cross linker for gsm timers

8/10/2019 Cross Linker for GSM Timers

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:: Cross Linker for GSM Timers ::

Afraz – 10060001 & Harris – 10060021

:: Introduction ::

In GSM networks, call setup and retainibility is divided into various phases, performed by

communication of various entities inside the network. These call phases are monitored by timers against

each such phase. Expiry of these timers can result in dropped calls or worse voice quality, meaning

degraded customer experience. There are numerous timers defined for call phases, spread across

network entities, to ensure proper communication between these nodes.

:: Our Project ::

The distributed location of these timers makes them extremely difficult to track. Various call flow

scenario are explained in text, but that is done with minimal set of timers. Our aim is to make a cross

linker for these timers, which can help track these timers and also capture the relation between timers,

even if they are spread across the whole network.

This information can help optimization teams to better comprehend problems in network and propose

wholesome approaches to eradicate problems and not symptoms.

We would focus mainly on timers of BSS end, and some from A interface as well, covering till OSI layer 3.

i.e.; Um interface, Abis, Ater and A interface.



Layer 3: The third layer of the GSM signaling protocol is divided into three sub-layers:

o

Radio Resource management (RR)

o

Mobility Management (MM) and

o

Connection Management (CM).

:: The MS to BTS Protocols ::

The RR layer oversees the establishment of a link, both radio and fixed, between the MS and the MSC.

The main functional components involved are the MS, the BSS, and the MSC. The RR layer is concerned

with the management of an RR-session, which is the time that a mobile is in dedicated mode, as well as

the configuration of radio channels, including the allocation of dedicated channels.

The MM layer is built on top of the RR layer and handles the functions that arise from the mobility of the

subscriber, as well as the authentication and security aspects. Location management is concerned with

the procedures that enable the system to know the current location of a powered-on MS so that

incoming call routing can be completed.

The CM layer is responsible for CC, supplementary service management, and Short Message Service

(SMS) management. Each of these may be considered as a separate sublayer within the CM layer. Other

functions of the CC sublayer include call establishment, selection of the type of service (including

alternating between services during a call), and call release.

:: BSC Protocols ::

After the information is passed from the BTS to the BSC, a different set of interfaces is used. The Abis

interface is used between the BTS and BSC. At this level, the radio resources at the lower portion of

Layer 3 are changed from the RR to the Base Transceiver Station Management (BTSM). The BTS

management layer is a relay function at the BTS to the BSC.

The RR protocols are responsible for the allocation and reallocation of traffic channels between the MS

and the BTS. These services include controlling the initial access to the system, paging for MT calls,

handover of calls between cell sites, power control, and call termination. The RR protocols provide the

procedures for the use, allocation, reallocation, and release of the GSM channels. The BSC still has some

radio resource management in place for the frequency coordination, frequency allocation, and the

management of the overall network layer for the Layer 2 interfaces.

From the BSC, the relay is using SS7 protocols so the MTP 1-3 is used as the underlying architecture, and

the BSS mobile application part or the direct application part is used to communicate from the BSC to

the MSC.



:: MSC Protocols ::

At the MSC, the information is mapped across the A interface to the MTP Layers 1 through 3 from the

BSC. Here the equivalent set of radio resources is called the BSS MAP. The BSS MAP/DTAP and the MM

and CM are at the upper layers of Layer 3 protocols. This completes the relay process. Through thecontrol-signaling network, the MSCs interact to locate and connect to users throughout the network.

Location registers are included in the MSC databases to assist in the role of determining how and

whether connections are to be made to roaming users.

Each user of a GSM MS is assigned an HLR that is used to contain the user's location and subscribed

services. A separate register, the VLR, is used to track the location of a user. As the users roam out of the

area covered by the HLR, the MS notifies a new VLR of its whereabouts. The VLR in turn uses the control

network (which happens to be based on SS7) to signal the HLR of the MS's new location. Through this

information, MT calls can be routed to the user by the location information contained in the user's HLR.

:: Call Scenarios ::

Attached Document explains the exchange of message between different entities in a call setup. It is for

a MTC case. For MOC, paging phase is removed and the rest is same.



:: GSM Timers ::

We have tried to gather almost all the timers that are used in the GSM system. However, if missed, the

list below still provides quite significant information regarding all the important timers that are used

within certain scenarios in the GSM system. Also, we have added signaling diagrams for situations and

scenarios where these timers are used. The table below also gives information regarding the figure

number among the figures to follow to correspond to that particular situation/scenario.

Timer Name Timer Description Used in Figure

T4 Supervises the sending of the RESET message from the BSC to the MSC Figure2

T7 Supervises the frequency of the HANDOVER_REQUIRED message. Figure3

T8 Supervises the external handover procedure in the serving BSC Figure4

T10 Supervises the channel assignment procedure Figure5

T13 Serves as a guard period for a local call clearing procedure Figure6

T17 Supervises the overload procedure in MSC overload Figure7

T18 Supervises the overload procedure Figure8

T3101 Supervises the immediate assignment procedure Figure9

T3103 Supervises the internal handover procedure Figure10

T3105_d Repeats the PHYSICAL_INFORMATION message during asynchronous handover on SDCCH Figure11

T3105_f Repeats the PHYSICAL_INFORMATION message during asynchronous handover on TCH Figure12

T3107 Supervises the assignment procedure and the internal intra cell handover Figure13

T3109 Supervises the channel release procedure Figure14

T3111 Delays the channel deactivation after disconnection of the main signaling link. Its

purpose is to let some time for possible repetition of the disconnection

Figure15

T3121 Supervises the external handover procedure from GMS to WCDMA RAN in the serving BSC Figure16

T3122 Delays the sending of another channel request Figure17

T3168 Used on the MS side to define when to stop waiting for a Packet Uplink Assignment

message after sending of a Packet Resource request message

-

T3192 Used on the MS side when the mobile station has received all of the RLC data Blocks -

T9101 Supervises the reception of the SCCP_RELEASED (RLSD) message Figure18

T9103 Supervises the channel activation procedure Figure19

T9104 Supervises the clear command that comes from the MSC Figure20

T9105 Supervises the SCCP connection procedure Figure21

T9108 Supervises the physical context request procedure Figure22

T9113 Supervises the external handover in the target BSC Figure23



:: Call Flow Diagrams ::

Figure 2-T4 activation




Figure 11-T3105_d activation

Figure 12-T3105_f activation




Figure 25 - T7 Expiry




Figure 31 -T3105_d Expiry

Figure 32 - T3105_f Expiry



:: Everyday Telecommunications Scenarios: QoS Perspective ::

The signaling diagrams above present the functionality of timers, whether successfully stopped or

expired, in different scenarios according to their use. However, the main issue remains that the

scenarios presented above are not complete. For example, they do let us know about the activation /

expiration of certain timers in certain scenarios such as the supervision of transmission of the RESET

message from the BSC to the MSC, or the supervision of the channel assignment procedure.

However, a full scenario would include a combination of the procedures presented in these signaling

diagrams. For example, take the scenario of a mobile originating/mobile terminating call. It not only

requires the channel assignment procedure at the time of voice/signal transmission but would also

require supervision of timers at the time of channel establishment, at the time of channel activation, at

the time of authentication etc.

These types of scenarios are a daily routine for QoS engineers. However, it is difficult to maintain arecord of which timer is activated at a certain time, or if perhaps there has been some failure, then

which of the timers had failed. This is because a scenario would be a combination of many procedures

and figuring out the timer(s) which had expired from a series of timers used during the procedure would

be highly cumbersome.

A combined signaling diagram would be the most helpful tool for these estimations to counter the issue

quickly. It would present a picture of all the timers used in the scenario and based on the actions taken,

this cross linker for timers would help in identifying the erroneous procedure.

Presented below are two such scenarios i.e. a handover failure scenario and a Mobile Call’s life cycle

which includes the steps taken to release the channel after call establishment after a mobile station has

suddenly disconnected i.e. in cases where the mobile station’s battery becomes dead.



:: Handover Failure – Signaling Diagram ::



:: Call Drop – Signaling Diagram ::

cross linker for gsm timers

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