Before we dive in too deeply, there's one thing to understand: Skype for SIP is not, as it
has commonly been described in the non-technical media, “a piece of software that connects PABXs to
Skype”. Skype is presenting a SIP-compliant interface to the outside world, which is (as I'll
discuss later on) a significant new aspect to its service.
To begin at the beginning: the world's
highest-profile Internet voice provider is trialing a service which would significantly expand its
business capability (presumably improving its revenue position as well).
The question is whether this will be
significant for business users, who have generally not been the core of Skype's market; for telcos,
which are already the target of “Skype's PSTN killer” speculation; or for Skype
To tackle these questions, we first need
to get under the skin of the announcement (or more accurately, what has been leaked and
What the market does not yet have is any
technical detail. However, at the recent eComm conference, Mark Spencer gave some detail of the
Skype offering. As creator of the popular Asterisk open-source VoIP PABX, Spencer has the profile
to be a Skype insider as well.
Spencer told the conference that Skype
is trialing a SIP gateway for its service – a significant development, since it means putting an
open face on what is otherwise a completely proprietary environment.
SIP – the Session Initiation Protocol –
is one of the key drivers behind the growing popularity of VoIP systems in the business world. SIP
created a standard that described the signaling part of IP telephony. This meant software could be
designed that replicated – and extended upon – the functions normally carried out by the PABX or
key telephone system.
By opening up the process of setting up
and tearing down calls over an IP network, SIP eliminated a pocket of proprietariness that held
back IP telephony. Certainly, PABXs could use data services before the emergence of SIP – ISDN,
Frame Relay and IP connections were deployed, for example, to connect systems in different offices.
However, with signaling standards lacking, most such applications depended on having the same
vendor's equipment at both ends of the link. The common signaling standard opens this up, since
anything that can generate standard SIP messages can send those messages to another system with
confidence that they'll be correctly handled.
The Closed World of
Skype, however, has always used its own,
proprietary signaling system. What is known about Skype signaling doesn't come from the company's
own documentation, but from analysis conducted by outsiders capturing and analysing Skype
One aspect of Skype's proprietary
protocol allowed the service to build up momentum and scale very quickly. Borrowing from other P2P
technologies, Skype decentralised the processing tasks to the users' machines: the more people sign
on to use Skype, the more processing is available to handle calls. This was a vital development.
Since Skype was initially a free service with no PSTN calling capability, its user-side processing
avoided the risk that popularity would kill the service either by flooding the servers or demanding
such processing power as to become uneconomic.
And because Skype gained its critical
mass relatively quickly, it has also been able to remain as a proprietary system. Its size made it
more important for other vendors to conform to Skype than for Skype to conform to a standard –
which is why so many hardware builders include a Skype client, even though it might be simpler and
cheaper to follow a standard.
The proprietariness of the Skype client
is also attractive to those whose business model has depended on some kind of walled garden. Mobile
handset vendors are the obvious example: their key clients are carriers, who resist the idea of
subsidising handsets with open VoIP clients that give their customers the chance to bypass their
voice networks. However, a Skype client is a little less threatening, because it gives the carrier
a way to construct plans around Skype capabilities without too far undermining their own
The problem is that this proprietariness
has been an obstacle standing in the way of the business market.
It may have seemed that Skype was
already a “force” in the business market: Skype contacts are already a common feature of business
cards, and Skype already has a suite of business services.
But clearly, it was missing a point of
demand. Vendors such as Spintronics offer SIP gateway boxes with SIP-to-Skype functionality, and
there are also a few open source projects addressing the same market.
Skype's entry into this segment clearly
offers an endorsement of its importance (albeit at the cost of attacking an existing market that
had been supporting Skype as a business tool). Interestingly, while Skype seems to acknowledge that
a SIP gateway is a useful tool, it's been somewhat overlooked by media commentating on the market,
who haven't noticed the pre-existence of such gateways.
To understand how Skype might
implement SIP support, there is an example at hand – Skype for Asterisk.
Asterisk supports SIP, but not
exclusively: it also has its own signaling protocol (IAX, the Inter-Asterisk
Skype for Asterisk is essentially a
Skype client designed to accept signaling messages from the Asterisk IP PBX software. For those
familiar with Asterisk, the Skype offering is presented to the Asterisk PABX as a “channel driver”
(in the same way as, for example, PSTN channels and SIP channels are presented to Asterisk). In
this way, Asterisk users can add Skype as a trunk, sending their Skype logins to the
The telephone user, in this case, would
treat Skype like a dial-out code (so you might dial 0 to access a PSTN line and 9 to access a Skype
trunk). Because Asterisk has some add-ons that support least-cost routing, it would even be
possible to leave it to the PABX to decide which outgoing trunk to use for a particular call, based
on whether the Skype trunks were available, whether a PSTN call might be free (because, for
example, it's a contract inclusion), whether the call is long-distance or international, and so
As with the consumer world, using Skype
as a trunk and adding Skype to the environment as a separate application allows the VoIP client to
remain proprietary. Skype is following essentially the same approach with Asterisk as it has done
with (for example) VoIP handsets, allowing its code to be embedded in a third-party
In Skype-for-SIP, the company is
presenting a gateway to the Internet that is SIP-compliant, so as long as the customer has a PABX
that can talk SIP and (for now at least) uses the G.729 codec, Skype will be able to set up and
tear down calls.
Why not offer SIP systems an installable
Skype client? Because there is a host of SIP applications and environments, running on a similarly
wide array of operating systems. SIP applications might run on Windows, Mac, Linux. The PABX might
be a standalone system from a major telephony vendor, running an embedded operating system
Skype has decided to avoid such an
open-ended development effort, and instead present a SIP interface to the outside
This is the most significant aspect of
the service, because it's such a fundamental departure from Skype's service history: users will be
able to connect to Skype without a proprietary client. Instead, the user will attach over the
Internet to a Skype-hosted interface and connect using their own software.
It's no surprise that this is a beta:
it's a completely different business model from that which Skype has followed in the
To make its service more attractive to
the business customer, Skype has decided to focus only on the service, and abandon the lock-in that
has been part of its culture since the beginning.
Whether or not the service succeeds,
however, is another question. The following questions are going to be critical:
- Scalability – Since the Skype for SIP service doesn't seem to offload processing to the
clients, its success as a business service will depend on Skype's ability to scale up to meet
- Security and Privacy – Skype will have to ensure that the service offers a level of
security suitable to business users. Skype for SIP users won't be vulnerable to Skype client
vulnerabilities, since they won't be using the existing clients, but protecting the businesses' SIP
logins will be critical.
- Competition – If a business is already using SIP in its PABX, it's probably already a
customer of a business VoIP service already. Businesses may well be prepared to add Skype as an
additional outgoing trunk – but that may not make them willing to move from their existing VoIP
provider to use Skype instead.
Finally, there's pricing to consider.
When Skype was first introduced in the early part of the decade, it was entering a
nearly-uncontested market. Its free on-network calls and (later) low-cost off-network calls were
In 2009, that's no longer the case. In
Australia at least, traditional PSTN carriers have noticed the threat posed by VoIP, and many of
their business services include various levels of free call allowances. In the meantime, the VoIP
market has blossomed, and all VoIP carriers now understand the important of offering free
on-network calls and cheap off-network calls.
VoIP is now a fiercely competitive
market. Skype has the brand in the consumer world, but that doesn't mean Skype for SIP will be an
automatic category killer.