GSM NETWORK ELEMENTS

MS (Mobile Station)

·         Responsible for voice encoding and transmition

·         Responsible for the frequency & time synchronization of the radio channel

·         Responsible for voice encryption – decryption

·         Capable of metering the power of the cell which is useful for HANDOVER function

·         Contains the IMEI (International Mobile Equipment ID) 

MS contains a card which is called SIM (Subscriber ID Module)

SIM is a portable smart card with memory that stores:

§  IMSI   (International Mobile Subscriber ID)

§  Ki authentication parameter

§  A3 & A8 authorization algorithms

§  Kc encryption key

§  TMSI  (Temporary Mobile Subscriber ID)

§  LAI (Location Area ID)

§  RAI (Routing Area ID)

BSS (Base Station SubSystem) = BTS + BSC

BTS (Base Transmitter Station)

·         Handles the radio interface

·         Receives data transcoding

·         Responsible for voice encryption – decryption

·         Responsible for “up Line” radio channel  power measurement

BSC (Base Station Controller)

·         Provides control functions & physical links between BTS and MSC

·         Responsible of monitoring and controlling BTS

·         Responsible of radio channel management

·         Responsible of switching between BTS and MSC

NSS (Network Switching Subsystem) = MSC + GMSC +HLR + VLR + AuC + EiR + SCP

MSC (Mobile switching Centre)

·         Responsible for switching between PLMN and PSTN

·         Responsible for MS “Handover” among BSCs , as well as allocating – deallocating radio resources from Old to New BSC

·         Responsible of Paging function for incoming call to Mobile Network

·         Provides the Billing records

GMSC (Gateway Mobile switching Centre)

The Gateway MSC (G-MSC) is the MSC that interfaces with the PSTN. All mobile to mobile calls and PSTN to mobile calls are routed through a G-MSC.

HLR (Home Location Register)

·         Stores subscriber’s information of all Subs which are registered in this geographic area

·         Stores IMSI

·         Stores MSISDN (Cellular number of the subscriber)

·         Stores VLR address

·         Subscription’s information (services)

·         Refers to authentication centre for getting “Ki” which is the authorization key

·         HLR called upon by MSC when a call comes from public (land line) to cellular network

VLR (Visitor Location Register)

·         Keeps the database that contains subscriber’s parameter and location information for every mobile subscriber which is in this geographical area that is being controlled by this VLR

·         IMSI

·         Copy of HLR subscriber’s information

·         Generates a TMSI for all subscribers which are located in this area.

·         Contains LAC

·         Provides the necessary data to MSC for routing  

AuC (Authentication Centre)

·         Stores the SRES (signed response) for authentication of mobile equipment

·         Stores the “Ki” (Subscriber authentication key)

·         Generates the “Kc”

·         Provides Random numbers, SRES and Kc on HLR request to authenticate a mobile in the network

EIR (Equipment Identity Register)

·         Stores IMEI of all mobile equipment in the market

·         Keeps a white list of IMEI that refers to “Good” phones

·         Keeps a black list of IMEI that refers to “stolen” phones

·         Keeps a grey list of IMEI that refers to “on observation” phones

Mobile Data Offloading

• is the use of complementary network technologies fir delivering data originally targeted for mobile networks
• rules triggering the mobile offloading action can be either set an end user or an operator
• Benefits for end user:
• data service cast control
• availability of higher bandwidth
• Benefits for operator:
• avoid congestion of the mobile networks
• no need of consecutive upgrade of mobile networks
• the main network technologies used for the mobile data offloading are WiFi of Femtocell.
• Access Network Discovery and Selection Function (ANDSF)
• is the most complete 3GPP approach to date for controlling offloading between 3GPP and non-3GPP access network (such as WiFi)
• the purpose of the ANDSF is to assist user devices to discover access networks in their vicinity and to provide rules to prioritize and manage connections to all networks.

SIGTRAN

SIGTRAN is the name, derived from signaling transport. The SIGTRAN protocols are an extension of the SS7 protocol family. It supports the same application and call management paradigms as SS7 but uses an Internet Protocol (IP) transport called Stream Control Transmission Protocol (SCTP), which is used to carry SS7 signaling over IP.

The SIGTRAN protocols are: SCTP, IUA, M2PA, M2UA, M3UA, SUA, V5UA and DUA.

M2PA provides an SCTP adaptation layer for providing an SS7 MTP signaling link over an IP network. It is specified in RFC 4165.

M2UA provides an SCTP adaptation layer for the seamless backhaul of MTP Level 2 user messages and service interface across an IP network. It is specified in RFC 3331.

M3UA provides an SCTP adaptation layer for the seamless backhaul or peering of MTP Level 3 user messages and service interface across an IP network. It is specified in RFC 4666.

SUA provides an SCTP adaptation layer for the seamless backhaul or peering of Signalling Connection Control Part user messages and service interface across an IP network. It is specified in RFC 3868.

MTP2 peer-to-peer adaptation layer (M2PA)

M2PA defines a protocol supporting the transport of SS7 MTP3 signaling messages over IP, using the services of the SCTP. M2PA allows for full MTP3 message handling and network management between two SS7 nodes communicating over an IP network.

M2PA supports:

– seamless operation of MTP3 protocol peers over an IP network connection;

– the MTP2/MTP3 interface boundary, management of SCTP transport associations and traffic instead of MTP2 links;

– asynchronous reporting to management of status changes.

The MTP specification requires that each node with an MTP3 layer is represented by an SS7 point code. In other words, each IP signaling point must have a separate SS7 point code.

M2PA receives primitives sent from MTP3 to its lower layer. These messages are processed or mapped to appropriate primitives at the M2PA/SCTP interface. In the same way, M2PA transfers primitives to MTP3 such as those used in the MTP3/MTP2 interface. Each M2PA link corresponds to an SCTP association. From a monitoring and measurement point of view, M2PA links are handled in the same way as standard SS7 TDM links.

Hence, signaling messages between SS7 nodes communicating over IP are measured and monitored in the same way as in a SS7 TDM network. All SS7 MTP features implemented within SURPASS hiS 700 are supported withM2PA as well.

M2PA can be used between:

– signaling gateways of the same or different networks,

– a signaling gateway and any SIGTRAN M2PA enabled end node.

MTP3 user adaptation layer (M3UA)

The MTP3 user adaptation layer (M3UA) has been defined in such a way that it provides the same services as MTPL3 to the upper layers (ISUP, SCCP). The procedures and message formats of the M3UA protocol are according to the requirements of the RFC3332 (recently RFC4666).

SCCP user adaptation layer (SUA)

The SCCP user adaptation layer (SUA) transports SCCP user messages over the IP network using the services of SCTP. SUA can thus be used for the transport of SCCP based signaling such as MAP (e.g. SMS), INAP, CAP, ... but not ISUP. 

ISUP message types

-       IAM

o   Initial Address Message (IAM) is sent in the forward direction by each switch in the circuit between the calling party and the destination switch of the called party.

o   Contains the called party number in the mandatory variable part and may contain the calling party name and number in the optional part.

-       SAM

o   Subsequent Address Message. In case the IAM did not contain the full called number, one or more SAMs may follow containing additional digits.

-       ACM

o   Address Complete Message is sent in the backward direction to indicate that the remote end of a trunk circuit has been reserved.

o   The originating switch responds to an ACM message by connecting the calling party’s line to the trunk to complete the voice circuit from the calling party to the called party.

-       ANM

o   Answer Message sent when the subscriber picks up the phone

o   Normally charging starts at this moment

-       CPG

o   Call Progress (CPG) can be sent by the terminating SSP after it has sent an ACM. For example, this could be used to signal that a call has been forwarded as a result of a busy line.

-       REL

o   Release Message is sent in either direction indicating that the circuit is being released due to a specific cause indicator

o   Is sent when either calling or called party hangs up the call (cause =16)

o   Is sent back to the calling party if the called party is busy (cause 17).

-       RLC

o   Release Complete Message is sent in the opposite direction of a REL to acknowledge the release of the remote end of a trunk circuit and to end the billing cycle.

ISUP Overview

ISDN User Part (ISUP) provides signaling support for all voice and non-voice services identified as part of ISDN. ISUP includes all of the flexibility found in the message-oriented signaling of ISDN’s Q.931 message format. It also extends beyond simple call establishment and disestablishment to include network management.

Then ISUP is part of the SS7 which is used to set up telephone calls in Public Switched Telephone Networks. It is specified by the ITU-T as part of the Q.76x series.

ISUP Message Format

-       ISUP information is carried in the Service Information Field (SIF) of an MSU.

-       SIF contains the routing label followed by a 12-bit Circuit Identification Code (CIC).

-       CIC indicated the trunk circuit reserved by the originating switch to carry the call

-       CIC is followed by the message type field

o   IAM, SAM, ACM, ANM, CPG, REL, RLC

SS7 Signal Units

-       Signaling information is passed over the signaling links which are called signal units. Signal Units are continuously transmitted in both directions on any link that is in service.

-       SS7 uses 3 different types of signal units:

o   Message Signal Units (MSU) :

§   All signaling associated with call setup and teardown, database query and response, and SS7 management requires the use of MSUs

o   Links Status Signal Units (LSSU):

§  communicate information about the signaling link between the nodes on either end of the link. This information is contained in the status field of the signal unit. They signal the initiation of link alignment, quality of received traffic, and status of processors at either end of the link.

o   Fill In Signal Units (FISU):

§  do not carry any information; they simply occupy the link when there are no LSSUs or MSUs. FISUs support the monitoring of link traffic because they undergo error checking. They can also be used to acknowledge the receipt of messages using backward sequence number (BSN)

SS7 Links

-          Is the physical transmission line that connects the individual nodes in an SS7 network

A-Links

-          Interconnect an STP and either an SSP or an SCP

-          their sole purpose is to deliver signaling to and from end points

B and D Links

-          bridge links (B-Links) are the quad of links interconnecting peer pairs of STPs

-          Diagonal Links (D-Links) are the quad of links interconnecting mated pairs of STPs at different hierarchical levels

C-Links

-          Cross Links (C-Links) interconnect mated STPs and are used to enhance the reliability of the signaling network

E-Links

-          Extended links (E-Links) connect an SSP to an alternate STP to provide backup connectivity to the network if the SSP’s “home” STP cannot be reached on it’s a-Link.

Linksets

-          Links are put into groups called linksets

-          Up to 16 links can be assigned to a linkset

-          All links in a linkset must have the same adjacent node

Routes

-          Route is a collection of linksets to reach a particular destination

-          A linkset can belong to more than one route

Routesets

-          A collection of routes that are assigned to destinations and also provide alternate routes

Destination

-          An address entered into routing table of a remote signaling point. A destination need not to be adjacent to the signaling point but must be a point code tha can be reached by the signaling point.

Point Codes

-          In SS7, addresses are assigned using a 3 level hierarchy:

o   Member: a signaling point within a cluster

o   Cluster: a collection of signaling points

o   Network: each cluster is defined as being part of a network

SLS

-          Signaling link selection is used as a label to route MTP level 3 messages in the SS7 network

-          Can be used in reference to the algorithm that an SS7 node uses for selection of the appropriate signaling link to use for communication with an adjacent SS7 node.

SS7 Components

The SS7 signaling architecture consists of three essential components, interconnected via signaling links. 

Signal Switching Point (SSP)

-are switches that have SS7 software and terminating signaling links

-can originate, terminate, and switch calls

-use the calling number party info to determine how to route the call

-looks up the dialed digits in the SSP routing table to find the corresponding trunk circuit and terminating exchange

-then sends an SS7 message out to the adjacent exchange requesting a circuit connection on the trunk which was specified in the routing table

-manages all of those connections until the destination is reached

Signal Transfer Point (STP)

-are packet switches and act like routers in SS7 networks

-messages are not usually originated by an STP

-three levels of STP:

            -National: exists within the national network. It can transfer messages that use the
             same national standard of protocol.

-International: functions within an international network. It provides for SS7 interconnection of all countries, using the ITU-TS standard protocol.

-Gateway: converts signaling data from one protocol to another. Gateway STPs are often used as an access point to the international network.

Signal Control Point (SCP)

-is usually a computer used as front end to a database syatem

-is an interface to telco database

SS7 Overview

Common Channel Signaling

is the transmission of signaling information on a separate channel from the data and more specifically where that signaling channel controls multiple data channels.

Signaling Modes

• Associated Signaling

•  Non-associated Signaling

• Quasi-associated Signaling