Cell Relay Archive

[rajeev@trillium.com (Rajeev Gupta)]

> From: albert.e.manfredi@boeing.com
> 
> This is most interesting to me at the moment. Also, this difficult problem,
> as far as I can tell, is identical to that faced by mobile IP. Do you have
> a brief description of the general approach taken by the ATM forum that you
> could share on here?

The procedures for location management are quite similar for mobile IP
and WATM. In mobile IP, the mobile host registers with its home agent
(via a foreign agent). This is similar to a mobile terminal registering
with its HLR (home location register) via a VLR (visitor location
register) at the visited MSC (swtich). 

In WATM, the mobile IP approach is referred to as the "integrated"
approach where the location register (and associated authentication
mechanisms) is collocated with the home switch. WATM also permits a
modular approach where the two are split (like in traditional cellular
systems).

Each time the mobile terminal enters a new location area, it registers
with the visited switch and obtains a new temporary address - this
address is then sent to the home switch to create a new association
between the mobile's permanent address and its current temporary
address.

For placing a call to a mobile terminal, the call setup message is sent
towards the home switch (normal PNNI routing). Any switch along the way
that recognizes that the call is for a mobile terminal (e.g. if the
address space was partitioned between mobile and non-mobile terminals),
can query the location server to obtain the current location of the
terminal. Obviously, there is at least one switch in the path (the home
switch) that can do this function.

Once the location is determined, the call can be redirected towards the
new location with or without teardown. In the former case, the call is
"released" back towards the calling party with a cause indicating that
the mobile is away and giving the current location. This can be used to
set up a more optimal path towards the mobile terminal. An alternative
is to extend the call from the switch that made the location query
towards the mobile terminal - this may result in sub-optimal route, but
call setup will proceed faster.

One can note that mobile IP provides for both "call setup" scenarios.
The default mechanism is that packets sent to the mobile host reach the
home agent and are then redirected towards the foreign agent/ mobile
host using tunnelling. An option is to convey the foreign location to
the correspondent hosts so they can send packets directly to the new
location, and thus avoid "triangular" routing.

For handover, the mobile IP problem is simpler, as there are no
connections to maintain. In fact, since IP allows packets to be lost and
misordered, handover can be as simple as the mobile host re-registering
with its home agent at the new location. This will result in the old
tunnel being torn down and a new tunnel setup to the new foreign agent.
Of course, since re-registration can take time, there are options
available to allow the old foreign agent to forward the tunnelled
traffic to the new location. There can be further complexities if the
correspondent hosts have to be updated with the new location.

In WATM, the handover issue is still far from being settled. The basic
architecture indicates that when the mobile terminal moves (it
determines this based on measured radio signal strength, etc.), it
requests the old switch to initiate a handover, giving to it a list of
target new radio ports. The old switch will then query the targets and
based on resource availability, select one of them to be the new port
(note that the new port can be on the same or different switch).

At this point, there are a number of ways in which the connection can be
rerouted to the new switch. One possibility is to extend the connections
from the old switch to the new switch (quick to execute but may waste
network resources). Another is to reroute connections from a fixed
anchor switch in the network (similar to anchor MSC routing in cellular
networks). Another possibiity is to build a new path all the way from
the calling party to the new switch (latency may be too high). Finally,
a partial reroute may be done by finding an intermediate cross-over
switch that can switch the connection from the old switch to the new
switch, such that the new path is optimal.

There is considerable debate in the WATM group on which method(s) to
choose or whether to standardize anything at all. Another area of
discussion is how the mobile terminal communicates with the switch to
conduct handover signalling e.g. directly with the switch on the UNI or
with the radio access point or some combination.

Hope this answers some of your questions.

______________________________________________________________________
Rajeev Gupta                            Trillium Digital Systems, Inc.
mailto:rajeev@trillium.com                     http://www.trillium.com