During early 1980s, European standardisation organisation, CEPT, started work on Group Speciale Mobile. The aim was to standardise wireless or mobile communication in 900 MHz frequency range. CEPT was evolved into ETSI (European Telecommunications Standard Institute). Later GSM was also renamed as Global System for Mobile Communications.
Let us see what different things we need to take care if we want to come out with mobile communication solution.
Landline phones are connected to local telephone exchange using telephone cables and a telephone exchange becomes an entry point to telephone network. We will also need similar exchange which would be an entry point to mobile telephone network, we will call it "mobile exchange". In case of landline phones, there is no question of finding location of phone as it is fixed and connected via telephone cable. For "mobile phone", the location need to be tracked and so we need an equipment or equipments in place between a mobile telephone and mobile exchange. Let's call it "location systems". These location systems will be in charge of talking to mobile phone over air. The location systems can consist of wireless antennas setup at various locations so that mobile phone can "move" at will and still be reachable.. The location systems need to inform whereabouts of mobile phones to mobile exchange as and when required. This way mobile exchange can do its work of network access without worrying about "mobility" of mobile phone. To cover a really big area like a state or country, we may need number of mobile exchanges. So to know the location of mobile phone, we will need a "common database" in which mobile phone's latest location can be stored. Mobile exchanges will have to refer to this database to complete (route) the call. SS7 already provide SCP which has similar functions of database query. We will call this database "location register".
So our mobile network will look like this:
SS7 provide signaling and transmission network backbone which fulfill our requirements. So it seems we can built our network on top of SS7 network. ME/G-ME would be SSPs, LRs would be SCPs. Note we will have to standardise number of things like the way ME and LR interact for location related signaling.
Network side looks achievable, now we need to look more at Location system.
Inside Location System
How do we track location ? Answer is simple: when we travel through city, we make out our location from road names, shop names etc. We can apply same principle here. Location system "broadcasts" its identification in its area. When mobile phone moves into different location system area, identified from broadcast identification, it informs to new location system about changed location (mentioning earlier location system's identification). Location system further inform this to mobile exchange. Mobile exchange in turn need to update it in location register (common among mobile exchanges). We can call it location signaling.
Location of mobile phone is known, how do mobile network contact it ? When mobile phone need to be contacted (e.g. incoming call), mobile exchange (to which mobile phone is currently present as per location register) will be contacted. Mobile exchange will inform location system in which mobile phone is currently present. Location system can then broadcast in its area that "there is a call for so and so mobile phone". This of course requires mobile phone to continuously listen to "broadcast" from location system (in which it is currently present). We can call this signaling "paging".
Air interface (frequency range) is shared, there is limit to number of simultaneous users that can be supported. So based on density of users, we need to setup antennas or covering circles.
Let us take an example:
We need to design location systems (assuming we use above procedure of location tracking).
First option is to map each circle to one location system. In larger density areas, covering circles will be smaller, so more chances of location change in terms of covering circle, leading to more location signaling.
Second option would be to use a single location system for all circles. But this defeats the basic purpose of location tracking as accuracy of mobile phone location is very low. Low location accuracy lead to paging in all the covering circles wasting valuable air interface.
Optimum approach would be to have location system for group of covering circles. Lesser amount of location signaling required compared to first option. At the same time location accuracy is improved compared to second option.
GSM network is designed in a very similar way; refer below the GSM architecture.
Mobile phone is known as Mobile Station (MS). Location system unit is Base Station System (BSS) consisting of a BSC (Base Station Controller) and connected BTSs (Base Transceiver Station). Mobile exchange is Mobile-services Switching Center (MSC) and G-ME is Gateway MSC (GMSC). Location register is split into Home Location Register (HLR) and Visitor Location Register (VLR). In practice, VLR is part of MSC. VLR has dynamic or latest BSC location of MS. HLR will have static information like services available to subscriber, and the VLR reference where MS is available.
References: GSM book by Mouly and Pautet, GSM Networks book by Heine
Let us study GSM in more detail in later articles.
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