GUEST COLUMN: VOICE OVER ATM. And the winner is . ATM!
nelsonpub.com
For quality and breadth of service, voice over ATM beats out voice over IP and voice over frame relay.
By Ben Plowman
Why consolidate voice services over a data network? The answer is simply financial. Leased lines cost U.S. end users an average $1,500 per month for a single T1 line and as much as $5,000 per month for E1 overseas. By consolidating multiple T1s into a single OC3c asynchronous transfer mode (ATM) connection, return on investment can occur within 12 months of installation, if not sooner. End users with an ATM network can take advantage of their current infrastructure to
consolidate multiple T1/E1s through a circuit emulation (CE) switch module or through an ATM access multiplexer.
In North Carolina, Branch Banking and Trust (BB&T) has deployed both private and public ATM networks. BB&T's network supports both T1 and T3 lines. In Winston-Salem. four sites were
connected to eliminate leased lines.
BB&T was able to reduce network costs by almost half when the networks were installed in 1995. Service providers have since made the bandwidth/transmission line costs more competitive, but big
savings are still available for end users with multiple leased lines.
The opposition
Voice over IP (VOIP) is designed to reduce long-distance charges for remote users. But IP networks still throw up a lot of obstacles. Product solutions can be proprietary so that one IP gateway will not be able to talk to another vendor's gateway. The quality of service can be far from toll-quality.
Most product solutions support fax modulation, but not all. You may need two types of interface cards, analog and digital, to support the different types of traffic that your PBX already handles. If
the IP network is busy, you will have to re-dial using the private switched telephone network (PSTN). If your IP solution is Internet, it will not support live voice traffic very well, unless you like
phone conversations with walkie-talkie quality. There is little doubt that VOIP can have a bright future-the key word is future.
Voice over frame relay (VOFR) has a longer history as well as standards to its credit compared to OIP. Like ATM, frame relay offers a common network with rerouting capabilities. Frame relay's
dynamic allocation of bandwidth, paired with its bursting technique for peak performance periods, manages the traffic quite well. But it will drop packets if the bursting goes on too long during high traffic congestion. You don't want to lose frames during a videoconference session or you'll have a jumpy picture with sound sync problems.
Do you want your voice or video frames to be sent first in the network traffic? Prioritization is not a priority in many frame relay networks today, but as in other network protocols such as Fast
Ethernet, prioritization will have to become standard to support voice and video services over the
network.
VOFR could be a gamble. The frames can be transmitted for free on the average bandwidth already purchased, as long as the information rate set chosen for the connecting sites is not
exceeded (which would cause the frames to be dropped). One solution to this overload is a backup leased line, designed to handle the overflow of communications. But the whole idea was to
eliminate leased lines, wasn't it?
The ATM Forum is preparing to release AAL-2, the Holy Grail of voice over ATM. The problem is that AAL-2 does not do anything for most ATM equipment today. Currently the only adaptation layers available for voice in ATM installations is AAL-5 and AAL-1. According to the ATM
Forum, AAL-1 functions in support of constant-bit-rate, time-dependent traffic such as voice and video. AAL 5 functions in support of variable-bit-rate, delay-tolerant, connection-oriented data
traffic requiring minimal sequencing or error-detection support.
The majority of T1/E1 services, whether plain CBR or circuit emulation (CE), are supported on AAL-1. ATM switches support AAL-1 as well as several ATM access products. Is there a
solution that is proven, standards-based, requires minimal additional equipment to the existing
data/telecom equipment, and can be purchased off-the-shelf today? Yes: ATM and AAL-1.
AAL-1 circuit emulation service (CES) standards mean equipment interoperability, not proprietary
solutions. ATM CE modules have talked to ATM access multiplexer CE modules and vice versa.
The standard that enables this communication is the ATM Forum Circuit Emulation Service
Interoperability Specification (CES-IS) Version 2.0. Many CE modules offer support of structured
and unstructured services on the same module. Most CE equipment solutions offer anywhere from
four to eight ports per module card, but some ATM multiplexers offer higher density solutions of up
to 80 T1s in a single chassis-killer savings if your network is that intense.
Prioritization is key to multiservice networks. For voice traffic, you want some guarantees. AAL-1
is the priority assignment for ATM traffic coming into the network-it's transmitted first. And again,
AAL-1 specifies that the traffic cannot be dropped, unlike other networks at this time.
Plowman is a consultant with Dakota-an Alliance of Friends, in Annapolis, Md. He has
more than 20 years' experience in networking, the last five exclusively in ATM solutions. |
