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Everyone is in favor of SS7 nowadays, just like Mom, Apple Pie and Cellphones (well, almost everyone). But why all the fuss? And, what about all signaling systems 1 through 6 – what happened to them? And, what is signaling, anyway?
While signaling can be used as a general term for any exchange of information (for example, the Semaphore Signaling with flags that boy scouts used to learn), in telecommunications it generally is restricted to the information used to control calls. Traditional forms of telecom signaling involved the exchange of special tones (known as MF – multi-frequency) between telephone exchanges. This is still commonly used between MSCs and wireline telephone switches – often known as R1, Type 1/2A/2B or Feature Group D. A notable characteristic of this type of signaling is that the signaling occurs on the same channel as the conversation (known as in-band signaling).
Newer telecom signaling protocols, such as SS7, generally provide a separate signaling channel, at least some of the time. The cellular protocols between a mobile and a base station use an out-of-band control channel for most signaling, although some mid-call in-band signaling occurs on the voice channels. SS7 is a pure ‘out-of-band’ (or ‘common channel’) protocol, with signaling occurring only on the dedicated channel. On a 24-channel T1, for example, being used for both signaling and voice, probably 2 channels (aka timeslots) will be used for signaling, with the remaining 22 used for voice.
SS7 has several characteristics that are desirable in modern telecommunications networks:
SS7 signaling links run at 64 kbps, a raw speed at least 1,000 times faster than MF tone-based signaling systems. This results in one of the biggest benefits of SS7 – faster call setup, an advantage that grows with the complexity of call setup.
SS7 is designed in several ways to be a highly reliable signaling protocol. Most SS7 links are duplicated, not to carry more traffic, but to allow a backup if a link fails. Also, unlike North American tone-based signaling, SS7 messages are usually acknowledged, allowing appropriate alternative action to be taken if a message does not reach the destination, is corrupted en-route (highly unlikely) or cannot be processed once it reaches the destination.
SS7 is flexible because it is based on a layered protocol architecture, although not layers that match the ISO 7-layer architecture very well. The bottom layer, known as MTP (Message Transfer Part) allows messages to be sent between any two signaling points within a national SS7 network, based on a numeric address (known as a point code). Intermediate routing is performed by Signaling Transfer Points (STPs) based on the point code.
The second SS7 layer, known as SCCP (Signaling Connection Control Part), provides enhanced addressing capabilities, known as “Global Title.” SS7 is perhaps unique in providing an open architecture for new address types, allowing them to be added to the protocol for new applications. International signaling for cellular and PCS applications, for example, is migrating towards the use of the ITU E.212 International Mobile Station Identity (IMSI) as not only a unique identifier for a mobile, but also a global title address that can be used to route an autonomous registration message for a roamer from the visited system to the home system, by using the first 6 digits of the IMSI (which identify the home country and the home carrier).
SS7 also provides greater flexibility than tone-based signaling due to its greater capacity. Any increase in signaling complexity using tones would come at too high a price in call setup delays. However, SS7 can afford to send a considerable amount of non-essential information related to calls (calling number and name, for example) that can be used for enhanced (i.e. extra cost) services. SS7-based call processing can also afford to take more side trips, to translate 1-800 and 1-900 numbers, for example.
SS7 allows greater efficiency in call processing, because the signaling and trunk setup can be performed in two separate phases. When setting up a call to a roamer, for example, cellular networks usually first transmit an IS-41 message over an SS7 network to determine whether the mobile is available. If the query indicates that the mobile is busy (or turned off, or not answering), then call forwarding can be performed at the originating wireless switch. Consequently, phoning a San Diego mobile roaming in Montreal, but currently unavailable, does not require a transcontinental trunk to be set up. Call forwarding can occur locally in San Diego, with only the expense of a transcontinental transaction on the SS7 network. SS7 networks are allowing call setup to be transformed as non-geographic numbering becomes the norm rather than the exception. Call setup to wireless phones, 1-800 numbers and portable numbers all require an SS7-based query to determine the precise location an d/or status of the destination.
Above the basic SS7 layers (MTP and SCCP) rides the application. Wireless systems most commonly use two applications: ISUP for call setup to non-wireless systems, and IS-41 for location management, call setup, fraud management, feature control and other capabilities of wireless phones. Until recently, IS-41 has been the application driving the migration to SS7. However, recent FCC mandates are herding the wireless industry towards ISUP – number portability and enhanced 9-1-1. Number portability requires a new address type to be associated with call processing for ported numbers (the Local Routing Number) and a flag to indicate whether the number portability query has been done. Enhanced 9-1-1 will require SS7 signaling to carry to the emergency services network the latitude and longitude estimate for the location of a wireless phone .
SS7 is a classic example of a behind-the-scenes enabling technology. Whenever your cellular phone rings while you are out of town – think SS7. In the future when you call 9-1-1 and they know where you are – think SS7. When you take your cellular phone number to a competing carrier – think SS7. When someone steals your mobile phone number and tries to clone you, and they get stopped – think SS7. SS7 is the power grid for modern, wireless, telecommunications.
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