TTM4133: Mobile Networks and Services
This compendium aims to explain the most important concepts in the subject. The content is mostly based on the lecture slides and the book: "From GSM to LTE-Advanced PRO and 5G".
The course focuses on: GSM, GPRS, UMTS, LTE, SIP & IMS, 5G and Over the Top Services. The compendium is structured around explaining each of these items in detail.
Everyone is allowed to edit this compendium, if you see any errors feel free to correct them.
GSM (short for: Global System Mobile Communications) was introduced in the 1990's and is still used by billions of people today. GSM has been through steady improvements throughout the years, which has led to reduced price for both infrastructure and mobile devices. Even though IP-based LTE (4G) has been widely delopyed, GSM is still used ouside of coverage areas. Knowledge of GSM is thus still required for a thorough understanding of mobile networks.
The mobile station is made up of two key elements: - The mobile equipment - The Subscriber Identity Module (SIM) Card
The mobile euqipment is the various hardware components of the mobile phone, for example:radio, screen, battery, processor, operating system, camera and so on. The SIM card is a component which mainly serves the purpose of identifying the user of the mobile equipment.
GSM has three different subsystems:
- The Network Subsystem (NSS), also called the "core network" contains all nodes and functionalities that are necessary for switching of calls, for subscriber management and mobility management.
- The Base Station Subsystem (BSS), also called the "radio network" contains all nodes and functionalities that are necessay to wirelessly connect mobile subscribers over the radio interface to the network.
- The Intelligent Netowk Subsystem (IN) comprises SCP databases that add optional functionality to the network. For example prepaid service, SMS mesages and data services via GPRS and UMTS.
The Network Subsystem
The most important responsibilities of the NSS are subscriber management, mobility management, call establishment, call control and routing to fixed networks (PSTN) and other public land mobile networks (PLMN).
The NSS consissts of the following nodes: Mobile switching center (MSC), Gateway Mobile Swithcing Centre (GMSC), VLR (Visitor Location Register) and HLR (Home Location Register).
Mobile Switching Center (MSC)
The MSC is the central element of a mobile telecommunication network and all connections between subscribers are managed by it.
The management activities to establish and maintain a connection are part of the Call control (CC) protocol, which is generally responsible for the following tasks:
- Registration of mobile subscribers
- Call establishment and routing.
- Forwarding of SMS messages.
As subscribers can roam freely in the network, the MSC is also responsible for the Mobility Management (MM) of Subscribers. This activity comprises the following tasks:
- Authentication of subscribers
- Location update.
The MSC is also responsible for billing, not prepaid however.
Gateway Mobile Switching Centre (GMSC)
The GMSC can be a an ordinary MSC with the interface to the PSTN or an MSC with no connected BSC or VLR.
The Home Location Register (HLR)
- The HLR is the subscriber database of a GSM network.
- It contains a record for each subscriber, with information about the individuals available services.
- It also has the knowledge of location (VLR) for each subscriber and the reachability of the subscriber (on/off).
- The International Mobile Subscriber Identity (IMSI) is an internationally unique number that identifies a subscriber and is used for most subscriber-related signalling in the network.
- The IMSI is stored in the subscriber's subscriber identity module (SIM) card and in the HLR and is thus the key to all information about the subscriber.
- The MSISDN is anbother intenrationally unique number which is the number hold in the phone/address book.
- The HLR has an associated Authentication Center (AuC) which handled authentication of the SIM through a secret algorithm called A3.
The Visitor Location Register (VLR)
- Each MSC has an associated Visitor Location Register (VLR), which holds the record of each subscriber that is currently served by the MSC.
- These records are only copies of the origianl records, which are stored in the HLR.
- The VLR is mainly used to reduce signalling between the MSC and the HLR. If a subscriber roams into the area of an MSC, the data are copited to the VLR of the MSC and are thus locally available for every connection establishment.
- Can be independent hardware, but is usually implemented as a software component in the MSC.
- When a subscriber leaves the coverage area of an MSC, their record is copied from the HLR to the VLR of the new MSC, and is then removed from the VLR of the previous MSC.
The Short Messaging Service Center (SMSC)
- Used to store and forward short messages.
Base Station Subsystem (BSS)
The BSS containts two central nodes: the Base Tranceiver Station (BTS) and the Base Station Controller (BSC).
Base Tranceiver Station (BTS)
- The BTS modulates/demodulates the air interface.
- BTS may communicate with several mobile stations due to access methods: FDMA and TDMA
Base Station Controller (BSC)
- Connection Establishment and release (signalling channels).
- Administration of prequency hopping.
- Handover decision.
- Controlling of transmission power.
- Controlling of timing advance.
Mobility Management and Call Control
There are three processes that allow a subscriber to roam throughout the network
Cell Reselection and Area Update
Location area update is performed whenever an MS registers with the network and whenever an MS roams into a cell with a new location area code (LAC).
Paging is the procedure of finding the exact cell of a mobile device.
The Signalling System Number 7
Signaling System No.7 (SS-7) is a telecommunications signaling architecture traditionally used for the set up and tear down of telephone calls. It has a robust protocol stack that uses out-of-band signaling to communicate between elements of the public switched telephone network (PSTN). In recent years it has been superseded by the Diameter signaling protocol on all-IP networks.
The SS-7 standard defines three basic types of network nodes:
- Service Switching Points (SSPs): Are switching centers that are more generally reffered to as network elements and that are able to establish, transport or forward voice and data connections.
- Service Control Points (SCPs): Are databases and application software that can influence call establishment of a connection.
- Signalling Transfer Points (STPs): Are responsible for the forwarding of signalling messages between SSPs and SCPs as not all network nodes have a dedicated link to all other nodes in the network.
Long Term Evolution (LTE)
- UMTS limits reached
- HSPA not enough
- inherent design limitations
- 5 MHz carrier bandwidth
- Increased speed -> am
Over the Top Services (OTT)
- Content provided over the internet
- Skype, Facetime, Viber, WhatsApp
- Youtube, Netflix, Hulu, ...
- Bypass traditional distribution
- no integration with existing systemms
- no revenue sharing
- Pre-installed some high-end phones or via apps
- Cheaper and customisable
- Poor quality of experience
- no handover support
- no dedicated bearer
- Energy impact
- Dependence on keep-alive messages
- No paging mechanism available
- Prevention of lower-energy RRC states (e.g DRX)
- Resource hungry
- No header compression mechanisms
- Net neutrality.
Session Initiation Protocol (SIP)
- SIP is a protocol used for initiating a session.
- A session is a communication.
- An IP communication using IP devices in an IP network.
- SIP is a signalling protocol.
- SIP is a protocol operating at the application level
- SIP can run on either TCP or UDP.
SIP user identification
- Public URI/Address of Record
- Used to identify users
- FQDN (Fully qualified domain names) are identifiers for individual devices.
A user may have several devices with a FQDN each, but only one Public URI. A SIP registrar is used to map the different FQDNs to a users Public URI. When a user logs on to a device, the device will send a SIP REGISTER message to the associated SIP registrar which stores the information. Whan another user calls this user, the registrar will retrieve the information and route the call to the device.
The public URI/Address of Record is analogous to the MSISDN or phone number in GSM.
SIP User Agents
- User agents (UAs) are SIP endpoints normally handled by users.
- Some UAs can establish sessions automatically without user intervention (e.g SIP voicemail).
- Examples are software running on a computer, SIP softphones, SIP-supported mobile phones.
- Proxy servers are also known as SP routers.
- A proxy server recieves a SIP message from a UA or another proxy and routes it to the destination.
- SIP messages can be routed based on different criteria.
- Time of day
- Nature of the call (e.g business or personal)
- Type of session (e.g video call or voice only)
- Proxies and registrars are logical roles.
- A proxy server can colocate with a registrar.
- If the service provider would like to charge for the duration of the session, the proxies must remain in the signaling path. The service provider can make sure REcord-route header fields are added to all SIP messages by the proxies.
- Normally, a proxy doesn't stay in the singnaling path once the INVITE has been accepted by the called party.
- Forking proxies
- A proxy can choose a single UA to route the SIP message to or it could decide to route messages to more than one destination at once. A forking proxy can route messages in parallel (all phones/terminals ring at once) and in sequence (try one after another)
- Stateful proxy server
- Remembers the state of a transaction until it’s finished. Can perform forking. Can perform complex operations such as billing and complicated routing rules. Limited performance
- Stateless proxy server
- Only forward messages. Faster than stateful. Cannot perform forking (because it is unable to do filtering to return one response out of many it receives)
SIP proxy vs Back-to-back user agent
There is a difference between a SIP proxy and a back-to-back user agent. INVITE forwarded from a proxy belongs to the same dialog as the invite it receives andthus retains the same CALL-ID. INVITE sent from a back-to-back user agent belongs to a differetn dialog and so can have a different CALL-ID.
SIP Redirect Servers
- Redirect servers also route SIP messages BUT do not relay messages.
- They only instruct the entity sending the message (UA or proxy) to try a new location.
- SIP is a textual request-response protocol.
- Communication is based on message exchange.
- Clients send request messages and servers send response messages (think of HTTP requests/responses)
A message has a: Start line, A number of header fields, empty line, optional message body.
Request messages: INVITE, ACK, BYE
- Provisional responses: 100 Trying and 180 Ringing
- Final responses: 200 OK
- Called status line
- Contains the protocol version (SIP/2.0) and the status of the transaction.
- Status is an integer from 100 to 699
- Called request line
- Contains the method name, Request-URI, and protocol version.
- Method name indicates the purpose of the request.
- Request-URI contains the destination of the request.
- Heaer fields come after the start line
- Both in request and response messages.
- Some fields are mandatory (appear in all messages).
- <header fieldname>: <header field's value>
- Some header fields can have more than one entry in the same messages
- Such as having two routes.
Here are the most important SIP methods that has been required to know in order to solve previous exams:
- Contains URI of the destination of the request BUT is not used to route the request. Only for filtering/displaying purposes.
- Contains URI of the originator of the request. Also for filtering/displaying purposes.
- Keeps track of all proxies a request has travered. Response uses these to traverse the same proxies as the request.
- Record route
- To tell the SIP entities that the specified route must be traversed by the subsequent SIP requests. Is added by a proxy that would like to remain in the signalling path.
- To force the SIP entity to pass the request to the specified route.
Transaction is basically an exchange of messages and the type depends on the initiating request. There are three types of transactions:
- REGULAR transactions
- May be initiated by any type of SIP method except INVITE, ACK or CANCEL. A regular transaction can for example start with a BYE request.
- INVITE-ACK transactions
- ACK is a request method sent in response to a final response of an INVITE. Consists of two sub-transactions when successful: INVITE transaction and ACK transaction.
- CANCEL transactios
- Inititated by CANCEL request. Always connected to a previous transaction
Note: ACK is concidered a seperate transaction when the final response of the invite is a success (code =2xx).
- A session description contains enough information for the remote user to join the session.
- Includes, for example, IP address and port number to send the media, codecs to use to encode voice and images.
- SIP does not restrict the format for session descriptions.
- The most common format used is Session Description Protocol (SDP).
- The name has "protocol" in it but is not actually a protocol.
SDP OFFER/ANSWER Model
- To establish a call between two parties, both parties must exchange SDPs.
- The caller makes an offer with an SDP which normally lists multiple codecs that the caller supports.
- The callee then answers with another SDP indicating the codecs he/she supports which are the subset of those offered by the caller
- The answer must contain the same number of media lines (m=)
- Any declined media can be specified by setting the port number to 0.
Event notification in SIP
- Three messages are related: PUBLISH, SUBSCRIBE, NOTIFY
- The person who would like to be notified of an interested event is a subscriber who sends SUBSCRIBE messages to indicate interests to a broker (for example a presence server).
- The entity responsible for the event (e.g a UA or voicemail system) reports the event or new information using PUBLISH messages that it sends to the broker.
- The broker, who is a notifier, maintains a mapping betweeen the subscriber and the interested events/information and sends NOTIFY messages to the subscriber upon reciving PUBLISH messages or when it's time (e.g periodically or based on some rules).
IP Multimedia Subsystem (IMS)
IMS is a global, access-independent and standard-based IP connectivity and service control architecture that enables various types of multimedia services to end-users using common Internet-based protocols.
- Network convergence
- A device operating on any access technology can receive IMS services.
Call Session Control Functions (CSCF)
A CSCF is basically a SIP server which processes signaling in the IMS. There are 3 types of CSCFs depending on the functionality:
- P-CSCF (Proxy-CSCF)
- I-CSCF (Interrogating-CSCF)
- S-CSCF (Serving CSCF)
- First point of contact in the signalling plane between the IMS terminal and the IMS Network.
- P-CSCF is allocated to the IMS terminal during IMS registration.
- Acts as an outbound/inbound SIP proxy server from a SIP perspective.
- (De-)compresses SIP messages.
- Apply integrity and confidential protection for SIP signaling.
- Asserts identity of the user to other nodes.
- Contact point within an operator's network for connections destined to a subscriber of that network.
- Address of I-CSCF often added in DNS
- Interfaces with Home Subscriber Service (HSS) to retrieve user location information.
- Assigns a S-CSCF based on capabilities received from the HSS.
- Routes incoming requests to an assigned S-CSCF or AS.
- Central node in the signaling plane
- Acts as a SIP registrar (maintain mapping between user identity and Fully Qualified Domain Names)
- Also interfaces to HSS
- Download user's service profile which may divert routing to AS.
- Update HSS to associate the S-CSCF with the user.
- Provides translation between telephone number and SIP URI.
- Enforces policies of network operators on service control.
Home Subscriber Server (HSS)
- Central repository for user-related information.
- Contains all the user-related subscription data required to handle multimedia sessions.
- User identities (public/private)
- Access info (e.g authentication, roaming authorization)
- User profile infor (e.g services that the user is subscribed to, required S-CSCF capabilities)
- Location info
- Allocated S-CSCF
AS (Application Server)
- SIP entity that hosts and executes services.
- Located in the home network or external 3rd party networks.
- May provide an interface to the HSS to downloads/upload user data when located in the home network.
- User Agents are end user devices, machines such as voice mail system and application servers.
- There are two types of UAs: UA Client (UAC) and UA Server (UAS)
- UAC is the one sending out requests (such as INVITE)
- UAS is the target of the requests and the one sending out responses.
- With the definitions above, of course applicationservers are also UAs because they send out requests and generate responses.
Identifying users, services, and network enities
User identities in IMS
- There are two types of user identities:
- Public user identity
- Private user identity
- Public User IDs are analogous to SIP URIs or Address of Records.
- Private User IDs are something else entirely.
- Public User ID
- Used for identifying a user and can be used by others to establish sessions with the user. Public user ID is on the following format: sip:email@example.com
- Private User ID
- Used for identifying a subscription and is used for authentication during registration. A private user ID is on the following format: firstname.lastname@example.org
Initial Filter Criteria (IFC)
A service profile may have one or more initial filter criteria.
An initial filter criteria may have 1 or more application servers.
- An initial filter criteria can have a service profile, but does not require one.
An initial filter criteria has exactly 1 trigger point.
A trigger point has 1 or 0 initial filter criteria.
A trigger point has exactly one service point.
A service point trigger has 1 or more trigger points.
An application server has exactly one initial filter criteria.