T325: Technologies for digital media

1. T325:Technologies for digital media Block III - Part 2: Network architecture Arab Open University – Spring 2012

2. Key protocols • User services and the network architecture • Authentication Outline Arab Open University – Spring 2012

3. Different protocols are used for 2G, 2.5G and 3G • 2G and 2.5G : different protocol stacks are used for the control plane and the user plane • 3G : sameprotocols are usedfor the control place and the user plane Key protocols Arab Open University – Spring 2012

4. The layered protocols approach Key protocols Arab Open University – Spring 2012

5. Each service provides data in blocks called data units. • The set of blocks at each node is often known as the protocol stack. • Each protocol has two end points • PHY protocol: UE and Node B • GPRS mobility management (GMM): UE and the SGSN Key protocols Arab Open University – Spring 2012

6. UE and SGSN are not adjacent on the diagram, which means the GMM messages are carried over the intermediate nodes (Node B and RNC) by lower level protocols; in this case the radio resource control (RRC) and radio access network application protocol (RANAP). • These transport the GMM messages but do not alter them in any way, so the intermediate nodes are invisible to the GMM protocol. Key protocols Arab Open University – Spring 2012

7. Each block can be thought of as communicating with the block of the same name at the other end point using the services of the layers below. • Radio Link control (RLC) block in the RNC communicates with the RLC block in the UE, using the services of the medium access control (MAC) and PHY layers. • Blocks communicating in this way are said to be communicating on a peer-to-peer basis and the two blocks are often referred to as peer entities. Key protocols Arab Open University – Spring 2012

8. Protocol interworking:A node converts from one protocol to another in each direction. • Example: At the lowest level, the Node B has blocks labeled PHY and ATM. Radio access network Arab Open University – Spring 2012

9. The blocks labeled PHY refer to the physical layer of the radio interface, which comprises a number of different physical channels. • In 3G, the primary means of sharing the allocated radio bandwidth between these channels is W-CDMA • The physical channels are used for: • user traffic • transport of Signaling messages • management of the radio link itself. • PHY interworks at the Node B with the ATM protocol • ATM protocol is used to transport user traffic and Signaling messages across the fixed part of the network from the Node B to the SGSN (and the MSC for circuit-switched traffic) via the RNC. Radio access network – Physical Layer Arab Open University – Spring 2012

10. Next group of protocols to consider are those which are used to manage the flow of data between the UE and the radio access networkand to control the allocation of radio channels. • The three most important of these are the RRC, RLC and MAC protocols • The end points for all of these protocols are the UE and the RNC (except for one specific case which need not concern us here). • This means that in virtually all cases the; Node B does not participate in these protocols it simply forwards the data to the UE Radio access network Arab Open University – Spring 2012

11. Key protocols Arab Open University – Spring 2012

12. MAC Layer : Principal functions • Prioritization between: • data flows addressed to a single piece of UE • data flows addressed to different pieces of UE. • Multiplexingdata units arriving from the upper layers onto the data units delivered to the physical layer and demultiplexing flows in the opposite direction. • Encryption(if this has not been carried out by the RLC layer) • Traffic volume measurement. Key protocols –MAC Layer Arab Open University – Spring 2012

13. RLC layer : Responsible for • Segmentation of data units received from the next layer up (the RRC layer) into smaller data units to be forwarded to the MAC layer. • Reassembling data units received from the MAC layer into larger data units for forwarding to the RRC layer. • If instructed to do so, the RLC layer will also routinely acknowledge packets and retransmitthose which are reported by the receiver to contain errors. Key protocols –RLC Layer Arab Open University – Spring 2012

14. Three types of communication channels (3G standards) • Logical channels:data flows between the RLC blocks • Transport channels:the data flows between the MAC blocks • Physical channels: the data flows transmitted across the radio interface by the physical layer Key protocols Arab Open University – Spring 2012

15. The top layer of the three is the RRC protocol, which handles most of the major functions in the radio network, including: • Setting up of physical radio channels • Reporting of radio measurements • Transport of Signaling messages between the mobile and the core network • It also instructs the RLC layer whether or not to acknowledge packets. • The three protocols (MAC/RLC/RRC) and the physical layer protocol PHY are classified together in the standards as theaccess stratum – the level which provides the UE with access to the core network. Key protocols –RRC Protocol Arab Open University – Spring 2012

16. Radio network controller needs to interact with the core network. • This is done using a protocol called RANAP, which is used to interact with the SGSN (and also the MSC in the circuit-switched domain). • RANAPmakes use of a version of the standard Signaling protocol used in the circuit-switched domain, Signaling System No. 7 (SS7). • The version of SS7 used in RANAP has been modified to run over ATM. RAN-Core network interaction Arab Open University – Spring 2012

17. RAN-Core network interaction Arab Open University – Spring 2012

18. At the very top of the stack is the non-access stratum (NAS), which handles direct Signaling between the mobile and the core network. • In the 3G packet domain, the principal constituent of the non-access stratum is the GMM protocol. RAN-Core network interaction Arab Open University – Spring 2012

19. IP addressing in fixed networks is hierarchical • Network/subnetwork address • Host address • This hierarchy is used for routing purposes • Problem of Mobility in IP-based Mobile networks (e.g. 3G) • If a device were to move from one subnetwork to another: • It could be come invisible at the subnetwork routing level, since the top-level routing would direct the packets to the previous subnetwork. • Since its IP address would almost certainly not match the addressing scheme of the new subnetwork, messages from the device might also be rejected by firewalls checking for valid source addresses. Support for IP in 3G networks Arab Open University – Spring 2012

20. Solving the mobility problem for mobile data communication, therefore, depends on using the location information the network holds, in order to make the IP addressing system continue to work as the mobile moves from one subnetwork to another. • A clever solution is for the network to put each IP packet inside another IP packet. • The outer packet can then be used to get the message to the router to which the mobile device is currently connected, at which point this router extracts the inner packet and sends it directly to the mobile device. • This approach is known as tunneling, and in GPRS and 3G it is provided using the GPRS tunneling protocol (GTP). Support for IP in 3G networks Arab Open University – Spring 2012

21. Support for IP in 3G networks Arab Open University – Spring 2012

22. When an IP packet arrives at the GGSN from an external IP network addressed to an active mobile: • GGSN adds the GTP header and the UDP header and puts the result in an IP packet addressed to the radio network controller in whose area the mobile is located.  GGSN sends the packet through the appropriate tunnel, which ends at the relevant RNC. • The RNC then extracts the inner IP packet and forwards it to the user equipment using the RRC protocol and the lower layers in the access stratum. • The UE then handles the IP packet just as if it had received it from a local IP router. Support for IP in 3G networks Arab Open University – Spring 2012

23. Packets from the UE to the GGSN follow a similar process and are routed by the GGSN to a specific external IP network depending on the session associated with the user traffic tunnel through which the packet arrived. • As the mobile moves around the network, it may pass from an area controlled by one RNC to that of another  If this happens, the two RNCs and the SGSN cooperate to move the tunnel end point from the old RNC to the new one. • If the move includes a change of SGSN, then the tunnel is also relocated from the old SGSN to the new one. Support for IP in 3G networks Arab Open University – Spring 2012

24. Inner levelrepresents the packet to be sent through the tunnel • Outer level consists of the headers added by the tunnelling protocol. • GTP uses the user datagram protocol (UDP) to send the tunnelled packets and also adds its own control header, which is used, among other things, to identify which tunnel is which. Support for IP in 3G networks Arab Open University – Spring 2012

25. Session information in the packet-switched side of the network is stored in a series of tables maintained by the SGSN and the GGSN called the packet data protocol (PDP) context. • Each session is directed to a particular network physically connected to one GGSN, and this network is identified by a uniform resource locator (URL) known as the access point name (APN). • The APN and the tunnel identifiers are held as part of the PDP context tables. • Typical APNs point towards the internet via a network belonging to an internet service provider (ISP), or to a corporate data network. Support for IP in 3G networks Arab Open University – Spring 2012

26. In addition to data transfers to single recipients, 3G also offers a multicast streaming and download service called the multimedia broadcast and multicast service (MBMS). • This allows an IP multicast tree to be constructed within the mobile network, eliminating the need to send multiple copies of the data across the network.  The individual pieces of user equipment subscribing to the relevant services form leaves on the multicast tree, so that each receives a copy of the data stream or the downloaded data. • The streaming service is particularly suitable for supporting mobile TV, and the download service can be used to distribute multimedia messages (for example, for advertising purposes). Support for IP in 3G networks Arab Open University – Spring 2012

27. Most networks offer additional facilities associated with the voice service, known as supplementary services. • There are many of these, but they include: • Call forwarding, call on hold, notification of a waiting incoming call, conference calls, prevention of calls to or from a mobile handset, voice mail • 3G networks allow video calls • The user requests a video call using the normal phone number and the network attempts to set it up with the called party. User services and the network architecture Arab Open University – Spring 2012

28. Second most popular service offered by mobile networks (after voice calls) is texting, orSMS (short message service). • The text messaging service is also provided by the circuit-switched part of the network • In the circuit-switched domain, the Signaling required to set up and clear down calls uses the ISDN SS7 protocol, which can be seen as an early packet-based protocol. • This is transmitted over one of the ISDN time slots which is reserved for Signaling. • Text messaging makes use of the SS7 protocol to deliver the messages, avoiding the need to reserve bandwidth to transmit a message. Text messaging Arab Open University – Spring 2012

29. Internet Access: the user is connected to the internet via an internet service provider working in partnership with the mobile network. • Access to corporate networks through a virtual private network (VPN): typically utilize an IPsec tunnel in order to provide an additional layer of security. • IPsec is a set of protocols for making IP communication more secure. • Multimedia Messaging Service (MMS): allows still pictures, video clips and audio messages to be sent, as well as much longer text messages than SMS permits. • Streaming services present audiovisual content to the mobile device for buffering and immediate replay. Data services Arab Open University – Spring 2012

30. Two types of video streaming service: • Mobile TV • User can choose from a number of channels but is constrained by a programme schedule • Content-on-demand. • Content is downloaded to the mobile device and can be replayed several times • The content provider may restrict the number of times the content can be played or specify an expiry date beyond which it is no longer available. Data services Arab Open University – Spring 2012

31. Internet Access: • The user is provided with a standard APN, which is a URL specific to the mobile network and which needs to be selected as a setting on a mobile device or on a USIM card associated with the device • The APN points towards the GGSN interface connected to the ISP’s network, and the mobile device can use this to set up web browsing sessions. How data services are provided? Arab Open University – Spring 2012

32. Multimedia Messaging: • Multimedia messaging is similar in one way to text messaging, in that it makes use of a multimedia messaging centre (MMSC) which is used to store and forward messages. • Unlike the SMSC, however, the MMSC is a fully IP-based device which can only be accessed through an IP network. • When the user wants to send a message, the user equipment sets up a session using an APN (URL) pointing to the MMSC. How data services are provided Arab Open University – Spring 2012

33. In 2G, authentication is the process used to prove to the network that the mobile device is genuine and allowed to use the network. • In 3G, the process is mutual; it also allows the mobile to prove to itself that the network is genuine. • In both 2G and 3G, authentication is a function of the non-access stratum • Takes place between the user equipment (actually the USIM card) and the core network without the intervention of the radio access network. • The RAN does not itself discriminate between users who are associated with a particular operator and those who are not. This means that all devices capable of operating on the frequency in question can connect to the RAN. Authentication Arab Open University – Spring 2012

34. Authentication Arab Open University – Spring 2012

35. On receipt of an authentication request, the SGSN sends a request to the AuC to begin the authentication process. • The AuC does this by calculating an authentication vector (AV), which is a sequence of, which includes the following five components: • RAND : a random number . • AUTN : the authentication token, which depends among other things on a sequence number SQN, which the UE and the AuCsynchronise between them, and the random number RAND . • XRES : an expected result, to be used by the SGSN to check the authentication response from the UE . • CK : a temporary cipher key, to be used to encrypt the user data until the next authentication request . • IK : a temporary integrity key, to be used to verify the integrity of Signaling messages to and from the UE. • SQN and RAND are used to prove that the AV has been freshly generated, to prevent replay attacks based on captured AVs. Authentication Arab Open University – Spring 2012