Demystifying the SS7(1)

Hello

I have been on a long long holiday you know, so I haven’t had time to update my blog lately. But today we will go technical and that will be on signaling system 7. I have had a series of delays in releasing this series. Anybody who has attempted to write a book will notice a lot of unexpected delays, here and there. Either that some drawings won’t just find its way as expected or the designer is not keeping up to date or you are unexpectedly out of fund to meet some obligation. It is neither here nor there. But one page at a time maybe all you need to get through. So this is what I have just adopted here.


Let’s look at the concept of Signaling first

Signaling

You remember the old railway signals on the railway junction crossings? Ok, I hope you do. The whole idea is to signal incoming traffic that a train is coming and that they need to wait to avoid a collision with the train.
Hopefully you never experience one of such horrific accidents that occurred in those days as a result of signal failure or drivers insubordination to obey the signals.

So signaling is not the traffic but is a message exchange necessary to keep the traffic flowing without trouble: in this case collision or congestion that will also result as a result of collision thereby bringing the whole traffic to a halt.
Without signaling the traffic won’t just flow. Ok, If you understand this far, you are not far from becoming an SS7 guru.

Signaling in an Ethernet Network

You remember the old time Ethernet; they are very much around and will be with us for as long as it will take new technologies will overtake it.
In this technology, there is also a kind of crude signaling taking place.
You know the Ethernet technology simply provides a pipe or a channel and then ask every connected node to be pouring their content on the channel. However before your pour yours, wait and check the channel to see if you hear a whistling content already on it so that you don’t get your mount full. Once you think the channel is free, go ahead and pour your content and then listen to see whether another person did pour his or her content at the same time. You know, the ALOHA stuff. If you detect a collision as a result of two or more nodes poring in there content simultaneously, then withdraw, wait for some randomly set time and try again.
Whiz kids call this CSMA/CD which stands for carrier sense multiple access/ collision detection.
This is signaling but a very crude one anyhow. This is the reason that an Ethernet network is heavily degraded when very many nodes connect to the same channel. Anyway they got around this problem through what they call sub-netting.
So signaling is to help keep the traffic flowing so that we don’t grind to a halt.
Well if you understand it this far you are almost there already.

The SS7

Now SS7 was not the original signaling system used in telephony. The earlier signaling system called R2 or so was an in-band signaling system in the sense that it used the same voice channel that we were going to talk on to signal across the network using a series of pulses or tones.
In-band signaling works in this way: The user dials a telephone number; the switch looks at the telephone number and determines that it needs to set up a voice circuit to that next switch connected to it. He then seizes an idle circuit.
He now needs to communicate with that next switch about the call by using the dialed digits and perhaps other information about the call.
Using this in-band signaling, traditionally using multi-frequency tones, it will use that voice channel to send those tones to communicate that signaling information. So it uses a series of tones across that voice channel and provides that information to the second switch. The second switch then takes that information, does the same thing, and figures out where to go next with the call, etc. We're setting the call up circuit-by-circuit and switch-by-switch.
There are some problems with using this in-band multi-frequency signaling. One of those problems is that it is expensive. It's expensive for two reasons: One is that at each end of that voice circuit, we need to have multi-frequency transceivers—electronics that can both send and receive those multi-frequency tones. The other problem with it is that it is fairly slow; it takes times to out pulse those digits.
And remember, we're not making any money on this phone call while the call is being set up, only when somebody answers the phone. If it takes a long time to set up the call, that means we're reserving resources in particular voice circuits for a longer period of time when we're not generating revenue. A long call setup time is another problem—called post-dial delay.

Another potential problem with this in-band signaling is we're using the voice channel to signal. Who has access to that voice channel? You do—the caller. So we had some fraud problems. If you've heard about blue boxes (or sometimes they're called black boxes), there's the capability—if you're smart enough—to actually fool the network by playing a correct tone at the right time and not charging you for that telephone call.
So having the signaling (which is really call control of the network) accessible by the user is a potential fraud problem.

Around the end of 1976, the Bell System (now AT&T) started deploying a different method of providing signaling. In this case, we're taking the signaling out of the voice channel (out-of-band signaling) and putting it on a separate network designed specifically to do signaling. This signaling is called signaling system 7. This separate network, called out-of-band (out of the voice band) common channel (in the sense that I can use a single signaling link or signaling channel to signal for a lot of different calls), became popular mostly in the backbone network—that is, the long distance network. It was expensive at the time, at least initially; but the advantage was that it was faster (call setup was much quicker).

Signaling system 7 is a very robust signaling system it was designed by those guys in AT&T originally adopted by ANSI and then CCITT as the signaling system used in an ISDN network to keep our calls flowing and not jammed, halted or even congested.

So let us look at it in detail
The signaling system is an important part of the communication network. As one of the key technologies of modern communication networks, signaling system 7 is applied in different networks, transmitting not only inter-office signaling necessary for circuit connection in the telephone network and ISDN, but also different subscriber roaming-related location information between communication entities in the mobile communication networks, as well as intelligent service information between service entities in the intelligent network. A good knowledge of SS7 signaling is very important for understanding different signaling-related problems in communication and telephony.

The purpose of constructing the communication network is to transmit different information including voice information and non voice information for the subscribers. Therefore, all kinds of “information” will be interacting between different equipment, thus enabling coordinated operation of the equipment in the network. Such information transmitted between equipment is called signaling. To be common, signaling is a “language” between the equipment for communicating respective states and aims.
Each language has its own conventional rules and stipulations, and signaling must also observe the rules and regulations stipulated by relevant organizations, that is, signaling protocols or signaling modes. Let us therefore discuss these signaling modes in three aspects; evolution, architecture and application next time.