Voice over asynchronous transfer mode (VoATM) uses ATM to carry digitized voice packets in fixed, 53-byte cells (5-bytes of header plus 48-bytes of payload). ATM switches are extremely fast, a very high quality of service. ATM offers various classes of service (CoS) such as constant bit rate (CBR) specifically for transporting voice. CBR provides a better quality of service by minimizing time variations in the transmission of voice cells, a phenomenon known as jitter. As with VoFR, VoATM is faster than VoIP since it is a Layer 2 protocol.
Where to Use It
ATM will never match gigabit Ethernet when it comes to LAN interconnections, though it can provide true QoS for voice and video. These inherent QoS capabilities allow ATM to outperform IP solutions in many bandwidth-intensive, time-sensitive applications.
VoATM is best used if you already have ATM networks in place, or to interconnect remote offices. The network in Figure 1 shows ATM supporting end-to-end calls.
Calls can originate in Baltimore, pass through Denver, and end in Los Angeles without having to encode or decode the call multiple times. ATM QoS features ensure the timely arrival of calls without any degradation in quality. Data and voice are placed in queues that are treated differently throughout the ATM network.
Figure 2 illustrates how priority can be given to voice traffic over data traffic. In this example, a CBR is specified for voice traffic, which means that a fixed bit rate is assigned to minimize jitter. The data traffic is relegated to an available bit rate (ABR) queue. The data traffic on the ABR queue does not have a guaranteed bandwidth, but it can be allocated more bandwidth than the voice traffic when bandwidth is available. This type of queuing scheme can satisfy both voice and data traffic within the same network.
When ATM's QoS measures are used, the tagging information is carried for the life of the cell. As long as the voice call is carried through the ATM network, it can retain its higher QoS tag. ATM is a completely different method of handling data flow.
One characteristic of ATM is that it segments data into fixed 53-byte cells. In a frame-based network such as Ethernet, the frame size can vary. This variance causes the switches to have to either wait for the entire frame (also called store and forward) or start sending the frame out the destination port as soon as it is received. Because ATM cells are always 53 bytes, the ATM switches know when the end of the cell has been received without an end-of-frame identifier or delimiter. This knowledge allows ATM switches to switch cells very rapidly.
ATM places as much data as possible in a cell, and if there is any space left over, it pads the cell, which potentially wastes space. The same voice call that was discussed in the VoFR section using G.729 for compression now takes up 14.13Kbps of bandwidth. Using G.729 results in voice being segmented into 30-byte payloads. This results in 23 bytes of overhead, almost as much as the payload itself (Figure 3). This overhead can consume a lot of bandwidth on slower connections, so it is important to understand the ramifications of using ATM on anything under T-1 speeds.
Continuous Bit Rate
ATM provides a CBR service to handle real-time information such as voice. Configuring an ATM link for CBR minimizes variations in time delay or jitter between successive voice cells, providing a high QoS to the end user.
CBR service provides a specific bit rate for voice traffic. There is an unavoidable delay between the transmission and delivery of a voice packet. Although important to minimize that delay, it is also important to minimize the variations in the time delay between successive voice packets. Ideally, each voice packet must arrive on cue, not late and not early.
Costs associated with ATM are still higher than Frame Relay. Equipment costs have come down quite a bit, and in a campus environment, ATM can be a very attractive solution. However, ATM service provider circuits are typically more expensive than Frame Relay circuits. This could make it difficult to justify ATM as opposed to Frame Relay. Keep in mind the following advantages that VoATM has over VoFR:
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Complete and detailed QoS measures
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End-to-end call routing without multiple encoding and decoding
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ATM popularity is still growing and services will be available everywhere in the near future
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ATM in the backbone as well as the WAN means a homogenous topology throughout
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