2G Layers,Power & Handover in Telecom

COMMUNICATION PROCESS

•        Each layer has its own specified functions and provides specific services for the layers above
•        It is important to define the interfaces between different layers and the functions within each layer.
•        The way a function is realized within a layer is not predicted.
•        Logically, the communication between functions always takes place on the same level according to the protocols for that level.

•        Only functions on the same level can “talk to each other”.
•        In the transmitting system, the protocol for each layer adds information to the data received from the layer above.
•        The addition usually consists of a header and/or a trailer.
•        In the receiving system, the additions are used, for example, to identify bits or data fields carrying information for that specific layer only.
•        These fields are decoded by layer functionality and are removed when delivering the message to the applications orlayers above.
•        When the data reaches the application layer on the receiving side, it consists of only the data that originated in the application layer of the sending system.
•        Logically, each layer communicates with the corresponding layer in the other system.
•        This communication is called Peer-to-Peer communication and is controlled by the layer’s protocol.

DESCRIPTION OF LAYERS

Application Layer

•        This layer provides services for support of the user’s application process and for control of all communication between applications.
•        Examples of layer 7 functions are file transfer, message handling, directory services, and operation and maintenance.
Presentation Layer
•        This layer defines how data is to be represented, that is, the syntax.
•        The presentation layer transforms the syntax used in the application into the common syntax needed for the communication between applications.
•        Layer 6 contains data compression.
Session Layer
•        This layer establishes connections between presentation layers in different systems.
•        It also controls the connection, the synchronization and the disconnection of the dialogue.
•        It allows the presentation layer to determine checkpoints, from which the retransmission will start when the data transmission has been interrupted.
Transport Layer
•        This layer guarantees that the bearer service has the quality required by the application in question.
•        Examples of functions are error detection and correction (end-to-end), and flow control.
•        The transport layer optimizes the data communication, for example by multiplexing or splitting data streams before they reach the network.
Network Layer
•        The basic network layer service is to provide a transparent channel.
•        This means that the application requesting a channel ignores network problems and the related signal exchange because that is the task of the lower levels.
•        It just requires an open channel, transparent for the transmission of data, between transport layers in different systems.
•        The Network Layer establishes, maintains, and releases connections between the nodes in the network and handles addressing and routing of circuits.
Data Link Layer
•        This layer provides an essentially error-free point-to-point circuit between network layers.
•        The layer contains resources for error detection, error correction, flow control, and retransmission.
Physical Layer
•        This layer provides mechanical, electrical, functional, and procedural resources for activating, maintaining, and blocking physical circuits for the transmission of bits between data link layers.
•        The physical layer contains functions for converting data into signals compatible with the transmission medium.
•        For the communication between only two exchanges, layers 1 and 2 are sufficient.
For the communication between all exchanges in the network, layer 3 must be added because it provides addressing and routing.

CALL FLOW

POWER CONTROL

RF POWER CONTROL

•        RF power control is employed to minimise the transmit power required by MS or BS while maintaining the quality of the radio links.
•        By minimising the transmit power levels, interference to co-channel users is reduced.
•        Power control is implemented in the MS as well as the BSS.
•        Power control on the Uplink also helps to increase the battery life.
POWER CONTROL IN THE MS
•        The RF power level employed by the MS is indicated by means of the 5 bit TXPWR field sent either in the layer 1 header of each downlink SACCH message block, or in a dedicated signalling block.
•        The MS confirms the power level that it is currently employing by setting the MS_TXPWR_CONF field in the uplink SACCH L1 header to its current power setting. The value of this field is the power setting actually used by the mobile for the last burst of the previous SACCH period.
•        The MS employs the most recently commanded RF power level appropriate to the channel for all transmitted bursts on either a TCH (including handover access burst), FACCH,SACCH or SDCCH.
•        When accessing a cell on the RACH (random access) and before receiving the first power command during a communication on a DCCH or TCH (after an IMMEDIATE ASSIGNMENT), the MS uses either the power level defined by the MS_TXPWR_MAX_CCH parameter broadcast on the BCCH of the cell, or the maximum TXPWR of the MS as defined by its power class, whichever is the lower.
TIMING OF POWER CHANGE BY MS
•        Upon receipt of a command on the SACCH to change its RF power level (TXPWR field) the MS changes to the new level at a rate of one nominal 2dB power step every 60ms (13 TDMA frames), i.e. a full range change of 15 steps should take about 900ms .
•        The change commences at the first TDMA frame belonging to the next reporting period . The MS changes the power one nominal 2 dB step at a time, at a rate of one step every 60 ms following the initial change, irrespective of whether actual transmission takes place or not.
•        In case of channel change the commanded power level is applied on the new channel immediately.
“Power control at BSS is optional.
The range over which the BS is capable of reducing its RF output power from its maximum level is nominally 30dB, in 15 steps of nominally 2dB.”

CELL SELECTION AND RE-SELECTION

•        In Idle mode (i.e. not engaged in communicating with a BS), an MS will do the cell selection and re-selection procedures .
•        The procedures ensure that the MS is camped on a cell from which it can reliably decode downlink data and with which it has a high probability of communications on the uplink. The choice of cell is determined by the path loss criterion. Once the MS is camped on a cell, access to the network is allowed.
•        An MS is said to be camped on a cell when it has determined that the cell is suitable and stays tuned to a BCCH + CCCH of that cell. While camped on a cell, an MS may receive paging messages or under certain conditions make random access attempts on a RACH of that cell, and read BCCH data from that cell.
•        The MS will not use the discontinuous reception (DRX) mode of operation (i.e. powering itself down when it is not expecting paging messages from the network) while performing the selection and reselection algorithm. However use of powering down is permitted at all other times in idle mode.
•        For the purposes of cell selection and reselection, the MS is required to maintain an average of received signal strengths for all monitored frequencies. These quantities termed the "receive level averages” is the averages of the received signal strengths measured in dBm.
•        The cell selection and reselection procedures make use of the "BCCH Allocation" (BA) list. There are in two BA lists which may or may not be identical, depending on choices made by the PLMN operator.
•        (i)   BA (BCCH) - This is the BA sent in System Information Messages on the BCCH. It is the list of BCCH carriers in use by a given PLMN in a given geographical area. It is used by the MS in cell selection and reselection.
•        (ii) BA (SACCH) - This is the BA sent in System Information Messages on the SACCH and indicates to the MS which BCCH carriers are to be monitored for handover purposes.
•        When the MS goes on to a TCH or SDCCH, it starts monitoring BCCH carriers in BA (BCCH) until it gets its first BA (SACCH) message.
CELL SELECTION - NO BCCH DATA AVAILABLE
•        The MS searches all 124 RF channels in the GSM system, takes readings of RSS on each RF channel, and calculate the received level average for each.
•        The averaging is based on at least five measurement samples per RF carrier spread over 3 to 5secs.
•        The MS tunes to the carrier with the highest average RSS & determines whether or not this carrier is a BCCH carrier.
•        If it is a BCCH carrier, the MS attempts to synchronise to this carrier and read the BCCH data. The MS camps on the cell provided it can successfully decode the BCCH data and this data indicates that it is part of the selected PLMN, that the cell is not barred (CELL_BAR_ACCESS = 0) & that the parameter C1 is greater than 0.
•        If the cell is part of the selected PLMN but is barred or C1 is less than zero, the MS uses the BCCH Allocation obtained from this cell and subsequently only searches these BCCH carriers. Otherwise the MS tune to the next highest carrier and so on.
•        CELL_BAR_ACCESS may be employed to bar a cell that is only intended to handle handover traffic etc. For example of this could be an umbrella cell which encompasses a number of microcells.
•        If at least the 30 strongest RF channels have been tried, but no suitable cell has been found, provided the RF channels which have been searched include at least one BCCH carrier, the available PLMN's shall be presented to the user, otherwise more RF channels shall be searched until at least one BCCH carrier is found.
•        30 RF channels are specified to give a high probability of finding all suitable PLMN's, without making the process take too long.
CELL SELECTION - BCCH INFORMATION AVAILAIBLE
•        The MS stores the BCCH carriers in use by the PLMN selected when it was last active in the GSM network. A MS may also store BCCH carriers for more than one PLMN which it has selected previously (e.g. at national borders or when more than one PLMN serves a country).
•        If an MS includes a BCCH carrier storage option it searches only for BCCH carriers in the list.
•        If an MS decodes BCCH data from a cell of the selected PLMN but is unable to camp on that cell, the BA of that cell is examined. Any BCCH carriers in the BA which are not in the MS's list of BCCH carriers to be searched is added to the list.
•        If no suitable cell has been found after all the BCCH carriers in the list have been searched, the MS acts as if there were no stored BCCH carrier information. Since information concerning a number of channels is already known to the MS, it may assign high priority to measurements on those of the 30 strongest carriers from which it has not previously made attempts to obtain BCCH information, and omit repeated measurements on the known ones.
Monitoring of Received Level and BCCH data
•        In Idle Mode an MS continues to monitor all BCCH carriers as indicated by the BCCH Allocation .
•        A running average of received level in the preceding 5 to 60 seconds is be maintained for each carrier in the BCCH Allocation.
•        For the serving cell receive level measurement samples is taken at least for each paging block of the MS and the receive level average is determined using samples collected over a period of 5 s or five consecutive paging blocks of that MS, whichever is the greater period.
Monitoring of Received Level and BCCH data
•        At least 5 received level measurement samples are required per receive level average value. New sets of receive level average values is calculated as often as possible.
•        The same number of measurement samples is taken for all non serving cell BCCH carriers, and the samples allocated to each carrier is as far as possible uniformly distributed over each evaluation period.
•        The list of the 6 strongest carriers is updated at least every minute and may be updated more frequently.
•        In order to minimise power consumption, MSs that employ DRX (i.e. power down when paging blocks are not due) monitor the signal strengths of non-serving cell BCCH carriers during the frames of the Paging Block that they are required to listen to. Received level measurement samples can thus be taken on several non-serving BCCH carriers and on the serving carrier during each Paging Block.
•        The MS includes the BCCH carrier of the current serving cell (i.e. the cell the MS is camped on) in this measurement routine.
•        The MS has to decode the full BCCH data of the serving cell at least every 30 seconds.
•        The MS attempts to decode the BCCH data block that contains the parameters affecting cell reselection for each of the 6 strongest non-serving cell BCCH carriers at least every 5 minutes.
•        When the MS recognizes that a new BCCH carrier has become one of the 6 strongest, the BCCH data shall be decoded for the new carrier within 30 seconds.
•        The MS attempts to check the BSIC for each of the 6 strongest non serving cell BCCH carriers at least every 30 seconds, to confirm that it is monitoring the same cell.
•        If a change of BSIC is detected then the carrier is treated as a new carrier and the BCCH data redetermined.
•        When requested by the user, the MS monitors the 30 strongest GSM carrier to determine, within 15 seconds, which PLMN's are available. This monitoring is done so as to minimise interruptions to the monitoring of the PCH.
CALL RE-ESTABLISHMENT
•        In the event of a radio link failure, call re-establishment may be attempted if it is enabled in the database.
•        The received level measurement samples taken on surrounding cells and on the serving cell BCCH carrier in the last 5 seconds is averaged, and the carrier with the highest average received level which is part of a permitted PLMN is taken.
•        A BCCH data block containing the parameters affecting cell selection is read on this carrier.
•        If the parameter C1 is greater than zero, it is part of the selected PLMN, the cell is not barred, and call re-establishment is allowed, call re-establishment is attempted on this cell.
•        If the above conditions are not met, the carrier with the next highest average received level is taken, and the MS repeats the above procedure.
•        If the cells with the 6 strongest average received level values are tried but cannot be used, the call re-establishment attempt is abandoned.

HANDOVER

A hard handover is one in which the channel in the source cell is released and only then the channel in the target cell is engaged. Thus the connection to the source is broken before or 'as' the connection to the target is made—for this reason such handovers are also known as break-before-make. Hard handovers are intended to be instantaneous in order to minimize the disruption to the call. A hard handover is perceived by network engineers as an event during the call. It requires the least processing by the network providing service. When the mobile is between base stations, then the mobile can switch with any of the base stations, so the base stations bounce the link with the mobile back and forth. This is called ping-ponging.

A soft handover is one in which the channel in the source cell is retained and used for a while in parallel with the channel in the target cell. In this case the connection to the target is established before the connection to the source is broken, hence this handover is called make-before-break. The interval, during which the two connections are used in parallel, may be brief or substantial. For this reason the soft handover is perceived by network engineers as a state of the call, rather than a brief event. Soft handovers may involve using connections to more than two cells: connections to three, four or more cells can be maintained by one phone at the same time. When a call is in a state of soft handover, the signal of the best of all used channels can be used for the call at a given moment or all the signals can be combined to produce a clearer copy of the signal. The latter is more advantageous, and when such combining is performed both in the downlink (forward link) and the uplink (reverse link) the handover is termed as softer. Softer handovers are possible when the cells involved in the handovers have a single cell site

The GSM handover process uses a mobile assisted technique for accurate and fast handovers, in order to: Maintain the user connection link quality. Manage traffic distribution

• The overall handover process is implemented in the MS,BSS & MSC.Measurement of radio subsystem downlink performance and signal strengths received from surrounding cells, is made in the MS.These measurements are sent to the BSS for assessment.

The BSS measures the uplink performance for the MS being served and also assesses the signal strength of interference on its idle traffic channels.Initial assessment of the measurements in conjunction with defined thresholds and handover strategy may be performed in the BSS. Assessment requiring measurement results from other BSS or other information resident in the MSC, may be perform. in the MSC.
The MS assists the handover decision process by performing certain measurements.
•        When the MS is engaged in a speech conversation, a portion of the TDMA frame is idle while the rest of the frame is used for uplink (BTS receive) and downlink (BTS transmit) timeslots.
•        During the idle time period of the frame, the MS changes radio channel frequency and monitors and measures the signal level of the six best neighbor cells.
•        Measurements which feed the handover decision algorithm are made at both ends of the radio link.
At the MS end, measurements are continuously signalled, via the associated control channel, to the BSS where the decision for handover is ultimately made.
•        MS measurements include:
•        Serving cell downlink quality (bit error rate (BER) estimate).
•        Serving cell downlink received signal level, and six best neighbor cells downlink received signal level.
•        The MS also decodes the Base Station ID Code (BSIC) from the six best neighbor cells, and reports the BSICs and the measurement information to the BSS.
•        The BTS measures the uplink link quality, received signal level, and MS to BTS site distance.
•        The MS RF transmit output power budget is also considered in the handover decision.
•        If the MS can be served by a neighbor cell at a lower power, the handover is recommended.
•        From a system perspective, handover may be considered due to loading or congestion conditions. In this case, the MSC or BSC tries to balance channel usage among cells.
During the conversation, the MS only transmits and receives for one eighth of the time, that is during one timeslot in each frame.
During its idle time (the remaining seven timeslots), the MS switches to the BCCH of the surrounding cells and measures its signal strength.
•        The signal strength measurements of the surrounding cells, and the signal strength and quality measurements of the serving cell, are reported back to the serving cell via the SACCH once in every SACCH multiframe.
•        This information is evaluated by the BSS for use in deciding when the MS should be handed over to another traffic channel.
•        This reporting is the basis for MS assisted handovers.