The mobile telephone switching office (MTSO), which is also called mobile switching center (MSC), provides the brains for cellular operations in a given
area. It is responsible for several or many cell sites, cell/channel assignment for a particular mobile subscriber, handover to another cell when appropriate, and handover to another MTSO or another cellular system included under the blanket term "roaming.'' Control information exchanged between/among MTSOs is carried out by data circuits based on ITU-T Rec. X.25*
Unless a MTSO shares a cell site, it does not have any direct radio functions. In other words, there are no cellular radios installed in a MTSO. One would expect to find microwave equipment used for links connecting the MTSO to cell sites and from the MTSO interconnecting with the PSTN. It is common practice that links connecting the MTSO with cell sites use the DS1 format (in the United States and Canada). In Europe, South America, and Africa, we would expect these links would use the E1 format and we would use the term MSC rather than MTSO. As one might expect, a MTSO has signaling, monitoring, and supervisory equipment, related to the setup, active supervision (including handover), and takedown of cellular telephone calls. If data service is available, the MTSO would also be responsible for these connectivities.
The MTSO is also responsible for billing of mobile subscribers and the maintenance of a "home location register (HLR),''which maintains records of those portable units permanently residing in the responsible area, and a visitor location register (VLR). Ideally, it should have communication capability with other HLRs to service " roaming'' customers operating outside their " home'' area to update their VLR.
* For a good description of ITU-T Rec. X.25, consult Practical Data Communications (Ref. 20).
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