By Eli Katz

VoIP peering is becoming an increasingly important part of VoIP providers’ strategies and operations, both from an engineering and a marketing perspective. Peering enables service providers to reduce off-net PSTN termination costs, expand network coverage for their services, and, more than ever, empower customers to experience higher quality broadband codecs and video on off-net calls.

As broadband and the IP communications applications running on it increase in popularity, the Session Initiation Protocol (News - Alert) (SIP) and Electronic Number Mapping (ENUM) standards are providing the foundations for the inevitable industry shift to multi-lateral VoIP peering.

What is VoIP Peering? 
(News - Alert)

Peering is a term that was established decades ago in the IP bandwidth space. ISPs either “peer” with each other, (i.e., freely exchange traffic), or buy transit from each other, where there is an imbalance of data flowing one way or another. For example if 100,000 users of a consumer broadband service are downloading MPEG video, large files and other content from a 10,000 Web sites around the world, then the consumer broadband service will have to buy transit IP bandwidth to accommodate.

In telecom it is minutes rather than data that flow between service providers. Classic telecom interconnects, whether achieved via VoIP or TDM technologies, are similar to IP transit, in that the source of the calls pay per minute to deliver traffic to the destination. Peering in the telecom environment, a free exchange of minutes, will only work if call minutes flowing in either direction is broadly balanced — and this is something that rarely happens in a traditional telecom environment due to the marketplace’s reliance upon transit carriers (especially for overseas calls).

Surprisingly, minutes between residential users on different networks are likely to be balanced, even when the size of the networks differs significantly. Imagine one VoIP service provider with one million consumers and another with only ten thousand consumers; as often as one of the million wants to call one of the ten thousand, one of the ten thousand will want to call one of the million. This natural tendency towards balanced traffic exists only when calls flow directly between the originating and terminating service providers, without the involvement of a transit carrier. This end-to-end delivery is referred to as peering and is complicated by one significant issue; you can only deliver a call directly to the terminating service provider if you know that a number belongs to them. Thus peering must be enabled by a “location” or “discovery” function that tells an originating operator whether a dialed number belongs to another VoIP service provider.

Modeling peering relationships on traditional telecoms interconnects implies that each bilateral peering relationship would require:

• Sharing of subscriber phone numbers.
• Independent commercial contract.
• Interconnection and interoperability testing process.
• Ongoing maintenance as new services and equipment are brought on line.

It is easy to see how scaling and security issues abound when undertaking multiple interconnects using this approach.

A more rational and scalable approach is multilateral peering. Each service provider has a relationship with only one entity, the central peering facility, which delivers the following improvements:

• Privacy, by acting as custodian of the phone numbers.
• Easier Interoperability, by providing signaling interoperability and insulating providers from technical changes implemented by others.
• Security, by defining standards and policing behavior.
• Simplicity, by defining a set of commercial and operational standards to which all members must adhere.

Counter-intuitively, signaling interoperability is often the most complex of these challenges from a technical perspective, even when everyone is using SIP.

SIP

Created from the ground up to be user friendly and easily extensible, SIP is becoming established as the protocol of choice as VoIP technologies are deployed. Large incumbents (such as British Telecom in the UK) and 3GPP IP Multimedia Subsystem (News - Alert) (IMS) have selected variants of SIP as their primary signaling protocol. Already the most popular VoIP signaling protocol available, SIP will probably be at the core of any peering relationship in the future — so why is interoperability a challenge?

While basic at its core, SIP has grown tremendously since its inception in the late 90’s. Contrary to its stated purpose of utmost simplicity, SIP today comprises myriad RFCs and drafts, and as such, interoperability between different flavors and implementations cannot be taken for granted. Each individual VoIP service provider may have a different SIP implementation, driven by different equipment vendors with their own preferences.

For example, in SIP there are several different fields in which to pass on Caller ID information. With each VoIP service provider using a different method, a multilateral peering service must identify and normalize these methods to ensure feature transparency, in this case by delivering the Caller ID to the terminating operator.

In addition to being used by endpoints to initiate communications, SIP is also used by network elements used to relay these requests. The network elements are tasked with relaying the signaling as well as fulfilling the “location” function by “finding” the remote party and relaying this information to the caller. In SIP this is accomplished by using a method called SIP redirect.

There are however, other protocols that can be used for this “location” function — ENUM being the primary one.

ENUM

ENUM, with its roots in DNS, enables originating networks to query a registry in order to discover routing instructions for e.164 numbers that are associated with IP devices on other networks.

As most VoIP networks are primarily connected to the PSTN for service, each end point is identified by a standard e.164 number. The ENUM registry allows the service provider to find the IP address corresponding to these E.164 number masks and route the call without utilizing PSTN termination.

Among the advantages over SIP in supporting the “location” function, is that ENUM is specialized for traditional phone numbers (whereas SIP can just as easily support e-mail addresses) and also provides support for augmenting routing information such as number porting and other similar functions.

Conclusion

ENUM and SIP provide key features and functions that facilitate peering, but also leave many options and decisions for each network architect. Due to the rapid growth of the market participants and the many different technical implementations, a neutral multi-lateral peering service is the logical path for the future.

Eli Katz is chief executive officer of XConnect. For more information, please visit the company online at www.xconnect.com.