Section 4 GSM Signaling BSSMAP BSS management messages (BSSMAP) between MSC and BSS (BSC/ BTS), which are necessary for resource management, handover control, paging order etc. The BSSMAP messages can either be connection less or connection oriented.
Section 4 GSM Signaling Initial MS Messages These messages are passed unchanged through BSS, but BSS analyses part of the messages and is not transparent like DTAP messages. Between BSS and MSC, the initial MS message is transferred in the layer 3 information in the BSSMAP. The Initial MS messages are: CM Request Location update request Paging response
Section 4 GSM Signaling LAPDm Link Access Procedures on the Dm channel (LAPDm) is the layer 2 protocol used to convey signaling information between layer 3 entities across the radio interface. Dm channel refers to the control channels, independent of the type including broadcast, common or dedicated control channels. LAPDm is based on the ISDN protocol LAPD, used on the Abis interface. Due to the radio environment, the LAPD protocol can not be used in its original form. Therefore, LAPDm segments the message into a number of shorter messages.
Section 4 GSM Signaling Data exchanged between the data link layer and the physical layer is 23 octets for BCCH, CCCH, SDCCH and FACCH. For SACCH only, 21octets are sent from layer 2 to layer 1. LAPDm functions include: LAPDm provides one or more data link connections on a Dm channel. Data Link Connection Identifier (DLCI) is used for discriminating between data link connections. It allows layer 3 message units be delivered transparently between layer 3 entities. It provides sequence control to maintain the sequential order of frames across the data link connections.
Section 4 GSM Signaling LAPDm Frame Structure info length command address N(R) P/F N(S) 0 0 0 1 SAPI CR 1
Section 4 GSM Signaling Sequence Number: N(S) send sequence number of the transmitted frame. N(R) is receive sequence number. P/F : All frames contain the Poll/Final bit. In command frames, the P/F bit is referred to as the P bit. In response frames, the P/F bit is referred to as the F bit. Service Access Point Identifier: Service Access Points (SAPs) of a layer are defined as gates through which services are offered to an adjacent higher layer.sap is identified with the Service Access Point Identifier (SAPI). SAPI = 0 for normal signaling of DTAP & RR SAPI = 3 for short message services
Section 4 GSM Signaling LAPDm has no error detection and correction. It is used in two modes: Acknowledge & Unacknowledged and having a different structure for both.
Section 4 GSM Signaling LAPD All signaling messages on the Abis interface use the Link Access Procedures on the D-channel. (LAPD protocol). LAPD provides two kinds of signaling: unacknowledged information acknowledged information LAPD link handling is a basic function to provide data links on the 64 kbps physical connections between BSC and BTS.
Section 4 GSM Signaling Links are provided for operation and maintenance (O&M) of the links, for O&M of the BTS equipment and for transmission of layer 3 Abis messages. Each physical connection can support a number of data links (logical connections). On each physical connection each data link is identified by a unique TEI/SAPI
Section 4 GSM Signaling LAPD has three sub signaling channels 1. RSL (Radio signaling Link), deals with traffic management, TRX signaling. 2. OML (Operation & Maintenance Link), serves for maintenance related info and transmission of traffic statistics. 3. L2M (Layer 2 Management), used for management of the different signaling on the same time slot.
Section 4 GSM Signaling LAPD Frame Structure Flag FCS info length command address Flag N(R ) P/F N(S) 0 TEI 1 SAPI CR 0
Section 4 GSM Signaling LAPD Frame structure is made up of: Flag: Indicates the beginning and end of each frame unit. Flag has a pattern of 01111110. FCS: Frame Check Sequence, provides the error checking for the frame. If error is found frame will be retransmitted. Command: It has two types of structure, in acknowledge mode it has N(S) and N(R ). N(S) is a sequence number of frame sent and N(R ) is the sequence number of the frame expected to receive next.
Section 4 GSM Signaling C/R: This bit indicates whether it is command or response. P/F: In command frames, the P/F bit is referred to as the P bit and the other end transmits the response by setting this bit to F. TEI: Terminal Endpoint Identifier, is a unique identification of each physical entity on either side like each TRX within a BTS have a unique TEI.
Section 4 GSM Signaling SAPI: Service Access Point Identifier, used to identify the type of link. SAPI = 0 for RSL SAPI = 62 for OML SAPI = 63 for L2ML Each LAPD link is identify by SAPI/TEI pair.
Section 4 GSM Signaling Exercise Q1. Name the protocol which is transparent to BSS and what information is used to transfer on this protocol? Q2. Name the protocols used between Mobile and BTS BTS and BSC BSC to MSC MSC to PSTN
Call Handling Section 5 Call Handling
Section 5 Call Handling Objective The Trainee will be able to understand: Basic call concepts Location Area concepts Call setup in different scenarios SMS routing Intra and Inter MSC handovers
Section 5 Call Handling Introduction Call setup is required to establish communication between a Mobile Station and Network Subsystem (NSS). The NSS is responsible for establishing a connection with the corresponded. Different types of calls require different teleservices. For the optimum utilization of the network, different location areas will be defined to reduce the paging load on the system.
Section 5 Call Handling Basic Types of Calls There are three basic types of call: 1. Mobility Management calls: Such as Location update. These are used to collect information about the MS and only signaling channels are used. 2. Service calls: Such as SMS. These calls passes very small information, therefore signaling channels are used. 3. User traffic calls: Such as speech or data. Large amount of data is exchanged hence traffic channels are used.
Basic Call Setup Section 5 Call Handling
Section 5 Call Handling Subscriber on switch A places a call to a Subscriber on switch B: 1. Switch A analyzes the dialed digits and determines that it needs to send the call to switch B. 2. Switch A selects an idle trunk between itself and switch B and formulate IAM 3. STP W receives a message, inspects its routing label, and determines that it is to be routed to switch B. 4. Switch B receives the message. On analyzing the message, it determines that it serves the called number and that the called number is idle. 5. Switch B formulates an address complete message (ACM), which indicates that the IAM has reached its proper destination.
Section 5 Call Handling 6. Switch B picks one of its links and transmits the ACM over the link for routing to switch A. 7. STP X receives the message, inspects its routing label, and determines that it is to be routed to switch A. 8. On receiving the ACM, switch A connects the calling subscriber 9. When and/or if the called subscriber picks up the phone, switch B formulates an answer message (ANM), 10. Switch B selects the same link it used to transmit the ACM 11. STP X recognizes that the ANM is addressed to switch A and forwards it over link
Section 5 Call Handling 12. Switch A ensures that the calling subscriber is connected and conversation can take place. 13. If the calling subscriber hangs up first switch A will generate a release message (REL) addressed to switch B. 14. STP W receives the REL 15. Switch B receives the REL, disconnects the trunk from the subscriber line, returns the trunk to idle status. 16. STP X receives the RLC, determines that it is addressed to switch A. 17. On receiving the RLC, switch A idles the identified trunk.
Section 5 Call Handling Location Registration When the mobile is turned on first time in the network, it has no indications in its data about an old Location Area Identity. MS immediately inform the network and request for the Location Update to the MSC/ VLR. After registration MSC/ VLR will consider the MS as active and marked the MS as attached.
Section 5 Call Handling Location Update When the MS moves from one LA to another, it has to register. This registration is performed when the MS detects another LAI than the one stored. This is called location updating. This function provides mobile subscribers with uninterrupted service throughout the GSM coverage area so that they can: Be called on a permanent directory number irrespective of their location at the time of call. Access the network whatever their position
Section 5 Call Handling There are four different types of location updating: Normal IMSI detach IMSI attach Periodic registration
Section 5 Call Handling Normal Update The Base Transceiver Station (BTS) of every cell continually transmits the Location Area Identity (LAI) on BCCH. If MS detects LAI is different from the one stored in the SIM-card, it is forced to do a location update. If the mobile subscriber is unknown in the MSC/VLR (new subscriber) then the new MSC/VLR must be updated, from the HLR, with subscriber information. It also consider the case of the location update timer runs out.
Section 5 Call Handling
Section 5 Call Handling 1. The MS requests a location update to be carried out in the new MSC/VLR. The IMSI is used to identify the MS. 2. In the new MSC/VLR, an analysis of the IMSI number is carried out. The result of this analysis is a modification of the IMSI to a Mobile Global Title (MGT) 3. The new MSC/VLR requests the subscriber information for the MS from the HLR. 4. The HLR stores the address of the new MSC/VLR and sends the subscriber data to the new MSC/VLR.
5. The HLR also orders the old serving MSC/VLR to cancel all information about the subscriber since the MS is now served by another MSC/VLR. 6. When the new MSC/VLR receives the information from the HLR, it will send a location updating confirmation message to the MS. Section 5 Call Handling