Picking packets

The business drivers for voice over packet are clear—the promise of reduced capital expenses, lower operational costs, optimization of network resources by converging voice and data, better use of network resources and increased revenue through new value-added services.  

What isn't clear is whether carriers will transport voice over pure ATM, IP over ATM, IP over MPLS or some combination of these for their short- and long-term packet network infrastructure. All of the choices have benefits, and next generation switches need to support these benefits and each carrier’s choice.

FROM OUR ARCHIVES Nov 5, 2001,TelephonyOnline.com Sprint to become first ILEC to make network all-packet Nortel wins $1.1 billion contract  for first phase of transition Nov 5, 2001,Telephony NEXT-GEN VENDORS CRACK THE BELLS Oct 15, 2001, Telephony  PACKET-SWITCHING CONTRACTS BUOY NEXT-GENERATION FUTURE by Donny Jackson Mar 13, 2001, TelephonyOnline.com  Qwest to deploy Sonus packet technology by Elizabeth Starr Miller

Carriers are moving voice services to packet networks both to reduce costs and to provide more value-added services in an increasingly competitive environment. A study by a major carrier found that packet equipment was 70% less expensive than traditional voice equipment, and data access lines were 60% to 80% cheaper than voice lines. 

Maintenance of packet networks was 50% less expensive, while provisioning was 72% lower.  So, given that the move to packet-based networks is beginning, why would carriers choose IP versus ATM for their voice network structure?

Why Voice over IP?

Two major different business drivers are fueling the voice over IP (VoIP) market.  The first driver has been the reason for many installations of long-distance VoIP networks--the low cost of long-distance VoIP.  And, in many cases, a flat rate pricing scheme provided an attractive offer for consumers to use such networks for communication especially for inter-continent communication.  

The second driver is next generation services.  As new services based on the convergence of data and voice become more mature, VoIP as a new access method becomes a viable choice for end users and especially for businesses.

VoIP is the ability to make telephone calls and access service over IP–based data networks with a suitable Quality of Service (QoS) and superior cost/benefit to PSTN–based calls. Today, most of the VoIP implementations are carried over ATM–based transport as shown in the second column of Figure 1.

FIGURE 1 Voice Network Choices
Voice and Telephony Over ATM (VToA)	VoIP/ATM	VoIP/Gig-E	POS
G.711/726/729	G.711/726/729	G.711/726/729	G.711/726/729
AAL1/2 ATM Sonet/SDH	RTP/UDP/IP RFC 2684 AAL5 ATM Sonet/SDH	RTP/UDP/IP Gig-E	RTP/UDP/IP PPP/HDLC Sonet/SDH

The benefits of implementing VoIP are mostly consumer-based and can be divided into the following three categories:

• Cost reduction—

  IP is everywhere. It is on our desktops and it is the technology behind the Internet. Many people view the Internet as a "free transport" for data and voice services. With the introduction of Net2Phone and other similar "free" services, many people are now making phone calls over the Internet. 
  
  In addition, businesses and individuals have turned to higher-quality commercial products and services to make voice calls based on IP. The prevalence of IP nodes and the abundant supply of better IP–based switches and routers continue to reduce the cost of providing VoIP.
  

• Simplification and consolidation—

  An integrated infrastructure that supports all forms of communication could allow more standardization and could reduce the total equipment complement. The differences between the traffic patterns of voice and data offer further opportunities for significant efficiency improvements. Universal use of IP for all applications, voice and data, holds out the promise of both reduced complexity and more flexibility.
  

• Advanced applications—

  Even though basic telephony and facsimile are the initial applications for VoIP, longer-term benefits are expected to be derived from multimedia and multiservice applications. For example, Internet commerce solutions can combine web access with information with a voice call button that allows immediate access to a call center agent from a PC. In addition, voice is an integral part of conferencing systems that could also include shared screens and white boards. Combining voice and data features into new applications will provide the greatest returns over the longer term.

Although ATM has been accepted by service providers for its ability to deliver high-speed data services, until recently its potential for deploying voice services was not fully used.

Using an IP–based network for voice traffic can offer advantages to consumers of reduced costs, simplification and consolidation due to the proliferation of IP–based applications and devices at the desktop. These advantages are compelling for consumers and are driving service providers to consider VoIP implementations. In contrast, voice over packet (VoP) over the ATM–based network offers distinct advantages directly to service providers and is still much more prevalent today.

Why Voice over ATM?

The ATM protocol, from the start, was designed to be a multimedia, multiservice technology. Although ATM has been accepted by service providers for its ability to deliver high-speed data services, until recently its potential for deploying voice services was not fully used. 

With the competitiveness of today's market though, network operators and service providers have been continuously striving to reduce operating costs and lift network efficiency and have turned to the ATM network to achieve these goals.   In addition, a strong driver for ATM on the line side has been Voice over DSL (VoDSL).  Early on, carriers adopted ATM to offer end users and businesses VoDSL service efficiently.

... the existence of an ATM infrastructure made IP over ATM a logical choice; however, in the long term--with the recent success of MPLS--it appears that pure IP over lambda may be the winner.

With hundreds of millions of dollars of ATM equipment infrastructure in the United States alone, service providers have recognized that significant economies of scale can be achieved if the data traffic and voice traffic are integrated onto a single network. To achieve this, service providers have started to use the circuit emulation services (CESs) of ATM switches to carry full or fractional E-1/T-1 circuits between end points. 

These CES mechanisms treat voice as a constant stream of traffic encoded as a constant bit rate (CBR) stream. In actuality though, voice is a combination of bursts of speech and silence and this increases the complexity of VoP.

Voice over ATM (VoATM) still is a viable choice especially in the cases where the objective is to replace existing TDM infrastructure or offer legacy voice services.  

The ATM Forum and International Telecommunications Union (ITU) came up with several advanced mechanisms to improve the efficiencies of transporting voice traffic, including:

• ATM trunking using AAL–1 for narrowband services

• ATM trunking using AAL–2 for narrowband services

• IP over ATM (AAL–5)

• Loop emulation service using AAL–2

Table 1 summarizes the benefits of utilizing the different methods for transporting VoATM.

TABLE 1 The benefits of using different methods for transporting VoATM
Standards	Voice Compression	Silence Removal	Channel Suppression	Switched Concentration
CES	No	No	No	No
BD-CES	No	No	Yes	No
ATM trunking using AAL-1	No	No	Yes	Yes
VoIP over ATM	Yes	Yes	No	No
AAL-2	Yes	Yes	Yes	Yes

The Debate

While it is clear that VoP is growing, there is still considerable debate about whether the underlying network technology will be ATM or IP.  The choice for replacement of tandem access, driven primarily by the regional Bell operating companies (RBOCs), appears to be ATM.  

Imbedded ATM QoS has made ATM an ideal choice for packetization of access tandems.  As IP-based QoS protocols such as DiffServ and resource reservation protocol (RSVP) at the edge, and MPLS for core traffic management, become available the concern with QoS associated with IP-based networks will diminish.

MORE INFO DiffServ Vs. MPLS RSVP MPLS Multiprotocol Label Switching is an IETF initiative that integrates Layer 2 information about network links (bandwidth, latency, utilization) into Layer 3 (IP) within a particular autonomous system--or ISP--in order to simplify and improve IP-packet exchange. more --from Webopedia The MPLS Forum MPLS Resource Center  VoIP Fine-Tuning Voice over Packet Services By Yuval Boger,  RADCOM Ltd. VoIP Howto Roberto Arcomano

In many cases, the existence of an ATM infrastructure made IP over ATM a logical choice; however, in the long term--with the recent success of MPLS--it appears that pure IP over lambda may be the winner. And certainly, IP at the application layer and the desktop is a more than just a viable near-term situation.

In addition to the challenges in architecting networks with end-to-end QoS, service providers must ensure that the rollout of such networks cause no disruption to their existing voice service revenue, which currently represent about 80% of their overall revenue source. 

With more than $650 billion of worldwide revenue generated by traditional voice and fax services and a more than $250 billion installed base of traditional equipment infrastructure in the U.S., carriers must deploy next-generation packet switches that seamlessly interconnect and competitively function as time division multiplexing (TDM)–based PSTN switches as well as support voice over ATM (VoATM) and VoIP.  

The packet switching industry is arguably still in the early states and the IP versus ATM debate will continue as the industry matures.  Carriers need to choose a solution that will protect them through the evolution—which means they must choose a solution that seamlessly interconnects to their TDM-based PSTN network today and also supports both ATM and IP voice services.  In addition, vendors need to provide solutions that support a carrier’s choice.

With underlying technology and chipsets evolving to support both ATM and IP, equipment vendors must support both types of data networks as well as TDM-based interfaces such as IMT/SS7 and ISDN PRI for interconnection to the PSTN. This flexibility will allow service providers to deploy packet switches for today’s TDM-based services, and eventually migrate to VoATM as well as VoIP services when it makes business sense and their network is ready.
Mehdi Ghasem is Director of Technology for Telica. He can be reached at mghasem@telica.com.

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