NMT [4] has been deployed in several European countries. There are two versions of the system: the first operates in the area around 450 MHz and the second operates in the area around 900 MHz. These variants, are known as NMT 450 and NMT 900, respectively.
3.3.1 NMT Architecture
An NMT system is made up of four basic parts:
† Mobile Telephone Exchange (MTX)
† Home Location Register (HLR), integrated in MTX or as a separate node
† Base Station (BS)
† Mobile Station (MS)
The MTX and HLR control the system and include the interface to the Public Switched Telephone Network (PSTN). This interface can be made at local or international gateway levels. BSs are permanently connected to the MTX and are used to handle radio commu- nication with the mobile stations. BSs also supervise radio link quality via supervision tones.
The set of BSs that are connected to the same MTX form an MTX service area, which in turn can be divided into subareas called Traffic Areas (TAs). The maximum number of BSs stations in a TA can be as high as 256.
MSs can be vehicle-mounted, transportable or hand-portable. In order to set up a call to a mobile, a paging signal must be sent out in parallel from all BSs in the TA in which the mobile station resides, instead of being sent out on all BSs in the service area. The aim of this approach is to reduce call set-up time and system load.
A number of network elements may also exist. These are:
† Combined NMT/GSM Gateway (CGW)
† Mobile Intelligent Network (MIN)
† Authentication Register (AR).
CGW is a gateway that can interrogate an NMT HLR and a GSM HLR. This is an optional feature for GSM MSCs that demands no new hardware. The HLR is used to store data about every subscriber, its services and location. In large networks where subscriber numbers are high, HLRs are preferably utilized as separate nodes, whereas in small networks, HLRs can be integrated with MTXs. The signaling protocol between MTSs and HLRs is according to CCITT Number 7 standard. Finally, The MIN adds intelligence to the network in order to enable introduction of new, customized services.
The radio network consists of cells, each having a Calling Channel (CC) and a set of Traffic Channels (TC). In order to enable frequency reuse, adjacent BSs obviously employ different operating frequencies. The frequency reuse schemes that are typically employed divide the available frequencies among groups of 7, 12, or 21 cells. The reuse plan is then built up by repeating these groups by trying to optimize the distance between BSs that employ the same
frequency. In order to adjust to variable traffic intensities, cell size may change correspond- ingly. Radio coverage is provided in the cells by placing BSs either at (a) the center of the cell or (b) at a corner of the cell (omni cells or sector cells). The latter option gives the advantage of using one BS for several cells, thus reducing the number of BSs used and obviously deployment costs. The coverage of a BS ranges from 15 to 60 km for NMT 450 and from 2 to 30 km for NMT 900, depending on the BS placement height and the actual environment.
3.3.2 NMT Frequency Allocations
Connections between BSs and MSs are utilized via full-duplex radio channels (ether in the 450 or 900 MHz band as mentioned before), which allow information to be exchanged simultaneously in both directions. These full duplex channels are utilized via a pair of uplink and downlink channels with BSs transmissions occurring in higher frequency bands than the transmissions of MSs. In NMT 450, 180 channels exist, separated via 25 kHz of spectrum. An optional extension band exists that can offer 20 more channels. With interleaved channels the system can use a total of 359 channels, which become 399 if the extended band is used.
3.3.3 NMT Channels
There are four channel types in NMT. These are (a) the Calling Channel (CC), (b) the Traffic Channel (TC), (c) the Combined Calling and Traffic Channels (CC/TC) and (d) the Data Channel (DC).
† Calling Channel (CC).Each NMT BS uses one channel as the calling channel. The CC is used by the BS for transmission of a continuous signal that identifies this BS to the mobiles. MSs within the cell of a BS lock onto the BSs CC. The CC is also used by the BS to page MSs under its coverage. Upon response of the MS, an additional channel, known as a TC, is allocated to the mobile. Finally, the CC may also be used for priority calls, meaning that messages over a CC can cause a user to terminate his call in order to receive one of a higher priority.
† Traffic Channel (TC).The purpose of the TC is to carry the voice traffic. A TC can be in three different states: (a) ‘free marking’ state, in which the TC is mainly used for setting up calls from mobile stations; (b) ‘busy’ state, in which the TC is occupied by a voice call and (c) ‘idle’ state, in which the TC is not occupied.
† Combined Calling and Traffic Channel (CC/TC).The CC of the BS can also operate as a combined calling and traffic channel. This is useful in cases where all traffic channels are occupied. In such cases, an MS can use the calling channel to set up a call. In such an event, the BS will completely lack a calling channel for some time. When a traffic channel becomes free, it functions as a combined calling and traffic channel. Thus, the BS’s CC will be used only when no other traffic channels are available.
† Data Channel (DC). The DC is used to make signal strength measurements on mobile stations that are involved in a voice call on order from the MTX. The results of these measurements are used by the MTX at handover decisions.
Every BS should have one CC, or some free TCs and one DC. Nevertheless, it is possible that a BS uses up to four DCs. This results in improved capacity for signal strength measurements, which is beneficial in situations characterized by increased traffic density or small cell sizes.
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3.3.4 Network Operations: Mobility Management 3.3.4.1 Paging
Paging is used to determine the position of a MS. The service area of an MTX can be divided into a number of traffic areas. Paging involves sending over all CCs in the traffic area where the subscriber is expected to be (the area where the last registration of this MS was made) a page with the number of the paged MS number. Paging only the traffic area that is known to contain the MS helps reduce paging load on the system. However, if the MS is not found, then paging will be reinitiated and performed on all traffic areas of the MTX rather than only in that where the MS is expected to be. Upon reception of the page message, the MS will respond to the BS of the cell where the MS is currently located. If a certain time period elapses without a reply from the paged MS, then the page is considered unsuccessful. If the paging is unsuccessful, it is repeated once more.
3.3.4.2 Handover
In order for handover to be performed, the radio connection quality is measured during the call. When the quality of the connection lowers, the BS that is currently serving the call signals the MTX. The purpose of this procedure is to investigate whether a BS with a better link quality to the mobile unit can be found. If such a BS is found and it has an available channel to serve the call, then a handover of the call to the new BS is initiated. If the handoff is to be performed, the MTX indicates to the mobile station that it must change its operating frequency to that of the new traffic channel selected in the new BS. The switch is made in the MTX at the same time as the mobile station changes its frequency. After a successful hand- over, the old channel is released. If, however, a BS with a better link quality to the mobile unit is not found, then the call continues with the current BS on the current channel and periodical signal measurements will be made in order to enable a successful handoff later. Normally 20–
30 s periods are used between successive attempts. If the handoff never takes place and the link quality continues to worsen (probably due to the subscriber moving far away from the BS) then the connection serving the call is dropped. A handover includes (a) seizing of the most suitable channel in the new BS, (b) supervision of the quality of the new channel, and (c) switching of the speech path towards the new channel.
3.3.4.3 Signal Strength Supervision
The MTX also performs continuous supervision of channel quality through signal strength measurements. This operation improves call quality, as handovers will be performed at an earlier stage.
3.3.4.4 Intra-cell Handover
This handover type involves moving a MS from a TC that experiences interference to another TC in the same BS. This procedure obviously improves call quality.
3.3.4.5 Handover Queue
In cases of highly loaded systems, handovers may be burdened with channel congestion, which imposes a difficulty when performing handovers. The handover queue tries to solve this problem. The MTX performs signal strength measurements on BSs surrounding the mobile and stores the first and the second best BS alternatives. If a handover is required and the BS with the best value does not have a TC available, the second best BS is chosen. If the second BS also does not have an available TC, then the handover is delayed and will be retried with the best BS alternative after a predetermined time period. This time period varies between 0 and 10 s and is adjustable. If no traffic channel becomes available during the waiting time period, the handover attempt is terminated and the call continues on the old channel.
3.3.4.6 Traffic Levelling
This feature increases the capacity and improves the success rate for call setups during peak time. The handover parameters are changed dynamically for the BS carrying high traffic load.
Thus, handovers of calls occur to less loaded BSs.
3.3.4.7 Location Updating
This function keeps continuous track of the MS in the network. It comprises two parts: (a) automatic location updating call from a mobile station and (b) updating of location data in the MTX. The location data indicates the current traffic area where calls to the mobile station can be directed.
3.3.4.8 Roaming Updating
Each MS subscriber is registered permanently in its Home Location Register (HLR) where all information relating to a mobile station is stored. Whenever an MS roams to the service area of another MTX which is controlled by an MTXV, then updating information is exchanged between the MTXs and the HLR. The HLR is updated to reflect the new subscriber location and the MTXV receives a copy of the MS subscriber’s categories, etc. Upon movement of an MS to another MTX area, the corresponding MTXV is ordered by the HLR to erase data that concerns that MS subscriber.
3.3.4.9 Inter-exchange Handover
This is an extension of the handover function that allows switching of calls in progress, even to BSs controlled by other MTXs. In conjunction with roaming, this feature makes the MS independent of the service areas of MTXs. The necessary signaling to perform this procedure is based on CCITT Number 7 and a specific Handover User Part (HUP). In inter-exchange handover more than one exchange is involved. These are (a) the ‘anchor exchange’, which controls the service area where the MS was at the original call set-up, (b) the ‘serving exchange’, which has radio contact with the MS and (c) ‘target exchange’ which involves the exchange to the BS which was identified by the BS as the most suitable BS for the
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handover. Furthermore, there are two different handover categories: (a) the basic handover, which is done from an anchor exchange to a target one and (b) the subsequent handover, which is done either from a serving nonanchor exchange back to an anchor exchange, or from a nonanchor serving exchange to a third exchange. The inter-exchange handover is controlled by the anchor exchange.
3.3.4.10 Subscription Areas
This feature enables operators to define mobility limits for MS subscribers. This procedure involves the definition of a restricted geographical area inside which the MS may place and/or receive calls. Representative examples of the usefulness of this feature are; (a) the subscrip- tion area is only one BS; this could be useful in cases where fixed telephony is more expensive than mobile telephony; (b) the subscription area is the coverage area of the whole system excluding large city areas; this can be used in the case of rural subscribers that enjoy a special tariff.
3.3.5 Network Operations
In NMT, subscribers are able to receive and originate calls both in their home and visited MTX. When a MS moves from one cell to another during a call, a handover will take place, enabling the call to continue.
3.3.5.1 Searching for a CC
A channel is selected randomly and the search starts from this channel. Then, additional channels are tested. The first time the search is performed, the searching MS operates at reduced sensitivity, in order to prevent itself from locking onto a channel with a weak signal.
If, however, the MS detects no channel during this search, it reinitiates the search, this time at an increased level of sensitivity. If the MS still detects no channel, it scans for a third time, operating at full sensitivity. When the MS has found a calling channel, the traffic area information is detected. The MS makes a comparison with the information stored in its memory. If the memory is empty or contains other information, the MS makes an updating call to the MTX on a traffic channel. If the MS is locked to the CC but experiences low quality for the CC, then it will start a search for a new CC as described above. However, the MS may have to check all frequencies in the CC band in order to find a high quality CC within the traffic area where the MS is registered for the moment. When a CC cannot be found, the MS locks itself to a calling channel in some other traffic area.
3.3.5.2 Searching for a Free TC or AC
This search operation uses the sensitivity reduction procedure mentioned above, however, with a maximum total number of 15 scans instead of three.
3.3.5.3 Transmission Quality Supervision
This function aims to ensure the best possible transmission quality of a call in progress,
irrespective of a subscriber’s movement within the service area. This is made possible by selecting the most appropriate BS to serve the MS calls. This selection is based on signal strength measurement performed at the current and all the neighboring BSs. Supervision of transmission quality is made by BSs in two ways: (a) measurement of the signal strength of the carrier from the MS; (b) measurement of the signal to noise ratio of a special supervision signal, which is transmitted by the BS and returned from the MS via the TC. The supervision signal is a tone above the speech band. It is also used as an identification signal to secure that handovers are being performed between the right BSs. Four different analog signals separated by 30 Hz are used: (a) signal number 1 at 3955 Hz; (b) signal number 2 at 3985 Hz; (c) signal number 3 at 4015 Hz; and (d) signal number 4 at 4045 Hz. Furthermore, it is possible to use an additional set of 35 digital supervisory signals. When transmission quality drops below a certain limit, the BS informs the exchange. In such a case, the MTX will request the RF carrier signal strength measurements of neighboring BSs. These results are then evaluated and ranked by the MTX. The result of this operation is a possible increase or decrease in the MS transmission power, or a handover to a new channel in the same or a different BS.
3.3.5.4 Blocking of Disturbed Channels
Idle traffic channels, which experience interference, either due to systems other than the network or the network itself (such as inter-channel interference) are automatically blocked for the duration of the disturbance. Therefore, they are not used for traffic, new calls, or handovers.
3.3.5.5 Discontinuous Reception
The purpose of this feature is to save battery energy in MSs. Battery saving is achieved by switching off the MS receiver most of the time, with only a clock function active during the low-power mode. Calls received during this time are buffered in the exchange. Once the MS has exited the power-saving mode, they are paged with information relating to buffered calls.
After the paging, MSs can re-enter low-power mode.
3.3.6 NMT Security
As in all cases of networking, security is a major issue of concern in NMT. The NMT features that aim to provide security are summarized below.
3.3.6.1 Mobile Station Identity Check
Unauthorized use of a MS can be prevented via use of a password that is attached to the identity of each MS. This password, which is stored in the MS and in the HLR, is a three-digit part of the mobile station identity, also known as the security code. Password validity is checked on calls to and from mobile subscribers and also on roaming updating messages.
When an incorrect password is detected, the call is disconnected and roaming is not performed. In order to prevent repeated call attempts with illegal passwords, thorough super- vision and logging of the identity check procedures are available.
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3.3.6.2 Subscriber Identity Security (SIS)
This feature improves the security of the subscriber identity beyond the level achieved by the three-digit password mentioned above. This feature protects subscribers from illicit use of their identities through an authentication mechanism based on a challenge-response method between the MTX and the MS including encryption of the dialed B-number. A Secret Authentication Key (SAK) is installed in the MS and the Authentication Register (AR), which is an external database that provides the HLR with authentication data. This data is generated in the form of a triplet comprising three values: (a) Key for B-number enciphering (B-KEY); (b) Random Number (RAND); and (c) Signed Response (SRES). This triplet is transferred on request to the HLR over a C7 signaling link. The HLR stores one or more triplets for every subscriber of the SIS system. The identity is checked every time a call is made from the mobile station. The check is performed by the MTX sending the random number to the mobile station, which computes an answer by using its SAK. The answer is sent to the MTX, which compares the answer to the result received from AR; when they corre- spond, access is allowed. If they do not correspond, the call is rejected. The MTX can handle MSs with and without SIS. Thus, SIS activation is optional. The operator can control the permission to roam for mobile stations with and without SIS. This flexibility is especially useful in international roaming in order to prevent illegal access to the network. SIS authen- tication can also be made for MSs that receive calls. This prevents unauthorized users receiving calls. The authentication is then done immediately after the call is set up and when the authentication indicates an unauthorized user the call is dropped. As in the case of the MS identity check, thorough supervision and logging of failed SIS authentication are available, in an effort to prevent repeated call attempts with illegal passwords and thus improve fraud prevention.
3.3.6.3 Location Dependent Call Barring
Selective call barring according to the location of the MS is also possible. The idea of this approach is to neglect location updating for MSs that are in a certain MTX service area or traffic area. Restrictions could then be put on outgoing calls and on roaming situations.
Placing restrictions on roaming could be useful in several cases, such as avoid expensive charging for forwarded incoming calls for MS roaming abroad.
3.3.6.4 PIN code
NMT provides an additional security method through use of a secret code at the MS known as the Personal Identification Number (PIN). PIN codes can be used to control roaming. For instance, when a subscriber is visiting a foreign MTX, he or she will only become fully updated in the visited MTX only upon dialing the PIN code. It is up to the operators to define whether it is necessary to dial the PIN code every time a call is made or only the first time a call is placed for a user that has entered a certain MTX service area or traffic area. The PIN code can also be used to control barring of outgoing calls, such as local or international calls.
The subscriber may control the type of barring by dialing a special service code including the PIN code.