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Communication systems have been evolved since the time when Alexander Bell first invented Telephone in 1978. In early days, the communication used to happen
through fixed lines, but now people want to communicate from anywhere, any time and to anyone. The cellular system provides data rates up to 144 Kb/s and a wide coverage area with almost universal roaming whereas IEEE 802.11 WLANS provides data rates upto 11Mb/s with limited coverage. To provide high data rates with universal roaming to the users, it is a need now to integrate both the systems.
2.1 FIRST GENERATION SYSTEM
The first generation analog cellular system was developed by Bell Telephone Company in USA and was implemented as, Advanced Mobile Phones Services (AMPS) in USA in 1978.In Europe; Nordic Mobile Telephones (NMT) introduced NMT-450 which was later developed into NMT900 analog cellular system. In UK, Motorola introduced The Total Access Communication (TACS) In Japan, Nippon Telephone and Telegraph (NTT) modified TACS to suit their frequency requirement and called it as JTACS and then introduced narrowband TACS (NTACS) in October 1990. Due to the incompatibility between these systems roaming forestalled, this forced the user to change their mobile as they move from one country to another. Furthermore, these systems can only provide voice services, were unable to handle increasing users and the quality of speech was not upto the mark.
2.2 SECOND GENERATION SYSTEM
To overcome all these problems, new cellular system was deployed which was based on digital radio transmission and was knows as second generation system (2G). 2G systems include CDMA1 (IS 95), IS 54 , Personal Digital Communication (PDC) and US TDMA (IS 136) and the popular GSM (Global system for mobile communication) which was employed worldwide .The GSM services were started in 1992 and used the frequency band of 900 MHz . The modified version of GSM: DCS-800 was deployed in 1994 with frequency band of 1.8 GHz and can handle data rate up to 9.0 Kb/s. The IS 54 system was introduced in 1989 in North America, later the standard was reintroduced as IS 136 /DAMPS to provide dual services between analog and digital cellular systems IS 95 is the first system based on CDMA was introduced in 1993. Around the same time PDC systems were started in Japan with frequency band of 800 MHz and 1.5GHz.
These communication standards allowed some sort of partial roaming in some regions which was not possible in 3G system. 2G systems provide basic services like voice, facsimile and low speed band data.
2.3 TWO AND HALF GENERATION
To overcome the low speed data issue the half generation follows 2G i.e., 2.5G which presents likeliness of 2G and 3G systems. 2G wireless system uses circuit switched transmission whereas high switched packet transmission is used by 3G systems. This generation includes GPRS (General Packet Radio Services), HSCSD (High-Speed Circuit Switched Data) and EDGE (Enhanced Data GSM Environment).
HSCSD uses circuit switch connection and can provide data rates up to 38.4 Kb/s which is much more than what 2G provides. EDGE is a GSM version, is fast and has ability to handle 384 Kb/s of data rates. GPRS uses packet switched connections and designed to work with digital cellular network. The packet data protocols are carried to and fro from GPRS terminals or other packet network. Being a packet service it still coexists with GSM services.
2.4 THIRD GENERATION
In 1995, to provide full global standard, high data rates, higher capacity and multimedia services, research and development for 3G communication system started. IMT-2000, WCDMA and cdma 2000 are main standards of 3G systems.
International Mobile Telecommunication (IMT-2000) aims for 144 Kb/s for high mobility, 384 Kb/s for low mobility and 2 Mbps for stationary environment. Furthermore, transmission rates up to 2 Mb/s users can have multimedia based communication alongside of voice and data communication. IMT 2000 uses variations of Code Division Multiple Access (CDMA) like direct sequence CDMA with frequency division duplex (FDD) / time division duplex (TDD) and Multicarrier CDMA with FDD. Wideband cdma and cdma2000 were submitted by NTT Docomo and Ericsson and Qualcomm respectively. WCDMA uses DSCDMA-FDD and MCCDMA-FDD.
2.4.1 3G NETWORK
Universal Mobile Telecommunications System (UMTS) network is divided into three domains: Mobile Station, Core Network (CN) and Universal Terrestrial radio Access Network (UTRAN). The functionality of CN includes assistance in routing, transit and switching of user traffic. It also includes databases and functions related to network management.
The CN architecture for UMTS is on the same line of GPRS in GSM network. The air interface is provided to MS by UTRAN, UTRAN contains BS and Radio Network Controller (RNC). The CN contains two domains: circuit switched and packet switched. Circuit switched domain comprise of Mobile Service Centre (MSC), Gateway MSC, Visiting Location Register (VLR). Packet switched domain comprise of Serving GPRS Support Node (SGSN) and GPRS Gateway Support Node (GGSN). Network elements: Equipment Identity Register (EIR), Authentication Centre (AuC) and Home Location Register (HLR) are shared by both domains.
2.5 WLAN STANDARDS
The WLAN standards are developed by IEEE and are mainly characterized as IEEE 802.11X series: IEEE 802.11a, IEEE 802.11b and so on. The IEEE 802.11 standards use different techniques for physical layer implementation:
- Direct Sequence Spread Spectrum (DSSS)
- Frequency Hopping Spread Spectrum (FHSS)
- High Rate Direct Sequence Spread Spectrum (HRDSSS)
- Orthogonal Frequency Division Multiplexing (OFDM)
The data rates provided by IEEE 802.11 standards range from 11Mbps to 54 Ghz. Basically IEEE 802.11 standards comprises of Basic Service Set (BSS). BSS contains more than two portable, fixed and moving stations/nodes that interact with each other in a limited geographical area over the air. There are mainly two configurations defined in the standards:
- Ad hoc Mode
- Infrastructure Mode
The Ad hoc mode is also called as Peer-to-Peer Mode. In this mode mobile stations directly interconnect to each other by passing Access Point (AP). The MSs in this mode are independent and equal. Without the use of internet, packets can be broadcast. It is easy to deploy this mode when the network cannot be accessed by the user or there is no need of the network.
However, in the second mode AP act as a medium to connect with, for the wired networks and the MSs. The BSS services can be linked to a distributed system. When a MS shifts from one AP's coverage area to another AP's coverage area the procedure of handoff takes place.
3. THINKING TOWARDS THE INTEGRATION OF 3G AND WLAN
3.1 IS 3G ENOUGH
Now a day, because of globalization business professionals are constantly on move and they are looking for an effective solution to remotely access the information system and databases using internet as backbone. However, some application demands high data rates and which call for high transmission capability.
The advancement in WLAN standards are going on worldwide, which aims for the data rates higher than 100 Mb/s and the success of WLAN in numerous places, justify that WLAN network will have an important role in wireless transmission. This is well understood by the cellular operators and is eager to exploit this technology and integrate with their networks. A question comes in mind that IS 3G sufficient for the ever increasing demand of wireless users. The answer seems to be more negative than positive. There are theories about 4G that has to come concurrently with 3G networks, but 4G is still not completed and not yet efficiently launched. Again 4G systems must have the capabilities that exceed far more than those is in 3G networks. The key factor distinguishing 3G and 4G will be the higher data rates. Therefore, it can be said that the future of wireless networks are beyond 3G. The data rate trends are shown in the figure and figure
its can be seen that the higher data rates are possible in WLAN networks.
Both the network possesses features that complement each other. As discussed above, cellular systems provides high mobility but are slow whereas WLAN are fast but have limited coverage. Therefore, it is now need to utilize beat features of both the technologies to address a new generation that can fulfil the ever increasing user demand of high data rates for data intensive applications and to provide seamless roaming anytime and anywhere.
3.2 3G VS WLAN
Comparision between both the networks is sumarrised in the table given below:
From the above comparison it is clear that the main advantages of 3G are: high range and mobility whereas WLAN has an advantage of high speed. These advantages can be summarised in the figure given below.
6. METHODS OF INTEGRATION OF WLAN-3G
According to the 3GPP release 6,the intention behind integration of WLAN-3G is to add more functionality to WLAN and extension of 3G services. WLAN then can be seen as a 3G complementary radio access. Either the 3G functionality can be in parallel with WLAN or behind WLAN.
The main methods proposed for integration of WLAN-3G are:
- Tightly Coupled Integration Method.
- Loosely Coupled Integration Method.
- The Mobile IP Architecture Method.
- The Gateway Architecture Method.
- The Emulator Architecture Method.
- Peer Integration Method
1. Tightly Coupled Integration Method.
The main idea behind this integration method is to create a scenario for 3G network, where 802.11 WLAN network will appear as some other 3G network. The WALN network would imitate the functions that are native to the 3G access networks. The ‘802.11 Gateway', employed by WISP No.1 in the figure below, appear as a PCF in cdma2000 or as SGSN in UMTS to the 3G core network (CN). The 802.11 Gateway apply all the protocols of 3G network such as, mobility management, authentication etc, which are required for radio access network in 3G and also obliterates the details of WLAN. Mobile nodes needs to apply the equivalent 3G protocols on the top of their network card and shift as needed from one layer to another.
In WLAN-UMTS integration a MS is allowed to connect to both WLAN and UMTS network. The packet switched services for data are provided by WLAN and circuit switched services for voice by UMTS. The core network includes SGSN and GGSN. The radio frames are converted into IP packets through a mediator RNC and IP packet again to radio frames through SGSN. The IP packets are routed between GGSN and SGSN and then through SGSN and RNC. The access router handles intra-AR handoffs and intra-domain handoffs are handled by Mobile IP. Protocols at IP layer like, RSVP,MMP are used for reservation and QoS signalling and mobility management in WLAN.
3G protocols are used to transmit all the traffic of WLAN network users in 3G CN. The infrastructure for authentication, signalling, billing and transport will be same for the two networks but will independent of protocols needed at physical layer.
Pros and Cons
The main advantage of the method, is control of quality of service is easy for time sensitive traffic.
The disadvantages includes: same operator need to own both the network. Modifications at physical and upper layer are needed for MSs. Network cards will be expensive as both interfaces are needed.
2. Loosely Coupled Integration Method
The loosely- coupled method needs an introduction of new element, the 802.11 gateway. The 802.11 gateway in this approach does not have any direct link with the PSDNs and GGSNs elements of 3G networks, but a direct connection to internet as shown in fig. The 802.11 gateway may have mobile users from other networks as well as those who are signed in locally. The data path is completely separate for 3g and 802.11 and 802.11 traffic is not transmitted into 3G but still seamless access is achieved.
There will be different protocols and mechanism for authentication, mobility management and billing in both the networks. However, they will interoperate to provide seamless access. In cdma200, the 802.11 gateway uses Mobile IP to allow mobility and AAA services to work with 3G home AAA services. This would help to 3G providers to collect accounting records from 802.11, prepare billing statements.
UMTS standards do not support Mobile IP and AAA protocols yet. For integration to happen Mobile IP needs to be modernised to suit GGSNs to allow seamless roaming. To assist billing and authentication for UMTS, a common database would be require to interact with HLR.
Pros and Cons
The main advantage is its low cost and less complexity. Deployment and traffic engineering is independent for 3G and WLAN. Little alteration needed on 3G networks. Little labour needed for accounting and billing issues.
The only drawback is to offer QoS guarantee for traffic i.e. time sensitive as QoS guarantee for internet is itself difficult.
3. The Mobile IP Architecture Method
In this approach, 3G and WLAN networks act as peer-to-peer networks. The Ms in 3G use session management and GPRS mobility management for roaming and to deal with Packet Data protocol sessions across WCDMA networks. In WLAN, IP is used directly by the MS, Mobile IP may be used in WLAN for mobility management. Dual mode protocol stack is employed in MS, which contain stack for both networks.
IP stack is used for handoff from 3G-WLAN, same IP is maintained for WLAN-3G handoff, to provide continuous connectivity. To allow the routers to forward and tunnel the data packets Foreign Agent (FA) and Home Agent (HA) are added in GGSN for 3G and AR of WLAN. A Care of Address (CoA) is used to identify the MS, when MS is outside home network. The de-encapsulation and packet delivery is managed by CoA associated with Foreign Agent (FA). The CoA is also given to HA. The HA intercepts the datagram sent to the MS's home address as well as encapsulate them to related CoA. The datagram addressed to MS are routed via HA while datagram from the MS are sent by the routing system of internet via an optimum path.
When a MS wants to move from 3G to WLAN, the handoff first starts at Layer1/Layer2. To avoid UMTS connection loss MS sends PDP/MM context (Packet data protocol/mobility management) standby message to the associated SGSN, to reduce the effort of reconnection if MS moves back after a time period. MS access the IP network and to locate FA it sends Agent Solicitation, MS receives the reply from FA and then sends a Registration Request to HA. After a CoA is updated in HA, packets sent to the home network are re-routed to the network MS currently in.
The WLAN -3G handoff is same as the above, the sole difference is, if MS has not initiated the PDP session, before getting any service it should activate the PDP session.
Pros and Cons
The advantage of this approach is that it is based on concept of the Mobile IP. The same Mobile IP is retained which resolves the issues related to address and provide sort of continuous connectivity.
The disadvantage is the triangle routing which is crucial for real time applications
4. The Gateway Integration Method
This approach uses a gateway for interconnection. The gateway act as a mobile proxy between WLAN and 3G, helps in mobility and routing. The MS in 3G uses session Management, GPRS mobility management and IP to link u with the WLAN network. The MS, in WLAN may use Mobile IP to carry out mobility within WLAN. All the control signals and data packets are sent via the Gateway. The aim of this is to allow separate operation of both the networks and ensure roaming between both the networks. Billing can be provided separately or combined.
In UMTS -WLAN handoff, the handoff procedure starts at L1/L2. The MS tries to incur a gateway address to execute inter-system handoff. It uses Dynamic Host Connection Protocol (DHCP). In visited network to get a Gateway address, M|S sends an DHCPDISCOVER. The Gateway replies with its IP address. After receiving Gateway address, the MS sends Router Area Update using 3G network IP address to the Gateway. The Gateway then sends update PDP context request to the GGSN to change its in use SGSN address. GGSN knows about MS movement to WLAN, when it receives PDP context from Gateway. The Gateway then acts as temporary
SGSN, packets are then forwarded to Gateway in place of old SGSN. The PDP-MM contexts are restored to avoid reconnection procedure. MS then sends packet using its 3G network IP address. If ingress filtering is not performed by the WLAN, then packets from MS can be sent to the internet through WLAN, otherwise it is forwarded via Gateway to the internet. IP routing is used to forward packets to MS.
When MS wants to move to 3G from WLAN, 3G procedures are performed by MS.
MS can start getting services if it is already attached to the 3G network, otherwise, it needs to activate PDP context. The Gateway is used as Access Point Name (APN) by the MS to tell SGSN, it wants to make use of WLAN IP. WLAN IP is used to request PDP context. When Gateway detects IP as WLAN IP, it responses MS with the same WLAN IP.The gateway acts as GGSN in 3G. The outgoing packets are sent to the gateway via SGSN and incoming packets are received by SGSN via Gateway.
Pros and Cons
All the signalling messages are sent through the internal network, so packet loss is less and handover procedure is fast. Routing inefficiencies and encapsulation is minimized as Mobile IP is not used. Both networks can take care of their single mode client independently. Reuse of gateway is possible and may be used for inter-technology roaming and mobility management.
To support gateway method 3G standards and internet are not sufficient. Additional controls in 3G protocols are needed. More detail is needed for exchange of HLR information and AAA. If a single gateway is applied and if it fails, entire network will fail. So better fault tolerance and gateway deployment is needed
5. The Emulator Integration Method
In this approach 3G and WLAN are peer-to-peer networks. The AP of WLAN is connected to the 3G-SGSN via RNC emulator, to facilitate packet transfer between WLAN and UMTS. MS is always consider as 3G user even if it connected to WLAN. WLAN is considered as a routing area related to 3G-SGSN. A dual mode MS is needed to access both networks. SM and GMM of 3G, are used to handle session and mobility management even if the MS is connected to the WLAN. The MS cannot connect to internet directly through WLAN. WLAN in this approach can be seen as a slave of 3G network.
For WLAN-3G handoff, MS needs to interface with UMTS Packet Data Convergence Protocol network via RNC Emulator. Signalling protocols related to UMTS are carried out between RNC emulator and MS. Packets are transmitted using IP protocols to the Iu interface and CN.
WLAN is considered as one routing area, if a mobile moves or come back, an update message is received by CN. Continuous connectivity is maintained as IP address is not changed.
Pros and Cons
The key advantage of this approach is roaming, billing, mobility management is taken care by 3G networks. The security and QoS used for real time services in 3G can now be used in WLAN, thus threat related to security in IEEE current standards. Handover delay is less than the two above mentioned approaches.
The main drawback is, each packet needs to pass through GGSN which becomes the chokepoint of the system. The higher data rates of WLAN degrade to suit the data rates of 3G terminal. WLAN terminals need to be modified which increases its cost. Thus two feature of WLAN are lost: speed and cost. Though this approach is efficient still it is not standardized, it is under study.
6. Peer Integration
Varma VK et al, proposed this approach, the summary as follows: It employs Mobile IP and both WLAN and 3G subscription will be available to user. The CN of UMTS will have both HA and AAA server functionality. The 802.11 WLAN Access Gateway (AGW) acts as an interface to core IP and is connected to several ESSs. To allow mobility WLAN also add HA and AAA functionalities and have HLR copy to facilitate mobility across UMTS.
In UMTS-WLAN handoff, the MS needs to first associate itself with the AP and then proceed with AAA functions. After acquiring CoA, Binding Update and Acknowledge messages are sent between MS and HA. The HA sends a location update message to Home Subscriber Service (HSS), to delete the last SGSN address and associated PDP context. CN network receives Binding Update from MS, to send packets directly to it rather tunnelling them. When MS returns back to the UMTS it need to execute PDP attach activation. The rest of the procedure is same as above.
In WLAN-UMTS handoff, the connection with UMTS is done by establishing UTRAN radio bearer and sending Attach message to UMTS. For authentication of MS SGSN communicate with HLR of WLAN and then PDP context activation is done. A CoA is obtained after completion of Mobile IPv6, Binding Update/Acknowledge takes place between MS and it's HA in WLAN. Packets coming to WLAN are now tunnelled via GGSN again tunnelled to MS. While returning to WLAN, MS needs to associate itself with, then execute AAA fuctions. Exchange of Binding Update/Acknowledge takes place between AA and GGSN. The PDP context is deleted and packets addressed to MS are forwarded to MS.
Pros and Cons
The key advantage is complexity is very low, as both networks are independent and only AAA link is attached. Mobility management issues are also less complex.
7. Other Possible Integration Methods
There are many other proposed possibilities for integration of WLAN-3G .We will just have a brief overview.
The one possibility is, use of Virtual Access Point (VAP) to integrate WLAN-3G. WLAN will be the main network and 3G will be the secondary. WLAN standards are used for mobility management.3G network will be like a service set connected to other AP. The main function of VAP is to interact with the MS linked with 3G, de-encapsulate and route the packets on WLAN. The packets are sent and received via the router. The large overhead of packets makes this approach inefficient.
Zhang,et al, has proposed a vertical handoff approach. The MS will use available high bandwidth of WLANs and shift to 3G network when the signals are not good in WLAN or the WLAN coverage is not available. A Virtual Connectivity Manager to keep the connection going using end-to-end principle without need of ay additional infrastructure support. All the handoff procedures need to be transparent to the upper-layer applications and should use handoff decision algorithms and metrics. Mobile IP will be used to facilitate mobility management.
Wireless communication scenario is continuously evolving since it started first. The user demands are increasing and 3G alone is not enough to meet these increasing demands. There is need for heterogeneous technology to support ubiquitous wireless access. Integration of WLAN-3G can provide variety of services and ubiquitous connectivity.
In this paper, i have given brief overview of 3G systems and WLAN, need of integration and explored more on different methods of integration of IEEE 802.11 and 3G systems.
The integration of both standards will be advantageous for both service providers and end users. As the two technologies use two different platforms, there are challenges in integration of both, especially, security, QoS and seamless roaming.
A further study is needed in areas including: fast authentication, billing, resource allocation and fast handoff procedure.