An Existing Interworking Architecture Computer Science Essay

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The motivation of next generation mobile network (NGMN) is the ubiquitous wireless access abilities which provide seamless interworking for the end users in the heterogeneous networks with different access technologies. Therefore, the integration of these dissimilar technologies using a common platform can enable user to freely move from one network to another. The focus of this work as mentioned in Chapter 1 is on mobile data network. Thus, this chapter will provide discussion on the current architecture that will serve as the foundation for the proposed architecture in Chapter 4. They are Compress Mode, Tight coupling, Loose Coupling, 3GPP IMS, Mobile IP and MIH.

During inter-frequency handover the UE's must be given time to make the necessary measurements on the different WCDMA carrier frequency. 1 to 7 slots per frame can be allocated for the UE to perform this intra frequency hard handover. These slots can either be in the middle of the single frame or spread over two frames. This compressed mode operation can be achieved in three different methods:

Decreasing the spreading factor by 2:1. This will increase the data rate so bits will get sent twice as fast.

Puncturing bits. This will remove various bits from the original data and hence reduce the amount of information that needs to be transmitted.

The higher layer scheduling could also be changed to use less timeslots for user traffic.

In compressed frames, Transmission Gap Length slots from N-first to N-last are not used for transmission of data. As illustrated below, the instantaneous transmit power is increased in the compressed frame in order to keep the quality unaffected by the reduced processing gain. The amount of power increase depends on the transmission time reduction method. What frames are compressed, are decided by the network. When in compressed mode, compressed frames can occur periodically, or requested on demand. The rate and type of compressed frames is variable and depends on the environment and the measurement requirements.

Figure 3.1 Inter-frequency transmission gap

The frame structure for uplink compressed frames is illustrated below.

Figure 3.2 Frame structure for uplink compressed frames

There are two different types of frame structures defined for downlink compressed frames. Type A maximises the transmission gap length and type B is optimised for power control. The frame structure type A or B is set by higher layers independent from the downlink slot format type A or B.

With frame structure of type A, the pilot field of the last slot in the transmission gap is transmitted. Transmission is turned off during the rest of the transmission gap.

Figure 3.3 Type A frame structure

With frame structure of type B, the TPC field of the first slot in the transmission gap and the pilot field of the last slot in the transmission gap is transmitted. Transmission is turned off during the rest of the transmission gap.

Figure 3.4 Type B frame structure

3.2.2 Tight Coupling

Tight coupling refers to hardware and software components that are linked together and dependent upon each other. For example, in a multiprocessing environment, where several computers share the workload, a tightly-coupled system would have to be shut down in order to add or replace a machine.

It is a method of interconnecting the components in a system or network so that those components completely depend on each other. Tight coupling increases the complexities in testing, maintenance and troubleshooting procedures because problems are difficult to isolate and the whole system is needed to be shut down.

3.2.3 Loose Coupling

Loose coupling is a method of interconnecting the components in a system or network so that those components, also called elements, depend on each other to the least extent practicable. Loose coupling simplifies testing, maintenance and troubleshooting procedures because problems are easy to isolate and unlikely to spread or propagate.

The extent or "tightness" to which the components in a system are coupled is a relative, qualitative notion. A loosely coupled system can be easily broken down into definable elements. The extent of coupling in a system can be informally measured by noting the maximum number of element changes that can occur without adverse effects. Examples of such changes are adding elements, removing elements, renaming elements, reconfiguring elements, modifying internal element characteristics and rearranging the way in which elements are interconnected.

Loose coupling minimizes unwanted interaction among system elements. However, loose coupling can also give rise to difficulty in maintaining synchronization among diverse components within a system when such interaction is desired. In some systems, a high degree of element interdependence is necessary for proper functionality. An example is the interconnection of alternating current (AC) utility power sources in the utility grid. The current from each source must be kept precisely in phase with the current from all the other sources. Otherwise, the sources will "buck" each other causing inefficiency, possible component damage or even catastrophic system failure.

3.2.4 3GPP IMS

The IMS is an architectural framework for delivering IP multimedia services. It was originally designed by the wireless standards body 3GPP, as a part of the vision for evolving mobile networks beyond GSM. Its original formulation, 3GPP R5 represented an approach to delivering "Internet services" over GPRS. This vision was later updated by 3GPP, 3GPP2 and TISPAN by requiring support of networks other than GPRS, such as WLAN, CDMA2000 and fixed line.

To ease the integration with the Internet, IMS uses IETF protocols wherever possible. SIP can be considered as the base protocol form IMS. According to the 3GPP, IMS is not intended to standardize applications but rather to aid the access of multimedia and voice applications from wireless and wire-line terminals, i.e. create a form of Fixed Mobile Convergence (FMC). This is done by having a horizontal control layer that isolates the access network from the service layer. From a logical architecture perspective, services need not have their own control functions, as the control layer is a common horizontal layer. However in implementation this does not necessarily map into greater reduced cost and complexity. A brief description of access network and core network for IMS is explained below:

Access network

The user can connect to an IMS network in various ways, most of which use the standard IP. IMS terminals can register directly on an IMS network, even when they are roaming in another network or country (the visited networks). The only requirement is that they can use IP and run SIP user agents. Fixed access (e.g., DSL, cable modems, Ethernet), mobile access (e.g. W-CDMA, CDMA2000, GSM, GPRS) and wireless access (e.g. WLAN, WiMAX) are all supported. Other phone systems like plain old telephone service (POTS -- the old analogue telephones), H.323 and non IMS-compatible VoIP systems, are supported through gateways.

3.2.4.2 Core network: The overall core network diagram is shown in Figure 3.5

Figure 3.5 IMS infrastructure

HSS: The HSS is a master user database that supports the IMS network entities that actually handle calls. It contains the subscription-related information, performs authentication and authorization of the user, and can provide information about the subscriber's location and IP information. It is similar to the GSM HLR and AUC.

A SLF is needed to map user addresses when multiple HSSs are used. Both the HSS and the SLF communicate through the Diameter protocol. Diameter is used to perform AAA operations.

Session Control Functions: Several roles of SIP servers or proxies, collectively called Call Session Control Function (CSCF), are used to process SIP signaling packets in the IMS.

A Proxy-CSCF (P-CSCF) is a SIP proxy that is the first point of contact for the IMS terminal. It can be located either in the visited network (in full IMS networks) or in the home network (when the visited network isn't IMS compliant yet). Some networks may use a Session Border Controller for this function. The terminal discovers its P-CSCF with either DHCP, or it is assigned in the PDP Context.

it is assigned to an IMS terminal during registration, and does not change for the duration of the registration

it sits on the path of all signalling messages, and can inspect every message

it authenticates the user and establishes an IPSec security association with the IMS terminal. This prevents spoofing attacks and replay attacks and protects the privacy of the user. Other nodes trust the P-CSCF, and do not have to authenticate the user again.

it can also compress and decompress SIP messages using SigComp, which reduces the round-trip over slow radio links

it may include a Policy Decision Function (PDF), which authorizes media plane resources e.g. QoS over the media plane. It's used for policy control, bandwidth management, etc. The PDF can also be a separate function.

it also generates charging records

A Serving-CSCF (S-CSCF) is the central node of the signalling plane. It is a SIP server, but performs session control too. It is always located in the home network. It uses Diameter Cx and Dx interfaces to the HSS to download and upload user profiles - it has no local storage of the user. All necessary information is loaded from the HSS.

it handles SIP registrations, which allows it to bind the user location (e.g. the IP address of the terminal) and the SIP address

it sits on the path of all signaling messages, and can inspect every message

it decides to which application server(s) the SIP message will be forwarded, in order to provide their services

it provides routing services, typically using Electronic Numbering (ENUM) lookups

it enforces the policy of the network operator

there can be multiple S-CSCFs in the network for load distribution and high availability reasons. It's the HSS that assigns the S-CSCF to a user, when it's queried by the I-CSCF.

An Interrogating-CSCF (I-CSCF) is another SIP function located at the edge of an administrative domain. Its IP address is published in the DNS of the domain (using NAPTR and SRV type of DNS records), so that remote servers can find it, and use it as a forwarding point (e.g. registering) for SIP packets to this domain. The I-CSCF queries the HSS using the Diameter Cx interface to retrieve the user location (Dx interface is used from I-CSCF to SLF to locate the needed HSS only), and then routes the SIP request to its assigned S-CSCF. Up to Release 6 it can also be used to hide the internal network from the outside world (encrypting part of the SIP message), in which case it's called a Topology Hiding Inter-network Gateway (THIG). From Release 7 onwards this "entry point" function is removed from the I-CSCF and is now part of the Interconnection Border Control Function (IBCF). The IBCF is used as gateway to external networks, and provides NAT and Firewall functions (pinholing).

Application servers: Application servers (AS) host and execute services, and interface with the S-CSCF using SIP. An example of an application server that is being developed in 3GPP is the Voice call continuity Function (VCC Server). Depending on the actual service, the AS can operate in SIP proxy mode, SIP UA (user agent) mode or SIP B2BUA mode. An AS can be located in the home network or in an external third-party network. If located in the home network, it can query the HSS with the Diameter Sh interface (for a SIP-AS) or the MAP interface (for IM-SSF).

SIP AS: native IMS application server

IP Multimedia Service Switching Function (IM-SSF): an IM-SSF interfaces with Customized Applications for Mobile networks Enhanced Logic (CAMEL) Application Servers using Camel Application Part (CAP)

3.2.5 Mobile IP

Mobile IP is an IETF standard communications protocol that is designed to allow mobile device users to move from one network to another while maintaining a permanent IP address. Mobile IPv4 is described in IETF RFC 3344 (Obsoleting both RFC 3220 and RFC 2002), and updates are added in IETF RFC 4721. Mobile IPv6 is described in IETF RFC 3775

The Mobile IP protocol allows location-independent routing of IP datagrams on the Internet. Each mobile node is identified by its home address disregarding its current location in the Internet. While away from its home network, a mobile node is associated with a care-of address which identifies its current location and its home address is associated with the local endpoint of a tunnel to its home agent. Mobile IP specifies how a mobile node registers with its home agent and how the home agent routes datagrams to the mobile node through the tunnel.

Figure 3.6 Mobile IP components

Mobile IP provides an efficient, scalable mechanism for roaming within the Internet. Using Mobile IP, nodes may change their point-of-attachment to the Internet without changing their home IP address. This allows them to maintain transport and higher-layer connections while roaming. Node mobility is realized without the need to propagate host-specific routes throughout the Internet routing fabric.

Mobile IP is most often found in wired and wireless environments where users need to carry their mobile devices across multiple LAN subnets. It may for example be used in roaming between overlapping wireless systems, for example IP over DVB, WLAN, WiMAX and BWA. Currently, Mobile IP is not required within cellular systems such as 3G, to provide transparency when Internet users migrate between cellular towers, since these systems provide their own data link layer handover and roaming mechanisms. However, it is often used in 3G systems to allow seamless IP mobility between different Packet Data Serving Node (PDSN) domains.

In many applications (e.g., VPN, VoIP), sudden changes in network connectivity and IP address can cause problems.

A mobile node can have two addresses - a permanent home address and a care of address (CoA), which is associated with the network the mobile node is visiting. There are two kinds of entities in Mobile IP:

A home agent stores information about mobile nodes whose permanent home address is in the home agent's network.

A foreign agent stores information about mobile nodes visiting its network. Foreign agents also advertise care-of addresses, which are used by Mobile IP.

A node wanting to communicate with the mobile node uses the permanent home address of the mobile node as the destination address to send packets to. Because the home address logically belongs to the network associated with the home agent, normal IP routing mechanisms forward these packets to the home agent. Instead of forwarding these packets to a destination that is physically in the same network as the home agent, the home agent redirects these packets towards the foreign agent through an IP tunnel by encapsulating the datagram with a new IP header using the care of address of the mobile node.

When acting as transmitter, a mobile node sends packets directly to the other communicating node through the foreign agent, without sending the packets through the home agent, using its permanent home address as the source address for the IP packets. This is known as triangular routing. If needed, the foreign agent could employ reverse tunnelling by tunnelling the mobile node's packets to the home agent, which in turn forwards them to the communicating node. This is needed in networks whose gateway routers have ingress filtering enabled and hence the source IP address of the mobile host would need to belong to the subnet of the foreign network or else the packets will be discarded by the router.

The Mobile IP protocol defines the following:

an authenticated registration procedure by which a mobile node informs its home agent of its CoA;

an extension to ICMP Router Discovery, which allows mobile nodes to discover prospective home agents and foreign agents; and

the rules for routing packets to and from mobile nodes, including the specification of one mandatory tunnelling mechanism and several optional tunnelling mechanisms.

3.2.6 IEEE 802.21 Media Independent Handover (MIH)

MIH is a standard being developed by IEEE 802.21 to enable the handover of IP sessions from one layer 2 access technology to another, to achieve mobility of end user devices

The importance of MIH derives from the fact that a diverse range of broadband wireless access technologies is available and in course of development, including GSM, UMTS, CDMA2000, WiFi, WiMAX, Mobile-Fi and WPANs. Multimode wireless devices that incorporate more than one of these wireless interfaces require the ability to switch among them during the course of an IP session, and devices such as laptops with Ethernet and wireless interfaces need to switch similarly between wired and wireless access.

Handover may be required, e.g. because a mobile device experiences a degradation in the radio signal, because an access point experiences a heavy traffic load.

The key functionality provided by MIH is communication among the various wireless layers and between them and the IP layer. The required messages are relayed by the Media Independent Handover Function, MIHF, that is located in the protocol stack between the layer 2 wireless technologies and IP at layer 3. MIH may communicate with various IP protocols including SIP, Session Initiation Protocol, for signaling, Mobile IP for mobility management and DiffServ and IntServ for QoS.

When a session is handed off from one access point to another access point using the same technology, the handover can usually be performed within that wireless technology itself without involving MIHF or IP. For instance a VoIP call from a WiFi handset to a WiFi access point can be handed over to another WiFi access point within the same network, e.g. a corporate network, using WiFi standards such as 802.11f and 802.11r. However if the handover is from a WiFi access point in a corporate network to a public WiFi hotspot, then MIH is required, since the two access points cannot communicate with each other at the link layer, and are in general on different IP subnets.

When a session is handed off from one wireless technology to another, MIH can provide the handover by passing messages among the wireless technologies and IP. Message are of three types:

Event notifications are passed from a lower layer in the protocol stack to a higher layer or between the MIHF of one device to the MIHF of another device. For instance "wireless link quality is degrading" is an event notification that is passed from the wireless layer to the MIHF layer.

Commands are passed down the protocol stack or between the MIHF of one device to the MIHF of another device. For instance "Initiate Handover" is a command in which the access point MIHF provides the mobile device MIHF with a list of alternative access points that it could use.

Information Service is of three types. A higher layer may request information from a lower layer, e.g. the MIHF may request performance information, such as delay from the wireless layer. A lower layer may request information from a higher layer, e.g. the MIHF may request to know the ISP Name from the IP layer. One MIHF may request information from another MIHF, e.g. the availability of location-based services

Short-lived sessions such as accessing a single web page typically do not require handover or QoS. Longer duration sessions, which may well require handover, such as VoIP, audio/video streaming (including live TV and VoD), and VPNs, typically have QoS requirements including delay, delay variation and packet loss.

It is important that QoS is maintained, not just before and after a handover, but also during the handover, and this can be achieved by using MIH to plan ahead. Before a handover is required, the MIHFs communicate to identify which access points using which wireless technologies are within range and what QoS is available from them. MIH can also be used to pre-authenticate the mobile device with alternative potential access points and to reserve capacity prior to handover. For instance WiMAX allows resources to be reserved for a session before they are actually allocated to that session. When a handover becomes necessary, much of the ground-work is therefore already in place and the session can be handed over with minimal delay and packet loss. Incoming packets to the mobile device that are delivered to the old access point after the handover can be forwarded via the new access point, thus further reducing packet loss.

QoS is handled differently by each technology, including both the wireless access technologies and also IP, which has two QoS approaches, DiffServ and IntServ. Some technologies divide traffic into "Service Classes", e.g. streaming, while others allow users to specify quantitative "QoS Parameters", e.g. transfer delay. WiFi, Mobile-Fi and DiffServ use the service class approach and although they do not have exactly the same service classes, it is possible to make a correspondence among them. WiMAX and IntServ use the QoS parameter approach, and UMTS uses both approaches. Again correspondences among parameters can be made.

MIH can be used to exchange information about service class and QoS parameter availability from one wireless technology to another and to the IP layer. One source of such information is performance measurements made by the wireless layer, e.g. 802.11k for WiFi and 802.16f for WiMAX

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