The IP over ATM Mailing List Archive by date

[<P82609@vnet.atea.be> (Lode Coene)]

> If you want to compare things to telephone systems then the problem to
> be solved here is not really the traditional "how do I phone somebody", but 
> rather "how do I phone somebody using their personal-800 (or national 
equivalent) 
> number" in a world where _everybody_ uses personal-800 numbers. This
> is a number which belongs to me, (say 1-800-GLEESON), which is independent
> of the actual phone number I use (say 1-408-945-8600), and which may
> map to different "real" numbers at different times (if I go on a business 
trip
> for instance). The mapping of personal-800 numbers to "real" phone numbers 
> is thus somewhat analagous to mapping IP addresses to ATM addresses. It is 
the
> handle by which I am known, the mapping is needed to find out where to place 
the
> call to, and the mapping is fairly static but may change. It would be 
interesting 
> to know how telephone companies currently handle this today, and how they 
would 
> handle it in a world where everybody used such a number. If they were 
allowed to 
> introduce some hierarchy into the 800 number space would they do so, or 
would it 
> stay flat ? (In the U.S. 800 numbers used to be hierarchical but now due to
> laws requiring 800 number portability between different carriers they are
> now "flat", I believe). Saying that very large IP-ATM flatnets make sense
> because the telephone system works is a bit simplistic, IMHO.
>
The best way to explain how a universal number works, is to use a mobile phone 
network. The public netwrok is just a special case were the boy/girl with the 
phone has to explictly notify a database via the phone of his location whereas 
the mobile network does this automatically without human intervention.

So we now dealing with a location-update operation(I explain later why I call 
it a operation) in the public or mobile network. The operation is trransported 
via a connectionless network(called signalling system nr7 or SS7) to the 
database concerned(be it called a signalling control point, a Home location 
register or database). The routing of that message is based on the digits of 
the universal number(take 1-800-GLEESON).  The orignator node put the number 
in the message and the network layer of the orginator node(=A) will translate 
this number based on the first 2 or 3 digits to a pointcode, of a node(B) 
which the originator node thinks is nearer to the final destination(=D).( It 
could be that the node is the final destination node but that is exceptional) 
Now the message is routed using the generated pointcode to the  B node( just 
the same as IP does with its DPC, OPC is left alone). When the message arrives 
in the B nodes then ihe networklayer will find that this is not the final 
destination, so yet another translation is performed on the number, yielding 
another pointcode(this time of the node C) The rules to which this translation 
confirms are under the discretion of the network operator. All the pointcodes 
generated are local to the operator network only, they have no meaning outside 
the operator network. The number or as we call it the Global Title is 
universal or global(it should conform to the E.164 numbering plan in a 
telephone network or to another numbering plan=some application do need a 
separete numbering plan for efficient routing).  Now in C the we cross a 
network border, that means  we are going from one operator to another operator 
network Here also a translation is done on the called part addres(=destination 
number), but the calling party address of node A  is also changed, because 
otherwise the return message would not be able to find A back.  Now the 
message finally arrives in D and it could be that it is directly routed to the 
application if theDPC was indeed the pointcode of D or it could also be that a 
translation is in order before it knows it has arrived.

Now the database is updated with the present location of the subcriber(say the 
number 1-408-945-8600) which from the point of the SS7 network is also a 
routable number)  and a result message is send back to A informing that the 
update was sucessfull. (Well a typical remote operation). Other operations are 
also posible: short message service, send-routing-info(needed for routing the 
telephone call through the network) and a lot more. So before the actual call 
is routed through the network to the person concerned, query must be done to 
know its present location. The database containing that information may be 
directly linked to the SS7 network, or may even be located in a private 
net(which  is running TCP/IP or even X.25).  

The only premise is that the application and the ASE supplying the remote 
operation concept must be the same.  This is not always the case: The rest of 
the world has standaized itself on a ITU SS7 stack with ITU or ETSI defined 
applications(called sometimes CCS7 for Common channel signalling nr 7). The US 
and Canada are using a ANSI SS7 stack with ANSI defined applications. The good 
news is that up to and including the network layer, the functionalty and 
syntax of the message is the same. The bad new is that the application layer 
protocols have a equivalent functionalty and a radicaly different message 
syntax. And what is worse internetworking between the US and the Rest of world 
is not considered usefull at this moment.(turning the US into a SS7 island) To 
a European used to crossing networks borders at will  in the SS7 networks, 
this is nearly impossible to believe.

That means that a 800 number dialled from outside the US, a call has to be 
setup until it reaches a switch in the US network which can handle the 800 
query, and then the setup of the call is continued. This surely not the most 
efficient way as it is the goal that the call setup must be done after that 
the query to the database has be done.

If everbody has a universal number, then everybody has a database entry 
somewhere in a node somewhere in the world. To obtain number portability, a 
commonly owned database can be used(distributed of course over a number of 
nodes) or a redirection to the second operator  in the database of the first 
operator could be installed. 

As for the number of entries in a database, in a GSM or IS-41 net, a HLR can 
contain more than 200 000 entries, and most of these people willl be moving, 
so the location update alone will be generating most traffic. So short 
transaction over a connectioless network are indeed used, even in a telephone 
system.

As for the hierachy in the number, practices are different between the US and 
the rest of the World. In the US only country code, area code and subscriber 
part(I think) No mention of the operator which is sering the subscriber.  
Numbers in Europe(not universal, every country has its own rules) are of the 
form country code, operator code, area code and subscriber number. Some 
prefixes are even reserved for universal phone numbers, so these may have yet 
another different format.
>
> Regards,
> Bryan Gleeson
> Adaptec.

As for ATM, I am still searching for the ATM network layer:-). With all the 
AAL and SAAL stuff, you can build  your favourite network stack on top of it.  
 IP over ATM is one of those, and SS7 is also a  ATM user. So anything goes. 



Yours sincerly,
Lode Coene

Siemens  ATEA
SW lab applied research & protocols - CCS7 software II      tel: 
+32-14-25-2081
Atealaan 32                                                                    
            fax: +32-14-25-3025
B-2200 Herentals                                      Belgium