IP Telephony

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The 7th protocol

What do you think of when ‘seven’ is mentioned?

 

What about the Seven Wonders of the World, the Seven Dwarves, or the Seven Deadly Sins? Seven is the fourth prime number and it is also a self number. And interestingly, the Pythagoreans called it the perfect number; three and seven, the triangle and the square, the perfect figures. Sounds a bit like the Da Vinci code!

Well, in this month’s column we’re going to discuss none of that collection of mystical and mathematical things. We’re going to discuss Signalling System Number 7 instead, which is much more innerestin’, agreed?

 

That’s SS7 for short, by the way, although in Europe, notably the United Kingdom, it is sometimes called C7 (after CCITT number 7 – the International Telecommunications Union (ITU-T) was formerly known as CCITT).

 

I thought it would be a clever idea to look at the demographics of SS7. Then I discovered that demographics is defined in terms of population characteristics, such as: age, sex, marital status, family size, education, geographic location, and occupation!

 

So not to be put off, I present here the SS7 equivalents, if you’re up for that?

SS7 is 31 years old; is asexual; married to telecommunications networks; has spawned a large family of national and international variants; is part of the advanced intelligent network; can be found in almost any public location worldwide; and its occupation is as a set of telephony signalling protocols, which are used to set up the vast majority of the world's PSTN telephone calls.

 

But before we get too carried away…

 

SS7 is a global standard for telecommunications, which defines functions to be performed by a signalling system and a protocol to enable their performance. It identifies an architecture by which elements in the inter-office telephone network exchange information to effect wireless and wireline call-establishment, billing, routing, control and information exchange functions.

 

SS7 performs these functions by passing control messages between telephone exchanges – signalling points (SPs)  and signalling transfer points (STPs). The SPs handle the signalling control network as well as the speech channel network, while the STPs deal only with routing SS7 control packets across the signalling network. Essentially, the signalling network tells the exchange which paths to establish over the circuit switched network.

The SS7 protocols that have been developed by AT&T (News - Alert) and others since 1975 are now defined as standards by various national and international organisations, such as the ITU-T (in its Q.7XX-series recommendations) and ANSI. SS7 was spawned to replace earlier, in-band signalling protocols, which were once in widespread international use. These, with the exception of R2 variants, are now largely defunct.

SS7 brought many advantages to carriers and service providers. For competitive local exchange carriers (CLECs) it meant cost efficiencies and performance gains. As one SS7 connection can service multiple trunks, cost savings are made when extra capacity is added. SS7 messages also contain all the information for call set up, which reduces post-dialling delay, improving revenue opportunities for service providers.

The SS7 intelligent network (IN) architecture was introduced as a means of deploying services in networks, while decoupling them from central office switches. The service logic was separated from the switching equipment and this meant that services could be added without having to redesign the switches to support them. The result was faster and more economical deployment of new and enhanced, vendor independent services in both wireline and wireless networks.

 

Services that we take for granted today, such as 0800/1800 Freephone numbers, calling card services, short message service (SMS), calling line identification (CLI/ANI), international roaming, mass calling (tele-voting), prepaid calling and network-based voicemail are all enabled by SS7.

 

This highly efficient, common channel signalling system is the foundation for getting call information from the origination exchange to the end terminating office. Because SS7 is also a data packet, message-based service, it is ideal for routing voice, data, packet, and video services; and is an effective means of transporting ISDN service offerings.

 

For those who are that way inclined, SS7 terminology is also an acronym fest. You can look up these in Google (News - Alert) or Wikipedia when you have a moment to spare: SSP, STP, SCP, CIC, ISUP, MTP3 – want any more?

 

Before VoIP – or BV for you acronym freaks – there was already a strong connection between SS7 and IP. This arose to some extent, because of the success of SS7 in enabling new IN services. With those new services came a rise in the volume of signalling message traffic on the network. Some of this was predicted, and if it had remained voice centric, perhaps the changes that occurred would have been deferred. It is unlikely that they would have been avoided, as one way or another, usage increases – rather like the relationship between expenditure and income in the familiar truism.

 

However, within the lifetime of SS7 (a short time, compared to the seven ages of man), we have also seen the proliferation of the Internet. In the days of dial-up (don’t get nostalgic on me now, will you!), the PSTN access network was used and SS7 came into play to route Internet subscribers’ traffic to the end offices of Internet service providers (ISPs). This occurred via trunks between local exchange offices in a local access and transport area (LATA) or between LATAs. All of this resulted in a further increase in signalling traffic.

 

Another phenomenon occurred during the same time period, which was the exponential rise in the use of SMS, which, as the name suggests, meant even more messages travelling on the network.

 

A third factor, once again subject to tremendous increases in popularity and corresponding usage during the same era, was the emergence of the cell phone. The SS7 network is used to convey messaging for key facilities related to effective management of services in mobile/wireless networks. Roaming, for example, depends on home location registers (HLRs) and other database elements; all of which utilise SS7 signalling protocols. And of course, the interface between wireless networks and the PSTN – at the mobile switching centre (MSC) – uses SS7.

 

The constant multiplication of the amount of message traffic in the network – by far greater than a simple order of magnitude – led to carriers quickly seeking alternatives to expensive, ongoing expansion of their signalling infrastructure.  

And Bob’s your uncle!

After pushing the resources of network facilities beyond the capabilities for which they were originally designed, carriers were left scratching their heads, metaphorically speaking.

 

An obvious preference to the constant expansion of proprietary gear from telecommunications equipment manufacturers (TEMs), was to use standards-based, IP-centric kit. All that needed to be done was to convert the already packet-based SS7 signalling into a packet format for transport over an IP network; and Bob’s your uncle!

 

Actually, Bob’s your SIGTRAN.

 

SIGTRAN is an architecture and suite of protocols, defined by the signalling transport working group of the Internet Engineering Task Force (IETF), for the transport of real-time signalling data (SS7 and ISDN messages) over IP networks. The protocol suite is made up of a new transport layer – the Stream Control Transmission Protocol (SCTP) – and a set of user adaptation (UA) layers, which mimic the services of the lower layers of SS7 and ISDN.

 

The result is a standardised solution with a consistent model being applied to the upper layers of SS7, where both ends of the connection use the same UA.

There are many vendors who provide SS7 stacks and some also offer SIGTRAN layers with a consistent application program interface (API) to IP switch clients. Applications of SIGTRAN include Internet dial-up remote access, and more recently it is aligned with IP telephony interworking with the PSTN.

 

SS7 support is also widely available in appliances, called signalling gateways, which provide SS7-to-IP interoperability between the PSTN and IP networks. By using signalling gateways, both legacy and new equipment can seamlessly continue to operate over high bandwidth, IP–based core networks, instead of burdening the TDM–based, legacy SS7 network. This helps network nodes, like short message service centres (SMSCs), HLRs and so on, to support heavy SS7 traffic loads.

 

Next generation carriers know the PSTN is here to stay for some considerable time and existing voice services will need to remain supported. Those carriers need to use SS7 technology to offer new IP-based services to PSTN users, and to access existing services – wireline and wireless – in the PSTN. Seamless interoperability of services and features across PSTN, SS7 and IP networks is needed to achieve widespread acceptance of IP telephony.

 

By the way, a 2006 report by analysts Venture Development Corporation indicated that shipments of SIGTRAN would grow at the expense of pure SS7 stacks over the period to 2008. It remains to be seen if this prediction will materialise.

 

An alternative to putting all your eggs in one basket is to have a basket on each arm. With the drive to next generation communications and the need for new technology to deliver on the promise, new protocols have come to prominence. One of these is SIP and with its promise, you might be forgiven for thinking that the days of SS7 and its cronies are numbered. Think again.

 

In a somewhat similar way to SIGTRAN, SIP-T (SIP for telephones) is also a method of conveying SS7 (and ISDN) signalling messages within an IP network. The main difference is that SIP-T started from the premise of operating in IP networks, rather than looking to replicate SS7’s architecture in an IP environment.  

 

SIP-T is a mechanism that uses SIP to facilitate the interconnection of the PSTN with IP. It defines SIP functions (see IETF RFC 3372) that map to ISUP interconnection requirements – encapsulating ISUP messages in the SIP body. This is intended to allow traditional intelligent network or advanced intelligent network (IN/AIN) services to be seamlessly handled in a next generation IP or Internet network environment.

 

It is essential that SS7 information can be available at the points of interconnection with the PSTN to ensure transparency of features that are not otherwise supported in SIP. Don’t forget, SIP is an embryonic protocol and although loaded with potential, it does not yet have the answer to life, the universe, and everything.

 

SS7 signalling messages – both for call set up and mid-call information – need to be available in their entirety and able to step across the PSTN-IP interface to cause their equivalent functions to be transparently enacted within the SIP network. SIP messages with ISUP dependencies need to be routed correctly, which involves the SIP header, and the SIP INFO method, for example, is used for the transfer of mid-call ISUP signalling messages.

 

Ah yes, my friends, SS7 is far from collecting its pension.

 

What can we make of all this?

 

We can argue that SS7 is already firmly entrenched in the IP arena.

The ultimate convergence of fixed and mobile voice, broadband Internet data and multi-channel television – the so called ‘quadruple play’ – is inevitable, but the communications industry must take care of what happens during the migration. Someone has to keep the lights on. Somehow interoperability has to be maintained while managing the interim network situation on the road to Valhalla.

 

SS7, quite simply, has indeed a part to play – and whether this is through SIGTRAN or SIP-T, or some new SIP-inspired protocol, I think it will retain its importance into the foreseeable future.

 

PS:  Don’t forget to avoid the Seven Deadly Sins in your daily lives. What are they? Well, lust, gluttony, avarice, envy, wrath, sloth, and pride of course.