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A Comparison of Voice Technologies (VoIP, VoFR, and VoATM)

  • November 24, 2003
  • By Fatima Ahmed
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Among the three packet-switching technologies, comparison and evaluation of the best is a difficult job because each one of them is suitable in different environments and needs of a network. In my article, I have tried to compare the three to some extent that will probably give a broader view to the reader.

What Packet Switching Is

Frame Relay, IP, and ATM are known as packet- or cell-switching technologies. This is in contrast to the public switched telephone networks (PSTN), which is a circuit switching technology, designed to carry analogous voice transmissions.

  • Frame Relay and IP insert data into variable-sized frames or packets. ATM chops data into small cells, which facilitates fast switching of data through the network.
  • They dynamically allocate bandwidth to various links based on their transmission activity while in circuit switching; a path is dedicated to the transmission for the duration of the call, which is sent in a continuous bit stream.

  • Packet-switching networks (except for ATM) are inherently less efficient than the circuit-switching network in dealing with voice because of the delay and network congestion.

  • These technologies use Access Devices (such as VFRADs/Routers and so forth) that employ sophisticated schemes to overcome the limitations. Prioritization, fragmentation, silence suppression, and voice compression are example of such schemes.

The following table describes the similarities and dissimilarities of the three voices:

VoFR VoIP VoATM
Definition:
VoFR (Voice over Frame Relay) enables a router to carry voice traffic, such as telephone calls and faxes, over a frame relay network. See figure: VoFR topology. VoIP (Voice over Internet Protocol), also known as VoP (Voice over Packets), allows the voice to travel simultaneously over a single packet network line with both fax and modem data. See figure: VoIP topology. VoATM (Voice over ATM) enables an ATM switch to carry voice traffic over an ATM network. See figure: VoATM topology.
Deployment:
Frame Relay is the most widely deployed and is commonly used in corporate networks. It is mainly recommended for star topology networks. VoIP is the technology that is used to transmit voice over an IP network—our Internet. Asynchronous transfer mode (ATM) is a multi-service, high speed, scalable technology but rarely found because of its expensive services.
Prioritization:
Voice Frame Relay Access Devices (VFRADs), such as RAD's MAXcess, has "tags" for different applications according to their sensitivity to delay. The VFRADs let the higher priority voice packets go first, keeping others waiting. Frame Relay Service Providers have also begun to offer different Quality of Service (QoS). The prioritization technique used for VoIP is different. They employ QoS scheme. The key IP QoS protocol was RSVP but now the Differentiated Service Model uses the ToS (Type of Service) octet field of the IP header to classify traffic at the borders between the customers and ISPs. ATM prioritization is implemented through QoS parameters.
Fragmentation:
The MAXcess and other VFRADs incorporates fragmentation schemes. Data packets are divided into small fragments for fast switching. IP fragmentation is performed in a similar fashion as in the Frame Relay but the size of the IP header is increased. Therefore, IP traffic consumes 50% more traffic than Frame Relay traffic. Fragmentation is built into ATM with its small, fixed-sized, 53-byte cells.
Variable Delay:
The variation in the packet arrival times, called jitter, causes non-smooth voice streams. This is controlled by a jitter buffer that avoids delay. Same technique of jitter buffer is employed in VoIP networks also. Dynamic bandwidth Circuit Emulation Service (DBCES) does not send a constant bit stream of cells but transmit only at an active voice call, reducing delays and variations.
Voice Compression:
Frame Relay access is usually at 56/64 kbps, low bit-rate voice compression algorithms such as ITU G.723.1 and G.729A are used to maintain high quality voice. Voice compression is vital in VoIP because traffic usually travels over low-speed links; for example, at 28.8 kbps. The same algorithms are used as in VoFR. In pure or simple ATM networks, voice compression is not essential but in hybrid (ATM & Frame Relay) networks, it is required.
Silence Suppression:
In circuit switched (telephone) networks, half of the connection line is used at one time; in other words, one person talks and the other listens. Therefore, this bandwidth can be used for transmitting other packets. The method of silence suppression is also employed in voice over IP networks saving approximately 60% bandwidth. Silence suppression is employed only in hybrid networks.
Echo Cancellation:
Echo Canceller eliminates the echo that results from the reflection of the caller voice back to him and improves voice quality. VoIP also employs echo cancellers between the ends of the voice transmitter and receiver. VoATM transports data, voice, and video at very high speed. The same method for echo cancellation is employed.
Header Size (in bytes):
Frame Relay frame's header is 2 bytes in size. IP headers are large, i.e. 20 bytes. ATM cells have 5-byte headers.
Protocol Standards:
The standard approaches specified by Frame Relay Forum in its FRF.11 are ADPCM (Adaptive Differential Pulse Code Modulation) and CS-ACELP (Conjugate Structure-Algebraic Code Excited Linear Prediction) for voice signaling. The other approach is FRF.12 for frame fragmentation. ITU H.323 defines interoperability standards for voice applications over IP. The upcoming standard is H.225.0 Annex G. ATM Adaptation Layer 1 (AAL1) protocol in ATM's CBR (Constant bit rate) service proved inefficient for voice transmission. AAL2's VBR (variable bit rate) service), specified by ITU-T recommendation 1.363.2, emerged as the standard choice.

Conclusion

Most of the organizations that deploy these technologies are in favor of the technology that costs the least and gives better performance. These technologies can be used in hybrid or combination form, giving a better chance for the user to accommodate for changes in his previous setup, if any. The widely used technology, VoIP, is very popular and is used to transmit both data and voice packets over a single line at the lowest cost. Similarly, VoATM provides synchronous transmission and is the fastest of all.

VoFR is another technology gaining immense importance in the Frame Relay networks and its combination with VoIP and ATM is being designed for mass usage. Hence, there is always a need to invent and explore these technologies so that the technology is delivered to all the users at a reasonable cost and provides the best possible service. Thus, each technology is better than the other technology in some way or another.

Figure 1: IP


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Figure 2: Frame Relay


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Figure 3: ATM


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