resources. Using ATM, information to be sent is segmented into fixed length cell, transported to and re?assembled at the destination. The ATM cell has a fixed length of 53 bytes. Being fixed length allows the information to be transported in a predictable manner. This predictability accommodates different traffic types on the same network.
The cell is broken into two main sections, the header and the payload. The payload (48 bytes) is the portion which carries the actual information?either voice, data, or video. The Header (5 bytes) is the addressing mechanism. Whereas each cell is a fixed?size, thus switching can be directly implemented in hardware. Since each cell header contains addressing and control information, the hardware can switch and route data at much greater speeds than is possible with software systems.
ATM is a layered architecture allowing multiple services like voice, data and video, to be mixed over the network. Three lower level layers are used to implement the features of ATM.
The ( AAL ) Adaptation layer assures the appropriate service characteristics and divides all types of data into the 48 byte payload that will make up the ATM cell. Plus performs the necessary mapping between ATM layer and next higher layer i.e. maps from native format into fixed?size ATM cells
provides transport for synchronous bit?streams
clock recovery system, using Synchronous Residual Time Stamp (SRTS)
Indication of lost or errors information which is not recovered by AAL 1, ifneeded
Asynchronous/synchronous circuit transport, video signal transport forinteraction/distribution, voice?band signal transport, high?quality audio signaltransport
takes variable?length frames/packets and segments them into cells
used for example in SMDS and CBDS
efficient AAL, no?mis?sequencing protection
designed for same class of service as AAL 3/4, but simpler, requiring lessoverhead
Signaling AAL to provide assured transport
needed because signaling protocols (such as Q.2931) assume messages are alwaysdelivered and do not offer retransmission facilities
Segmentation and Reassembly Handles adapting rates and cell?jitter
The ATM layer takes the data to be sent and adds the 5 byte header information that assures the cell is sent on the right connection. Also performs routing, traffic management, multiplexing. Statistical multiplexing allows greater aggregate bandwidth than capacity of channel. Once cells are discarded, there is no retransmission in the Convergence Layer (sometimes called “Transmission Convergence Sublayer”). The ATM layer maps cells onto the transmission medium being used, and is responsible for cell delineation, and simple management functions relating to cell mapping. It decouples the rate of cell transmission from the physical media (insertion/removal of idle cells to pad bit?rate to transmission rate being used).
The Physical layer defines the electrical characteristics and network interfaces and transports the bits that make up ATM cells. This layer Converts to appropriate electrical or optical format “puts the bits on the wire.” ATM is not tied to a specific type of physical transport.
ATM has several key benefits:
1.One Network?ATM will provide a single network for all traffic types?voice, data, video. ATM allows for the integration of networks improving efficiency and manageability.
2.Enables new applications?Due to its high speed and the integration of traffic types, ATMwill enable the creation and expansion of new applications such as multimedia to thedesktop.
3.Compatibility?Because ATM is not based on a specific type of physical transport, it iscompatible with currently deployed physical networks. ATM can be transported overtwisted pair, coax and fiber optics.
4.Incremental Migration?Efforts within the standards organizations and the ATM Forumcontinue to assure that embedded networks will be able to gain the benefits of ATM incrementally?upgrading portions of the network based on new application requirements and business needs.
5.Simplified Network Management?ATM is evolving into a standard technology for local, campus/backbone and public and private wide area services. This uniformity is intended to simplify network management by using the same technology for all levels of thenetwork.
6.Long Architectural Lifetime?The information systems and telecommunications industriesare focusing and standardizing on ATM.
ATM coexists with current LAN/WAN Technology. ATM specifications are being written to ensure that ATM smoothly integrates numerous existing network technologies, at several levels (IE, Frame Relay, Ethernet, TCP/IP).
The early evolution of communications systems through history was first influenced by the simple desire to communicate messages. With increasing influence from computer technology, communications started to take a new direction towards mass data communication. The development of multimedia computer technology is starting to have a profound effect now on communications, with the possibility for mass multimedia communication systems coming into our lives.
The networking technologies for these new applications must be powerful and flexible to provide a large scale and long term solution. ATM has been developed to provide these features by evolving the system concept to give high speed data transfer. With the support of fibre technologies, ATM is the optimum transfer mode and most widely accepted transfer mode for future communications.