Quite recently T-Mobile has started to make use of the GPRS Network Mode of Operation (NMO) 1 feature in southern Germany. I haven’t seen any other operator using NMO-1 in Germany so far and only few in other countries so this came as quite a surprise. In this network operation mode, the circuit switched part of the network used for voice calls and SMS and the packet switched part of the network used for GPRS and EDGE data transmissions are connected via a signaling interface. This interface, referred to as the Gs interface, has a number of subtle but important advantages:
- During an ongoing GPRS / EDGE data transfer (TBF established), mobiles can’t detect incoming voice calls and SMS messages as they are focused on receiving packets and thus can not observe the paging channel. In NMO-1 (sometimes also abbreviated as NOM-1), the circuit switched part of the network forwards the paging message to the packet switched side of the network which then forwards the paging message between the user data blocks while a data transfer is ongoing. Mobiles can thus receive the paging message despite the ongoing data transfer, interrupt the session and accept the voice call or SMS.
- Location/Routing area updates when moving to a cell in a different location/routing area are performed much faster as the mobile only communicates with the packet switched part of the network. The packet switched network (the SGSN) then forwards the location update to the circuit switched part of the network (to the MSC) which spares the mobile from doing it itself. This is especially important for ongoing data transfers as these are interrupted for a shorter period of time.
- Cell reselections from UMTS to GPRS can be executed much faster due to the same effect as described in the previous bullet. Whithout NOM-1 an Inter RAT (Radio Access Technology) cell reselection with Location and Routing Area update requires around 10 to 12 seconds. With NOM-1 the time is reduced to around 5 to 6 seconds. An important difference as this reduces the chance to miss an incoming call during the change of the radio network. Also, ongoing data transfers are interrupted for a shorter time,an additional benefit that should not be underestimated.
Hi Martin,
Do you know in which areas T-Mobile has upgraded the networks to NOM I? I’d believe that the upgrade wasn’t cheap… wouldn’t make a lot of sense if they already have UMTS coverage in these areas…
Also, do you know if T-Mobile is the only operator that is interested in DTM? I think they offer the DTM in their US market too
Hello Red,
no, I don’t know, I’ve just observed it where I live. I don’t think that it has cost a lot since NOM I means installing a signaling connection in the core network between the MSCs and the SGSNs and to activate the functionality in the core and access networks. The functionality itself has been available in the software for some time so I don’t think it has been a huge step.
I think it is useful to have GPRS NOM I in areas with UMTS coverage as UMTS to GSM cell reselection speed benefits.
Not sure if you are mixing up NOM I with DTM. DTM = Dual Transfer Mode, i.e. phone calls + GPRS simultaneously. DTM and NOM-I have nothing to do with each other. I haven’t heard of any operator that has introduced DTM yet. Also, there aren’t a lot of mobiles supporting DTM yet. Noteworthy exceptions seem to be the latest Nokia N-series mobiles such as the N73, N93i, N95, etc.
Cheers,
Martin
Hello Martin,
Suppose i ve a mobile which support either PS or CS at a time(not DTM), i.e, if PS call is initiated and receive a CS call later, then PS gets paused and gives priority to CS. To send paging for CS call network should operate in NMO-1, am i correct? Will there be any difference if i use Vodafone (NM0-2) or T-mobile (NMO-1)in this particular scenario?
Hello Srinu,
In NMO-I, CS paging is performed via the SGSN and PCU for GPRS attached mobiles. In NMO-II, CS paging is done via the BSC. If a mobile is not GPRS attached, CS paging is done via the BSC in both operation modes.
Cheers,
Martin
Hello Srinu,
In NMO-I, CS paging is performed via the SGSN and PCU for GPRS attached mobiles. In NMO-II, CS paging is done via the BSC. If a mobile is not GPRS attached, CS paging is done via the BSC in both operation modes.
Cheers,
Martin
Hi Martin,
Regarding paging for CS service( Voice) as you said it will be done via BSC in NMO-II if mobile is GPRS attached.
Can you please explain how will it page mobile for voice call in NMO-II,
case 1: mobile is idle
case 2: PDP is active but , but no data transer
case 2: mobile is busy doing packet transfer
Will apprecite your help in clarifying my doubts.
-Mrin
Hi Mrinmoy,
As per my understanding, i answer u.
NMO – 2 (SGSN and MSC are not interconnected, so SGSN have no idea of incoming call info from MSC)
Case 1: Mobile is Idle
—It gets Paged for Incoming call
Case 2: PDP active, but idle
Case 3: Active Data transfer
For Case 2 n 3, Mobile is attached to PS services, so it wont get paged for incoming calls.
Please let me know i am not right
-Srinu
Hi,
In case 1 and 2 the mobile will see the paging and accepts the voice call. Only case 3 is critical since the paging is done but not noticed by the mobile. Hence the call is missed.
Cheers,
Martin