Bahan Kuliah IVBahan Kuliah IV

JARINGAN JARINGAN TELEKOMUNIKASITELEKOMUNIKASI

S1-TES1-TE

Sekolah Tinggi Teknologi Telkom

Bandung2005

P L M P L M NN

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PLMN PLMN Konsep Dasar, Elemen & Struktur Jaringan Konsep Dasar, Elemen & Struktur Jaringan

PLMN: PLMN: Cell, Base Station (BTS), BSC, MSC, GMSCCell, Base Station (BTS), BSC, MSC, GMSC

Multipple AccessMultipple AccessRadio Channel : Traffic Channel, Control ChannelRadio Channel : Traffic Channel, Control Channel

Attachement, Detahhement, RegistrationAttachement, Detahhement, RegistrationRoaming, Registration & PagingRoaming, Registration & Paging

Location Area, Location Updating, PagingLocation Area, Location Updating, PagingRoamingRoaming

Radio ChannelRadio Channel

Proses Komunikasi (panggilan) Proses Komunikasi (panggilan)

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D.1.1.1 Mobility

Mobility in a public telecommunications network is no unequivocal concept. (See Volume 1, Chapter 6, Subsection 6.2.4.) We differentiate between portability, movability and (complete) mobility.Portability represents the simple case in which only the terminal is moved and then connected again at another point in the network. Movability implies that the subscriber moves his personal access; for example, when logging onto a data network from different network positions. Mobility refers to the state of complete ambulatory capability in which both the terminal and subscriber access can be moved, while the network automatically keeps track of all movement. In other words, this means both terminal and service mobility.Mobility requires radio access via base (or radio base) stations. The physical access in a mobile network is arranged to enable a terminal to connect itself anywhere in the network and move about while a call is in progress. (Of course, the subscriber's subscription must be available at all access points.) This movability presupposes specially designed access ("cells" instead of connection points). It also requires that the terminal be able to maintain continuous radio contact with the network.

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D.1.1.1 Mobility

• Portability : terminal is moved.

• Movability : moves his personal access.

• Mobility : terminal and subscriber access can be moved.

Mobility requires radio access via base (or radio base) stations.

The physical access in a mobile network is arranged to enable a terminal to connect itself anywhere in the network and move about while a call is in progress.

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D.1.1.2 Primary PLMN functions - Main network elements

It is necessary to be somewhat familiar with the specialised terminology to understand mobile networks and their functions. Examples of basic concepts include location updating, roaming, handover and paging. To elucidate these concepts and the handling of mobile traffic, we should have used animated illustrations. For practical reasons, we must leave the animation to the reader's imagination when we refer to Figure D.1.1, which illustrates the salient elements of a fixed network and of a PLMN.Mobile networks require functions for network intelligence, even when handling "ordinary" calls. Figure D.1.1 shows two of these functions: the home location register (HLR) and the visitor location register (VLR). The figure also makes clear that access to the PLMN is significantly different from access to fixed networks. Each base station controller (BSC) includes a switching function allowing it to switch to another base station as the terminal moves (roaming). In the figure, imagine the terminal having moved from location area 1 (LA1), through LA2, to LA3, where it has been called via the associated BSC. The next destination is LA4. Such movement also involves a number of mobile switching centres (MSCs).

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• Location updating• Roaming• Handover and paging.

D.1.1.2 Primary PLMN functions - Main network elements

Basic concept function of PLMN (Figure D.1.1):

Elements of PLMN :

• MSC (Mobile Switching Center)• HLR (Home Location Register)• VLR (Visitor Location Register)• BSC (Base Station Controller)• LA1, LA2 … (Location Area 1, 2, ….)

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Figure D.1.1 Comparison of a fixed network and a PLMN having cells grouped in location areas (LA)

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D.1.1.3 An orientation - Common concepts

The following concepts are described in this section : Cells and base stations - Multiple access Radio channels between base stations and mobiles -

Control channels and traffic channels Attachment and detachment Roaming Registration and paging - Location area Locating and handover

The various network elements - MSC, BTS, HLR, VLR - are clarified in more detail in Chapter 2, Section 2.3.

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D.1.1.3 An orientation - Common concepts

Some concepts in PLMN : Cells Base Stations Multiple access Radio channels Control channels and traffic channels Attachment and detachment Roaming Registration and paging Location area Locating and handover

The various network elements - MSC, BTS, HLR, VLR - are clarified in more detail in Chapter 2, Section 2.3.

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Cells and base stations - Multiple access

Radio access offers subscribers a number of radio channels for communication. However, radio channels are in short supply. To effectively utilise the frequency spectrum allocated for use by mobile subscribers, every radio channel should be reusable, which requires well-defined and separate geographical areas that have access to a range of frequencies. Such areas of service are referred to as cells. The nomenclature has given rise to the term cellular system that we find in a system name such as personal digital cellular (PDC).The number of radio channels in a cell is significantly less than the number of mobiles, since - in the normal case - only a minority of the mobiles are active at the same time. The technique used to assign idle traffic channels to calling or called mobiles is referred to as multiple access. (See also Volume 1, Chapter 5, Section 5.10.) Three variants of multiple access are described in Chapter 4, Subsections 4.3.5 - 4.3.7, of this Part.Base stations use either omnidirectional or directional antennas. The antenna of an omnidirectional cell radiates (more or less) an equally strong signal in all horizontal directions, thereby covering a circular area. A mobile station located in this area will normally experience good radio contact with the base station. The circle's radius can be modified by changing the output power of the base station, which in most cases is done in connection with cell planning (see Chapter 10, Section 10.5). As a rule, maximum cell size is determined by the mobile's available output power.

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Base stations use either omnidirectional or directional antennas.

The antenna of an omnidirectional cell radiates (more or less) an equally strong signal in all horizontal directions, thereby covering a circular area.

The circle's radius can be modified by changing the output power of the base station, which in most cases is done in connection with cell planning

As a rule, maximum cell size is determined by the mobile's available output power.

• Cells : the coverage of a radio channel (s) transmitted by an (omni/sectoral) antenna of a base station

• Multiple Access : the technique used to assign idle traffic channels to calling or called mobiles (there are three variants of MA)

• Base Stations : TRx + antenna

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Figure D.1.2 Hexagonal patterns are easy to work with

Figure D.1.2 shows a system made up of omnidirectional cells. The figure also demonstrates the origination of the well-known hexagonal pattern. Hexagonal patterns are easy to work with: graphically, geometrically and logically. However, since the hexagonal model provides an idealised representation of coverage one must always complement this model with actual coverage measurements.

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Figure D.1.2 Omnidirectional cells

• Hexagonal model provides an idealised representation of coverage.

• Hexagonal patterns are easy to work with: graphically, geometrically and logically.

• However, since the one must always complement this model with actual coverage measurements.

• Omnidirectional cell & Hexagonal patterns

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A base station that uses three directional antennas, where each antenna covers an angle of 120°, has three sector cells around it. Figure D.1.3 illustrates the appearance of the corresponding cell pattern. It is not always necessary to have three sector cells together. Occasionally, one sector cell will suffice; for example, when covering a section of a road or highway.The transmitters of each of the cells have their own frequencies. Cell pattern planning is closely related to the use and reuse of frequencies. (See Chapter 5, Subsection 5.2.1.)

Figure D.1.3 Three sector cells

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Figure D.1.3 Three sector cells

• A BS can have three directional antenna covers an angel of 120o

• To cover a road or highway, one sector is sufficient• Each cell has it’s own frequencies• Cell pattern planning related to frequency reuse attern

• Sectoral cell (directional antenna)

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Radio channels between base stations and mobilesThe mobile telephony service is assigned special operating frequency ranges (which vary depending on the country and the standards employed). These frequency ranges are in turn subdivided into radio channels, commonly 25-30 kHz wide (channel separation). Duplex mode is employed for traffic over radio access, meaning that the base stations and the mobiles must be capable of simultaneous transmission and reception, requiring two frequency ranges sufficiently separated from one another. The separation between them is referred to as the duplex separation; its size, determined by technical factors, varies as a function of the frequency range being used. The combination of two frequencies (or portions of frequencies) constitutes a duplex radio channel. As an example, Figure D.1.4 shows frequency assignment and utilisation for the NMT 450 mobile telephone system.The channels of a mobile network are divided into two primary groups: control channels and traffic channels.

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Figure D.1.4 Frequency range for NMT 450

Every cell employs at least one channel as a control channel, on which the base station continuously transmits an identifying signal that is used by the mobiles to lock into that particular cell. Control channels are also used for paging calls; if the called mobile is in the cell, it will respond over the same (or another) channel. The number of control channels in a cell varies as a function of the access technique employed and the expected call intensity.

After having completed call connection signalling, the mobile is assigned another channel - a traffic channel - for the call. The number of traffic channels in a cell varies with the cell's expected traffic intensity.

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Figure D.1.4 Frequency range for NMT 450

• A mobile telephony service has certain operating frequencies range (regulated by goverment)

• A radio channel commonly need 25-30 kHz of range• Duplex mode is employed to separate a pair of up-link and down-link to

enable simultaneous transmission (full duplex)• An example : NMT 450 has (453-457,5) MHz up-link and (463-467,5) MHz

down-link

• Radio channel(s) between BS - MS• Analog cellular (1 G) FDMA/FDD

• Radio channels of a cell consits of traffic channels and control channel(s)• Proportion of each depend of traffic intensity• Radio channel = physical channel, traffic channel / control channel = logical

channel

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Control channels and traffic channels are also referred to as logical channels. These logical channels are mapped onto physical channels.A physical channel can be a radio broadcasting frequency, a pair of frequencies (including duplex separation) in an analog mobile system or a time slot on a pair of frequencies in a digital mobile system.Traffic channels are addressed in greater detail in Chapter 4, Subsection 4.3.5. Control channels are described in Chapter 7.

Attachment and detachmentAs soon as a mobile is turned on, it establishes contact with the network. It thus has "access" to the network, and the network registers its movements.A user can turn his mobile off occasionally to conserve battery power. Since it would not make much sense to attempt to call an idle mobile, the system includes functionality to keep track of whether the mobile is ON (attachment) or OFF (detachment).

RoamingRegardless of its location, a mobile that is turned on must maintain constant radio contact with the network. Both the network and the mobile include special functionality for this purpose: the roaming function.

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Note : in digital cellular (GSM for istance) an RF divided into 8 time slots FDMA/TDMA, FDD

- Attachement = turn on the power of MS MS estabilishes contact with the network (MS “access” to the network)

- The network register its movement- Detachement = turn off the power (occasionally to conserve battery

power)- The mobile system includes functionality to keep track of of whether

the mobile is ON (attachment) or OFF (detachment)

• Attachement and detachement

• Roaming- Roaming = movement of an MS accordingly of its location (an area at

which an MS that turn on is maintain constant radio contact contact with the network).

- Both the network and the mobile include special functionality for this purpose : the roaming function.

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Location updating and paging - Location areaA terminal in a fixed network is connected to a fixed access point, which is also associated with a subscriber number. Information about this association is stored in the local exchange responsible for the particular access point. If a terminal is moved, it will normally be assigned a new number depending on which local exchange it is moved to. This movability places no demands on the network in terms of routing or connection control.Fixed access points do not exist in the world of mobile networks. When a mobile is called, the network must be able to determine its position, and that requires special intelligence. Registration (or location updating) is the intelligent network function that keeps track of the mobile's position. Paging is the actual search operation performed in all or some of the network cells.

Figure D.1.5 Location area

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Radio resources would be greatly overworked if, for every incoming call, the paging function were activated to locate the position of the called mobile all over the network. The solution lies in forcing the mobiles to report their positions, that is, to register. The question is: How often should a mobile report - upon entering a new cell or less frequently? The size of the area within which the mobile need not register becomes a trade-off between location updating and paging. Updating locations per cell would load the network with too many registrations, while a large area - for example, an MSC service area - might very well load the network with too many "paging assignments". The group of cells in which a mobile need not register is referred to as its location area.The location area can correspond to a BSC service area (as shown in Figure D.1.5) but can also consist of cells from several different BSC service areas located in the same MSC service area.While the use of a traffic channel is related to specific, non-adjacent cells, call channels are a common resource for a given location area. A location area must not be made so large as to allow the number of calls in the area to cause call-channel congestion.Since the registration and paging functions require network intelligence, they are also addressed in Chapter 6, Section 6.2.

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Locating and handoverThe channel used for a call - or for control - must be capable of being switched from cell to cell as the mobile traverses the cells. The system must be able to detect whether or not switching is necessary (normally coinciding with the fact that signal strength has dropped below a given value or the signal-to-noise ratio has become unsatisfactory). This function is referred to as locating.The technical term for actually switching from cell to cell - which preferably occurs without the user noticing it - is hand over.Handover is addressed in greater detail in Chapter 3 (switching aspects) and in Chapter 5 (transmission aspects).

D.1.2 ServicesD.1.2.1 TelephonyD.1.2.2 Data D.1.2.3 Telefax D.1.2.4 Supplementary servicesD.1.2.5 Emergency call servicesD.1.2.6 Messaging services

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Kayaknya sampai sini deh

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The basic idea of mobile services is to offer the moving subscriber the same services that are offered to fixed-network subscribers. Subscriber movement requires sophisticated solutions to maintain service continuity throughout the network. Information on the individual subscriber's access to a specific service and on the status of this service must be transmitted between the exchanges of the mobile network in step with the movement of the subscriber.

D.1.2.1 TelephonyThe most important function of a mobile network is the creation of a good and dependable telephone service. Under favourable radio transmission conditions, the quality of the telephone service is comparable with fixed-network telephony. Digital mobile networks are capable of delivering telephony of varying quality, depending on the voice-coding method employed over radio access. The GSM digital mobile system uses the terms "full-rate coding" (13 kbit/s) and "half-rate coding" (6.5 kbit/s).

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D.1.2.2 DataThe speed normally used by GSM is 9.6 kbit/s, but higher speeds are being developed (see Chapter 2, Subsection 2.4.4).The use of modems enables analog systems to offer data services with bit rates up to 19.2 kbit/s.

D.1.2.3 TelefaxAll larger mobile systems support Group 3 telefax.

D.1.2.4 Supplementary servicesMobile network supplementary services are similar to their counterparts in the fixed network, even if services such as call barring require a greater number of variations. A subscription for official use can be barred so that incoming calls are not accepted; for instance, when the mobile is used on an assignment in another country. This protects the company against the risk of having to pay the high cost of private calls being made from home to the mobile.

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D.1.2.5 Emergency call servicesMany mobile networks offer an emergency call service. All the user has to do in an emergency situation is contact an emergency centre - no knowledge of the telephone numbers of different centres in the area is required. Even black-listed mobiles and mobiles that are ordinarily unable to pass authentication can use this service.

D.1.2.6 Messaging servicesMessaging services are particularly important in increasing accessibility in a PLMN, because terminals can be turned off or can be in an area where buildings or hills create radio shadows. Voice mail, telefax and short message service (SMS) are examples of messaging services.SMS allows callers to leave short text messages (GSM allows up to 160 characters). A message that cannot be delivered immediately will be stored in a short-message service centre until the mobile can be reached.

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D.1.4 Security

Networks that utilise radio communications are especially sensitive to unauthorised use of terminals and to tapping along the radio path. Mobile networks therefore require the institution of special security measures. Both the user and the network operator must be protected against any unauthorised intrusion by a third party. This protection can either consist of a supplementary service selected by the user; for example, a smart card (with a personal code) for systems that use such cards, or of various network functions such as encryption and authentication.The following functions have been enhanced to protect the network:

authentication system that protects against unauthorised use of the network's services;

encryption to protect against unauthorised tapping of radio access; terminal identification that protects against the use of stolen mobiles;

and temporary telephone numbers that protect against unauthorised

access to a mobile's identity.

Security is addressed in greater detail in Chapter 6, Section 6.4.

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D.1.5 Terminals

Figure D.1.6 The most important key functions on a mobile telephone

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The development of the mobile telephone has been characterised by two dominant trends: size reduction and increased intelligence. Both trends have the same origins, namely the endeavour to make components ever smaller and more advanced and the constant development and refinement of the design. Also, the mobile telephone has already passed three initial phases: the car-mounted model, the portable model and the current pocket model.In digital networks, the mobile assists in the handover process by continuously measuring base station signal strength and then reporting the measured values to the network. The mobile's ability to control the handover process (mobile-controlled handover) will mark the next step in its development (see Chapter 5, Section 5.4).A pocket telephone has a number of facilities. The most common are:

alphanumeric display; memory for many abbreviated numbers; signal strength indicator; battery indicator; and electronic lock.

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The mobile office is a concept that has developed in step with the increase in teleworking. In addition to the mobile telephone, an important tool is the laptop PC which can be equipped with a modem card. The laptop can then be connected directly to a mobile telephone's modem port. The portable fax is another terminal that can be used over a PLMN.The development towards more advanced terminals as described in Section 1.3.1 is illustrated in Figure D.1.7.

Figure D.1.7 The development of PLMN terminals from simple mobile telephones into intelligent mobile terminals

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D.2.2.2 GSM voice coding

The following is a brief description of the voice coding employed by the mobiles of a GSM system. A number of electronic filters are used to simulate the operation of the human organs of speech and to extract the vocal cords' original frequencies, called excitation sequences.

Figure D.2.2 GSM voice coding

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Information about the filter characteristics and the excitation sequences is sent to the receiver, where it is used to reproduce the original signal. Figure D.2.2 and Figure D.2.3 illustrate the principle.

The first step of the analysis is performed through linear predictive coding (LPC). The LPC analysis unit is designed as the inverse of the speech organs' filtering model. When a 20 ms voice block from the segmentation unit is allowed to pass through the filter for LPC analysis, this filter will deliver the excitation sequence for the sample.

Since two consecutive blocks have similar excitation sequences, the difference between them is calculated with the long-term prediction (LTP) methods.

The resulting excitation difference passes through a low-pass filter and is then input to the residual pulse excitation (RPE) grid selection unit, which is a waveform coder (similar to the one used in PCM). The filtered excitation difference is sampled and every third sample is coded. The resulting bit stream is 9.4 kbit/s.

The RPE bit stream and the LPC and LTP parameter values are transferred to the receiver, where the original speech is reproduced through a reverse process.

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The LPC and LTP parameter values generate 3.6 kbit/s, making the total bit stream from the voice coder 13 kbit/s (260 bits per 20 ms sample).

Figure D.2.3 The original frequencies (excitation sequences) of the vocal cords are extracted using LPC/LTP analysis

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D.2.3 Network elements

D.2.3.1 Network elements for (user) trafficD.2.3.2 Network elements as databases D.2.3.3 Network elements for additional network intelligence D.2.3.4 Network elements for operation and maintenanceD.2.3.5 Network elements for signallingD.2.3.6 Network elements for transport and transmission

For the most part, the same types of network element are found in all mobile networks, even if they are named differently in different standards. In Figure D.2.4,we use GSM as an example.

36Figure D.2.4 GSM network elements

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