How do Mobile IP and CDPD compare? We can compare these systems from a variety of perspectives including:
¥ Objectives, goals and design assumptions
¥ Technical architecture and design
¥ Model and terminology
¥ Operational assumptions
¥ Standardization process
¥ Potential
We will follow our comparison with a prediction and a discussion of issues surrounding co-existence and convergence between these standards.
Obviously, the goal is to objectively compare Mobile IP and CDPD. In particular,
¥ For the most part we limit ourselves to a comparative analysis of the CDPD specifications and current Mobile IP specifications. In other words we do not compare existing or future implementations of mobile devices or networks.
¥ We limit the comparative analysis to the mobility dimension of CDPD. This limitation is necessary for a meaningful comparison. In addition to mobility support, the CDPD specifications cover the airlink protocols, network management, network administration, accounting and conformance testing. Thus, we limit ourselves to Part 500 (Mobility Management), Part 501 (Mobile Network Location Protocol), Part 507 (Mobile Network Registration Protocol) and Part 406 (Airlink Security) in our analysis of CDPD.
Both CDPD and Mobile IP require mobile hosts to be able to communicate with other systems that do not implement mobility functions. No changes or enhancements are required for systems that do not support mobility, to be able to communicate with mobile hosts.
Mobile IP makes no assumption about any particular link layer technology. One of the driving requirements for design of Mobile IP was that it should be completely independent of the data link.
In the case of CDPD, there were no external requirements for support of data links other than the CDPD airlink (an overlay on AMPS). From the onset the CDPD architects recognized that mobility for CDPD could be independent of CDPD's airlink. To this end, CDPD was designed under a self-imposed requirement for CDPD mobility to be independent of the airlink.
CDPD treats the airlink as a precious resource and minimizes the number of bytes transferred over the air. Mobile IP recognizes that the link by which a mobile node is attached to the Internet may often be a precious wireless link and Mobile IP considers optimization over that link.
When trade-offs were made between layering integrity (Layer 3 and Layer 2) and airlink efficiency in CDPD, they were in favor of airlink efficiency. Mobile IP is contained strictly within Layer 3.
CDPD was designed to not only support IP, but also to be a multi protocol mobility solution. Mobile IP is a pure IP solution.
Both CDPD and Mobile IP require that mobility be supported without the mobile system needing to change its IP address. This is a departure from existing IP networks.
CDPD assumes that the network is centrally administered, managed and operated by cooperating cellular Service Providers. Mobile IP assumes no additional constraints beyond the existing mode of operation of the Internet. This is probably the most fundamental difference between CDPD and Mobile IP and has serious ramifications on address assignment and security.
CDPD is based on early Mobile IP work, and thus resembles but does not exactly match Mobile IP.
The triangular routing mobility approach is essentially the same in both CDPD and Mobile IP.
The following enumerates major areas of design differences between the two approaches:
¥ The user's IP address must be assigned by the CDPD service provider. Mobile IP makes no such assumptions.
¥ Mobile IP allows for co-location of mobile node and the foreign agent. Combining the M-ES and the Serving MD-IS was not considered and is not practical in CDPD.
¥ CDPD's mobility tunnel is based on CLNP. Mobile IP's mobility tunnel is IP-based.
¥ Mobile IP is a pure IP design. CDPD is a multiprotocol design.
¥ Mobile IP operates completely above the data link layer. CDPD mobility is mostly above the data link layer.
¥ Since the infrastructure of the CDPD network is closed there are less security considerations for CDPD.
The terminologies of CDPD and Mobile-IP are different. CDPD is following the OSI reference model terminology.
Table 10.1, maps key concepts of Mobile IP to the corresponding terms in CDPD.
Mobile IP uses the Internet as an operational model. Operational assumptions for Mobile IP are the same as those for the Internet, which can best be described as "managed chaos".
On the other hand CDPD assumes a clearly defined network with well defined boundaries of authority and responsibility. The CDPD internetwork is a collection of CDPD service provider networks. Based on bilateral agreements, each Home MD-IS interoperates with various serving MD-ISs, which may be administered by various CDPD service providers.
The infrastructure of the CDPD network is a closed network. This implies that some level of trust, order and accountability can be expected."Control and Order" expresses the general flavor of CDPD's operational assumptions.
These operational assumptions had a direct impact on many of CDPD's protocol design decisions, particularly in the areas of security, manageability and scalability.
Because the mobility tunnel begins and ends within the CDPD networks, to some extent it does not require the level of security that is necessary between the home agent and the foreign agent in Mobile IP.
Even though, securing the network layer of the CDPD network is not required in the CDPD specifications, the CDPD specification team recognized that providing data confidentiality and authentication for the mobility tunnel was important. Network Layer Security Protocol (NLSP), which can be considered an adjunct to CLNP, provides comprehensive security services. Unfortunately, it is a lot more theoretical than real.
Mobile IP specifications are the product of one of IETF's Working Groups. The standardization process is completely open and based on volunteers' effort. This process is also complex and dynamic. "Rough Consensus and Running Code" best expresses the general flavor of Mobile IP's standardization process.
Sometimes, IETF Working Groups are highly efficient and produce high quality specifications in short time frames. That has not been the case with the Mobile IP Working Group.
The CDPD specification effort was funded by a group of cellular service providers. The specification was developed by a small team of paid consultants under the direction of cellular service provider representatives. There was significant schedule pressure in the CDPD specification development process. The CDPD Release 1.0 specification was developed in less than a year, and the following Release 1.1 was developed in about one year.
The value of coupling mobility with Internet access is significant. Any solution that provides mobile connectivity to the Internet is likely to be in high demand and heavily used. From this perspective, the opportunity for widespread deployment of both CDPD and Mobile IP technologies is immense.
However, in some ways these technologies are in competition with one other. Unless CDPD services are widely deployed soon and unless the subnetwork-independent characteristic of CDPD mobility is further developed and adopted by users of other than cellular-based media, it is unlikely that CDPD mobility will become the mainstream solution to Internet mobility.
The Mobile IP standardization process has been quite slow and the base specification for Mobile IP has not yet reached RFC status as of this writing. However, Mobile IP enjoys certain characteristics which seem to ensure its survival until it becomes widely adopted. These characteristics include complete openness, subnetwork independence, user orientation, and proven correct specifications.
Another key factor in the widespread deployment of any network is the fitness of the network operators and equipment manufacturers for the job at hand. CDPD network operators are cellular service providers and CDPD equipment manufacturers are typically tele-communication equipment suppliers. Mobile IP network operators are likely to be Internet Service Providers and Mobile IP equipment manufacturers are typically data-communication equipment suppliers. We note that the entire Internet phenomenon took place essentially independently of the public telecommunications providers.
It is important to recognize that even though CDPD mobility has the potential to become a generalized mobility solution and compete with Mobile IP, it was not designed as such. Since CDPD's mobility management scheme is similar to that of Mobile IP, either could be adapted to interoperate with the other. Depending on the degree of integration required, this could be a relatively easy or a significant job.
Finally, CDPD devices are native IP hosts which can communicate with Mobile IP hosts without any modifications. The whole point of the Internet is that the local subnetwork connection is largely irrelevant to communications with the rest of the IP-based world. This is the foundation and the benefit of layered communications protocols. It is the communication itself and not the protocol used that is important.