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Thursday, December 7, 2006

IMS vs. 3G vs. Wimax – Part I (Basics)

Disclaimer: I don’t know if and when there will be a part II, but I have a lot to say in this area, and I get the feeling that a single post will not be enough. So let me at least start the article naming in the right way. As I decide to author followups, I will link each post to the other.

Updated Dec 20 2006: Link to part II

There is a lot of confusion and misunderstanding in the market today on technologies that could potentially co-exist or replace one another. This article presents my view of how IMS, 3G and WiMax fit into the larger equation. I

First, let us talk about these conceptual layers: (You would find most network architectures are based on derivations of these layered principles)

Starting from bottom up:

Access Layer: Simply put, the Access Stratum involves all the physical characteristics and the mechanisms needed to connect devices with each other. This may be WiMAX radio engineering (802.16, 802.16e etc.), WiFI engineering, UMTS RAN or other technologies. The job of the access stratum is to define and implement the required protocols to be able to physically connect multiple nodes of a network together. An analogy would be, when you connect your desktop to an ethernet cord, the ethernet related protocols (defined in IEEE 802.3 and related) are responsible to ensure that my computer is able to ‘physically reach’ my wired router sitting somewhere in the back end.

IP Connectivity Layer: Most next generation network architectures have chosen to us IP as their connectivity layer. In other words, designers would like abstract the upper layers from the Access Stratum such that when people build applications on top, they see IP packets, IP addressess and the IP based mechanisms that have become the de-facto standard of the Internet. Of course, what that means is someone needs to figure out how to run IP over the Access Stratum – and there are a lot of ‘convergence subsystems’ that define elaborate technologies to achieve just this. This layer should be able to create an “IP pipe” over the “physical pipe” such that layers on top can use IP addressing to communicate without worrying about the intricacies of how the communication is really happening.

Session Management Layer: Now that we have an end-end IP pipe, where I can reach elements using IP addressing, how does one communicate with them ? How do I discover another user ? How do I request a phone call ? Or a Video call ? How do I, as a network provider set policies (example, Bob is barred from calling), charge (example, $10 flat rate for push-to-talk calls, $17 flat rate for 2 way audio) and provision the network ? This is what the Session Management Layer defines.

Mapping 3G, WiMAX and IMS to these layers:

3G is the full set of layers – it defined protocols from the Access Stratum through the Session Layer. Specifically, in the Access Stratum, it defined UMTS and W-CDMA as popular cellular technologies. As time passed, they realized that other disruptive technologies such as Metro-Wifi and WiMax could be alternate access technologies and that it is potential in the future, as these alternate access stratums get laid out, there may be large subscriber base that do not need cellular protocols but only use WiMAX, for example.

WiMax as it stands today, specifies the protocols for the Access Stratum (IEEE 802.16, 802.16e and related families). To add to this, the WiMax Forum defined a WiMAX based Network Reference Model which uses the ‘Access Stratum’ of the 802.16x specifications and specifies mechanisms of how to run IP over it. In addition, it also specifies the convergence subsystem in great depth, that is required to run a WiMAX based system and end up with an end-end IP pipe with good stuff like Mobility Management, QoS management, Resource Management and the rest, at the IP layer.

IMS is the ‘top most’ stack of 3G protocols. It uses SIP as a preferred choice for session management and specifies Policy, Charging, Location, Routing, Services and Session management so that devices connected to the network via an IP pipe can ‘do things’ like place calls, send IMS and Networks can figure out how to charge, bill, provision and manage.

So when people say ‘IMS is dead, long live WiMAX’ or the other way around, they are probably mixing things.

As an example, WiMAX forum has for long considered to adopt the work done by 3GPP/IMS to see how IMS as a session management layer could fit on top of the WiMAX NRM. WiMAX needs some session management layer, correct ? Now one could create proprietary session management layers on top which are not standardized (many of the current wimax trials use their own walled in session management layers) but there are a few problems with this:

A network architecture takes years to ‘thaw out’. Any network architecture that accounts for session management (so please don’t sight IP and HTTP, please), subscriber management, and heterogenous network interworking issues (example wimax caller calling a PSTN user, or wimax-gprs etc.) requires a lot of thought and design. Are you sure that if you start on your own path, four years down the line you will not re-invent the same thing that IMS already is ?
The motivation for an open standards network is just that – something that is validated and vetted by multiple organizations so that both the users and networks could benefit from muti-vendor interoperability.

On the other side, it is true that the IMS layer carrier within it a lot of ‘baggage’ that may or may not apply to a pure WiMAX network (call continuty anchoring at the IMS level is one example).

The key therefore, is to select a relevant subset of the IMS that is useful for WiMAX as perceived today, and then continue to build on top as the network evolves.

In short, while I think it is reasonable to say ‘long live WiMAX, UMTS is dead’ or ‘long live UMTS, WiMAX is dead’, it is incorrect to say that for IMS vs. WIMAX. They don’t play in the same layer !!


Right, I do have a lot to say. And I think I should end this article here. In subsequent parts, I would like to talk about how WiMAX and IMS coexist and the interworking mechanisms, including ‘Why you may eventually re-invent a lot of IMS even if you think you don’t need it’.

So there you have it, a promise for Part II and Part II at least...

Updated Dec 20 2006: Link to part II


  1. Good read.
    I believe the future will have IMS (or the likes) as the Core Network whatever be the access technology. Also co-existence of Wimax and 3G depends a lot on their interworking without which they are always going to be rivals.
    Waiting for part II (Interworking stuff).

  2. Excellent read. Very clear. Congratulations!

  3. Interesting to read. I think that some mobile operators are concerned on the SCIM layer functionality provided by next generation IN to have a brokering between the legacy and next generation protocols like Camel Phase 2 to ISC interface. Do you have any thoughts on this.

    -Subbzz, New Zealand