What protocols work best for VoIP?

Voice and data convergence is still in its infancy. Adoption rates are steadily increasing, and the technology has improved. However, uniting voice and data traffic over a WAN using transport backbones such as IP, frame relay, Asynchronous Transfer Mode (ATM) and broadband is still a complex task.

One reason voice and data have long run on separate networks is their unique characteristics. Voice networks operate in real time, with users expecting a particular set of behaviors from the network such as a dial tone every time they prepare to make a call or smooth speech quality. Voice is sensitive to delays (latency) and jitter, though it can withstand a small loss of information as long as real-time interactive behavior remains consistent. On the other hand, data networks aren't sensitive to delays or jitter, but they can't tolerate information loss.

For a converged network to meet desired performance levels, someone must manage the network so that voice packets have priority over data packets. Because of the differences between voice and data, network managers must first tag voice packets with a class-of-service mechanism such as resources-reservation protocol (RSVP), differentiated services (DiffServ), type of service (ToS) or port prioritization. These prioritization measures occur in the last mile.

A second layer of prioritization usually occurs when service providers use IP-enabled multiprotocol label switching (MPLS) networks to prioritize the traffic based upon tags on each packet. Companies can also compress voice packets to maintain bandwidth levels to a remote site. This compression ensures quality and efficient bandwidth utilization.

Next, managers should employ quality of service (QoS) techniques that set preferences through queuing mechanisms in routers and switches. Then, before the information leaves the LAN, they should use link fragmentation and segmentation or similar techniques to shorten the size of packets to minimize delays across WAN links. These steps are important for ensuring high QoS.

Today's WAN technologies

The most common WAN access-protocol technologies that support converged voice and data networks are frame relay and ATM. But this data likely moves from Ethernet LANs to the WAN and back, so companies must consider the added complexities of connecting Ethernet to IP-enabled backbones running MPLS. Each of these technologies presents its own set of benefits and challenges for managing voice and data.

Frame relay is a connection-oriented service employing permanent virtual circuits and is the most widely accepted transport protocol used in data networking today. New bandwidth-hungry applications such as Citrix, SAP and HTTP-based applications using the frame-relay transport protocol require more capacity to ensure proper voice and data quality on a converged network.

Like frame relay, ATM is connection-oriented technology that organizes information into 53-byte cell units for transmission. Because of this fixed length, network performance can be more predictable. ATM cells have a header (5 bytes) and a payload (48 bytes). When information flows through an ATM network, the packets, bytes or frames are segmented into smaller cells. This queuing allows ATM's QoS capabilities to follow traffic-policing guidelines so that critical information has priority. In addition, the smaller consistent cell size, in comparison to variable frames, uses less processing overhead, improving the performance of the network. ATM's fixed-cell structure may not properly handle VoIP compression standards if not designed correctly.

IP-enabled services give customers QoS features for a more predictable IP traffic delivery over the backbone. ATM and frame relay are the access protocols, and IP is the transport across the backbone. Companies should also consider that they will need to monitor and manage seven layers of QoS (based on throughput and bandwidth) as the technology evolves, further complicating the task of ensuring quality.


One of the most widely deployed connectivity technologies, Ethernet, is based on a bus topology where end points (PCs, phones, printers, etc.) compete for access to the network. With the high bandwidth that an Ethernet network can offer (in the gigabit range today), one would think that little is needed for traffic to pass between points A and B -- unless Point B is across the country and a WAN is in between. Typically, Ethernet traffic is in your building, and it crosses to a frame-relay, ATM or virtual private network service to get to remote locations. So when adding voice to such a segmented, routed, converged, unlimited endpoint network, management can be unwieldy, and information can get misplaced.

Since each transport technology carries both pros and cons, many companies will want to mix and match them to cherry-pick the best service options at the right price.

Matt Costanza is a network consultant at Mansfield, Mass.-based Vanguard Managed Solutions LLC, which offers managed network and security services.

Copyright © 2004 IDG Communications, Inc.

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