Voice over Asynchronous Transfer Mode (ATM)

Asynchronous Transfer Mode (ATM) is a technology that can transmit voice, video, data, and graphics across LANs, metropolitan-area networks (MANs), and WANs. ATM is an international standard defined by ANSI and ITU-T that implements a high-speed, connection-oriented, cell-switching, and multiplexing technology that is designed to provide users with virtually unlimited bandwidth. Many in the telecommunications industry believe that ATM will revolutionize the way networks are designed and managed.

Today’s networks are running out of bandwidth. Network users are constantly demanding more bandwidth than their network can provide. In the mid 1980s, researchers in the telecommunications industry began to investigate the technologies that would serve as the basis for the next generation of high-speed voice, video, and data networks. The researchers took an approach that would take advantage of the anticipated advances in technology and enable support for services that might be required in the future. The result of this research was the development of the ATM standard.

How VoATM Works

Using a WAN switch for ATM, customers can connect their PBX network and data network to a public or private ATM network.

One attractive aspect of ATM is its ability to support different QoS, as appropriate for various applications. The QoS spectrum ranges from circuit-style service, where bandwidth, latency, and other parameters are guaranteed for each connection, to packet-style service, where best-effort delivery allocates bandwidth for each active connection.

The ATM Forum developed a set of terms for describing requirements placed on the network by particular types of traffic. These five terms (AAL1 through AAL5) are referred to as adaptation layers, and are used as a common language for discussing what kinds of traffic requirements an application will present to the network. 

  •   – AAL1—Connection-oriented, constant bit rate, commonly used for emulating traditional circuit      connections.
  •    – AAL2—Connection-oriented, variable bit rate, used for packet video and audio services.v    – AAL3/4—Connection-oriented, variable bit rate.
  •    – AAL5—Connectionless, variable bit rate, commonly used for IP traffic as it provides packetization      similar to that done with IP.

Choosing VoATM

VoATM is an ideal transport for multiservice networks, particularly for customers who already have an ATM network installed. ATM handles voice, video, and data equally well.

One attractive aspect of ATM is its ability to support different QoS features as appropriate for various applications.

The ATM Forum has defined a number of QoS types, including:

Constant bit rate (CBR)—
An ATM service type for nonvarying, continuous streams of bits or cell payloads. Applications, such as voice circuits, generate CBR traffic patterns. The ATM network guarantees to meet the transmitter’s bandwidth and other QoS requirements. Many voice and circuit emulation applications can use CBR.

Variable bit rate (VBR)—An ATM service type for information flows with irregular but fully characterized traffic patterns. VBR is divided into real-time VBR and non-real-time VBR, in which the ATM network guarantees to meet the bandwidth and other QoS requirements. Many applications, particularly compressed video, can use VBR service. It is fairly common in real networks that will never receive the ceiling value.

Unspecified bit rate (UBR)—An ATM service type that provides “best effort” delivery of transmitted data. It is similar to the datagram service available from today’s internetworks. Many data applications can use UBR service.

Available bit rate (ABR)—An ATM service type that provides “best effort” delivery of transmitted data. ABR differs from other “best effort” service types, such as UBR, because it employs feedback to notify users to reduce their transmission rate to alleviate congestion. Hence, ABR offers a qualitative guarantee to minimize undesirable cell loss. Many data applications can use ABR service.

How Packet Technologies Stack Up for Voice

Because Frame Relay technology was originally designed and optimized as a data solution, you could dedicate a public or private Frame Relay network to data and pay separate dialup or Virtual Private Network (VPN) rates for intracompany phone calls. Provided you can afford the different types of equipment, services, and staff resources required to manage both networks, this choice assures you of the highest quality for each type of traffic today. This option is most likely desirable for sites that are very data-heavy. 

Another option is to achieve some level of integration by using one piece of circuit- switching equipment, such as a time-division multiplexer (TDM), to connect both the PBX and LAN server to a wide-area network. Customers gain economies by running all WAN traffic over a single service (rather than receiving multiple WAN bills) and avoiding paying phone company rates for intra-enterprise phone calls. 

The costly downside is that within the network, bandwidth is likely to be wasted, because you are still reserving circuits for certain types of traffic, and those circuits sit idle when nothing travels across them.

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