ITS has a wide range of applications, but not all of them are currently practical. Many applications are still at the conceptual stage. Much research has focused on Ad-hoc networks concerning ITS communication. However, the Ad-hoc network had some problems still to remain unresolved. UMTS is a universal and reliable technology but does not fully applicable to the ITS. Because such that the article attaches importance to the realization of these applications. we have introduced some useful characteristics to be applied to ITS in the field of telecommunications, and we utilized the existing UMTS techniques to support them. However, we proposed two practical protocols to service the needs of ITS.
In this section, we will analyze and discuss the benefits of our proposal. We will first simply reiterate the basic characteristics of ITS communication, and then explain why our proposal is suitable in ITS environments. In conclusion, security is taken as a point to discuss briefly.
According to the recipient of the message, ITS wireless communication can be classified into two types.
(1)Vehicle-to-roadside communication: it indicates that the vehicle and ITS deliver data by means of wireless infrastructure. Generally it is the ITS that delivers relevant data to the vehicle. Those data may regard road conditions, video, or audio.
(2)Vehicle-to-vehicle communication: it indicates that the relevant data is delivered between vehicles. In this mode, one can view the vehicle as the router. It resembles the Ad hoc network as it can automatically search and link the neighboring vehicles to form a topology. Moreover, neighboring vehicles also can link themselves together to exchange messages.
ITS wireless communication can also be classified according to the data contents.
Common messages: this type of message is usually when many vehicles transmit the same message, such as traffic reports or multimedia. Some common messages must be kept private from nonauthorized users; only a legal user may receive them. This is due to the fact that legal users must pay for the contents of the data, and nonauthorized users may want to steal it. According to the data provider, data can be classified into two types.
Content provider is the centralization. Their data source is fixed and the connection is continuing, such as audio, video, or movie.
Content providers are the localization. Their data source is not fixed but will follow the vehicle location to change, such as traffic information, travel information, traffic control messages, or traffic management messages.
Private messages: these must always be kept confidential.
These characteristics help us understand the suitability of our scheme.
Currently, developments in wireless communication on ITS concern an exclusive network called VANET. VANET was constituted by an Ad-hoc network which automatically links the neighboring vehicles to form a topology. However, Ad-hoc is an exclusive network technique under development. The biggest problem is that it is not stable. Its topology is constituted by mobile nodes (vehicle) that may change at any time. An inherent drawback is that vehicles that are too close or too far cannot communicate. To solve this defect, fixed access points have been used to strengthen the signal, since the outdoor wireless network's coverage is still not comprehensive. However, it will be quite expensive to build the necessary infrastructure to solve this problem. These problems are completely avoided with UMTS. More importantly, its infrastructure is nearly complete, thus avoiding the problems of lack of signal coverage.
Seamlessly UMTS is the best platform to use with ITS. It can support vehicle-to-roadside as well as vehicle-to-vehicle communications when our modifications are used. The original UMTS was supported by IMS and MBMS to transfer private and common messages. However, when applied to the environment of ITS, it will have some additional considerations, especially the fast movement caused by rapid changes in message routing and multicasting members of the frequent changes. Both of them will cause a heavy load on the MBMS. For this, our protocol introduces the concept of group keys. Moreover, in the UMTS, IMS and MBMS are two separate systems, and the MBMS must be used with the IMS. This has caused wastage of resources and authentication delays. For this, our protocol integrates them into a single protocol. However, to solve the bottlenecks, our protocol is an expansion of the existing IMS-AKA protocol using support group key. This does not only combine IMS and MBMS but also solves the problems of MBMS in the ITS.
Two protocols are proposed in this paper. One is an improvement over the IMS-AKA protocol; another is a novel V2V-AKA protocol. The improvement over the IMS-AKA protocol was achieved by integrating a vehicle key on the UMTS-IMS-AKA protocol. The V2V-AKA protocol is an innovative design. It makes direct communication between vehicles possible on the UMTS. The suitable collocation of our IMS-AKA protocol with our V2V-AKA protocol can get all-round development on ITS. Generally messages delivered by our IMS-AKA protocol and interaction with vehicles are shared with our V2V-AKA protocol. Some of the applications can be accomplished with slight revision. For example, ETC (Electronic Toll Collection) system can to regard our protocol as their charging solution. In this scenario, P-CSCF corresponds to the tollbooth. When a vehicle passes through the tollbooth, the tollbooth sends the authentication request to the vehicle. Our IMS-AKA protocol can be adopted in this scenario. With our protocol, authentication and authorization are assured, and the billing program can also be solved.
4.3. Security Analysis
Our proposal has the structure of the original IMS-AKA protocol and inherits its security features. All the fundamental conditions of security  on the IMS-AKA protocol are also attained in our protocol, including anonymity and untraceability. Moreover, our protocol can achieve three objectives (confidentiality, integrity, and availability)  of ITS to resist four general threats (deception, disruption, usurpation, and unauthorized disclosure) . Only authorized vehicles can have the secure key and pass the authentication and get the relational key and . All unauthorized vehicles intercept unreadable secret content. Moreover, as in the IMS-AKA protocol, our protocol has the enhanced feature of mutual authentication.
On vehicle-to-roadside communication, in order to retain compatibility with the original IMS-AKA protocol, we have fine-tuned the original protocol. The main change is that we have joined a vehicle key, . However, is generated by and ; is a random number selected by the vehicle; is a secret key shared between the vehicle and HSS. By the way, the vehicle has participated in the decision of because is offered by him. The advantage is that a vehicle can very easily identify fresh , and prevent a malicious attacker from stealing and reusing it. The group key, , used for encrypting the group messages is encrypted by and delivered to the corresponding vehicle. Therefore, will not leak it in the process of transaction, and group messages can also maintain secrecy.
On the vehicle-to-vehicle communication, our protocol combines UMTS and Ad-hoc. Both the technologies rely on the UMTS authentication mechanism that verifies the identity of vehicles and uses the Ad-hoc network architecture to communicate between the vehicles. It has the advantage of UMTS's security and Ad-hoc network's convenience. Moreover, vehicles know each other, and only the IMPU can ensure the anonymity of the two sides. Using to encrypt the meeting key, , and the transaction can guarantee that will not leak during the transmission.
Keeping with these discussions, a conclusion can be derived that our proposed is based on the original IMS-AKA protocol and continues to develop. In our expanding function, the random number, , and the secret key, , making the vehicle key, , has privacy and security. Since the is secure, and protected by are also secure; therefore, our protocol is secure.