How the LTE Core Network talks to UMTS and GSM

An important functionality that has to be in place when LTE networks are launched from day one is the ability for mobiles to roam from LTE to other types of radio access networks. In most parts of the world except for the US and Canada, that is UMTS and GSM. While doing some research on this topic as to how that works from a network point of view, all books I have come across so far point to the new S3, S4 and S12 interfaces between the 2G and 3G network nodes (the SGSN and RNC) and the LTE core network nodes (or the Evolved Packet Core (EPC) to be precise), i.e. the Mobility Management Entity (MME) and the Serving Gateway (S-GW).

One might be happy with this answer from a theoretical point of view but in practice this approach might be a bit problematic. As the functionality has to be there from day one, using the new interfaces means that the software of the 2G/3G SGSNs and RNCs need to be modified. Now one thing you don't want to do when introducing a new system is to fiddle with the system that is already in place as you've already go enough work at hand. So I was wondering if there was an alternative to introducing new interface, even if only for Inter-RAT (Inter Radio Access Technology) cell reselection triggered by measurements on the mobile side.

It turned out that there is. After some digging, annex D in 3GPP TS 23.401 provided the answer (sometimes I wonder what is more important, the specification text or the annexes…). Here, a network setup is described where the 2G and 3G SGSN is connected to the LTE world via the standard Gn interface (Gp in the roaming case) to the MME and the PDN-Gateway. To the SGSN, the MME looks like an SGSN and the PDN-Gatweay looks like the GGSN. No modifications are required on the 2G/3G side. On the LTE side, this means that both the MME and the PDN-Gateway have to implement the Gn / Gp interface. But that's something that has to be done on the new network nodes which means its not a problem from an real-live network introduction point of view. With the Gn / Gp interface support in place, the introduction of LTE and roaming between different radio access networks could be introduced as follows:

Cell Reselection Only at First

To make things simple, LTE networks are likely to be launched with only cell reselection mechanisms to 2G and 3G networks instead of full network controlled handover. That means that the mobile is responsible to monitor signal strengths of other radio networks when connected to LTE and autonomously decide to switch to GSM or UMTS when leaving the coverage area of the LTE network. When using the GSM or UMTS network the mobile also searches for neighboring LTE cells and switches back to the faster network once the opportunity presents itself (e.g. while no data is transmitted).

Handovers Follow Later

The advantage of cell reselection between different types of access networks is that they are simple and no additional functionality is required in the network. The downside is that when a network change is necessary while a data transfer is ongoing the mobile will either not attempt the change at all or the change results in an temporary interruption of the ongoing data transfer. The answer to the downside is to perform a network controlled handover between the different radio systems. This makes the change between access networks a lot smoother but requires changes in both the new and the old radio networks. On the GSM/UMTS side, the software of the base stations and radio network controllers have to be upgraded to instruct the mobile to also search for LTE cells while the mobile is active and to take the results into account in their existing handover mechanisms. As far as I can tell, no modifications are required in the SGSN, as transparent containers are used to transfer non-compatible radio network parameters between the different networks.

Packet Handovers Today

At this point I think it is interesting to note that packet handovers are already specified today for GPRS/EDGE to UMTS and vice versa. However, I haven't come across a network yet that has implemented this functionality. Maybe it is the speed difference between the two radio access networks that makes the effort undesirable. Between UMTS and LTE, however, such packet handovers might finally make sense as in many scenarios, the speed difference might not be that great.

The GGSN Oddity

One last thought: In annex D, the 2G/3G GGSN functionality is always taken over by the PDN-GW. That means that an LTE capable mobile should never use a 2G/3G only GGSN when first activating a PDP context in GPRS/EDGE or UMTS. If this was done I don't see how it would be possible to reselect to the LTE network later. This is due to the fact that the GGSN is the anchor point and can't change during the lifetime of the connection. If an "old" GGSN would be the anchor point, then the MME and S-GW would have to talk to the "old" GGSN after a cell reselection or handover from GPRS/EDGE or UMTS to LTE instead of a real PDN-GW. That's a bit odd and I don't see this described in the standards.

There are several ways how that could be achieved. Using a special APN for example that triggers the use of a combined GGSN/PDN-GW when the connection is established could be a possibility or the analysis of the IMEI (the equipment ID). While the first idea wouldn't require new software in the SGSN, the second one probably would and then there is always the chance that you miss some IMEI blocks in the list on the SGSN, especially for roamers, so it's probably not such a good idea after all. Another option would be to replace the GGSNs in the network or upgrade their software so they become combined GGSNs/PDN-GWs. However, there some risk involved in that so some network operators might be reluctant to do that at the beginning.

If you know more about this or have some other comments or questions in general, please leave a comment below.

Jumping from EDGE to EDGE on the Train with SFR

I had an interesting wireless experience recently when I took the TGV from Stuttgart to Paris. There isn’t a lot of 3G network coverage along the railway track so while in France I used SFR’s network with a Vodafone Websession from Strasbourg to Paris. So far I thought that  SFR did not have EDGE in its network. Looks like that assumption was not quite correct.

Every now and then one of the cells along the railway track was EDGE activated and data rates suddenly jumped from a meager 60 kbit/s (typical GPRS) up to 200 kbit/s, even at 300 km/h. The bad thing is just that EDGE was only available in few places so it’s not really worthwhile to go online and download eMails and do some web browsing. I can only speculate that SFR tries to cover some towns along the railway track but they certainly don’t try to do anything in terms of higher rate data for the railway track in particular. What a shame…

At first I thought it might be a mobile device problem. So I tried with a second mobile which has an engineering mode which confirmed that EDGE is only sporadically activated. The pictures on the left shows the throughput of a file download in a standard GPRS cell and with EDGE when it was available.

Have to try with Orange next time.

T-Mobile activates EDGE in Southern Germany

T-Mobile has announced a couple of months ago that it will upgrade most parts of its GSM/GPRS network in Germany to EDGE. Since then I’ve seen reports that EDGE has been activated here and there but not where I live. Today, I noticed that T-Mobile has also switched to EDGE in the South of Germany near the Lake of Constance where I live. Not that we wouldn’t have excellent 3G coverage here already by all network operators but since the first iPhone won’t be 3G capable and we’ve still got a good number of rural spots in this part of Germany without 3G coverage it’s a welcome move.

Looks like T-Mobile is quite active of continuously improving their 2.5G network.

Direct Tunnel – GPRS Core Network Streamlining

While work is ongoing on 3GPP LTE (Long Term Evolution) and SAE (System Architecture Evolution), current 3G networks continue to be enhanced as well. Since the 3G air interface is in the process continues to evolve with HSPA (High Speed Packet Access) it was felt in the standards groups that the 3G core network should be streamlined to handle the increasing network traffic more efficiently.

One part of the network in particular has been waiting for optimization for quite some time. In today’s 3G packet core architecture the SGSN (Serving GPRS Support Node) which is the gateway between the radio network and the core network handles both signaling traffic (e.g. to keep track of a users location) and the actual data packets exchanged between the user and the Internet. Since the users location can change at any time, data packets are tunneled (encapsulated) from the gateway to the Internet (The Gateway GPRS Support Node, GGSN) via the SGSN over the radio network to the mobile device. The current architecture uses a tunnel between the GGSN and the SGSN and another one between the SGSN and the Radio Network Controller (RNC). All data packets thus have to pass the SGSN which has to terminate one tunnel, extract the packet and put it into another tunnel. This requires both time and processing power.

Since both the RNC and the GGSN are IP routers this process is not really required in most circumstances. The one tunnel approach now standardized in 3GPP thus foresees that the SGSN can create a direct tunnel between the RNC and the GGSN and thus remove itself from the chain. Mobility Management remains on the SGSN, however, which means for example that it continues to be responsible to modify the tunnel in case the mobile device is moved to an area served by another RNC.

The approach does not work for international roaming since the SGSN has to be in the loop in order to count the traffic for inter-operator billing purposes. Another case where the one tunnel option can not be used is in case the SGSN is asked for example by a prepaid system to count the traffic flow. A small limitation since in practice it’s also possible to connect such a system to the GGSN (via Diameter).

For the details have a look at the following documents:

  • Direct Tunnel 3GPP Work Item Description SP-060142_S2-060545
  • The TR (Technical Recommendation) describing the overall design and impact on existing functionalities: TR 23.809
  • The Change Request (CR) for 3GPP TS 23.060
  • And the latest version of the ‘GPRS Service Description;  Stage 2’ which contains the enhancements. TS 23.060 7.4.0

GPRS and EDGE Multislot Classes

The upload and download speeds that can be reached with a GPRS and EDGE mobile phones depends on a number of factors. First, the capacity available in the cell. Usually, voice calls take precedence over GPRS traffic and in case not enough time slots are available for both, some GPRS timeslots are sacrificed for voice calls. Second, the capability of the mobile, or it’s multislot class. Third, the network has to match the abilities of the mobile device. And fourth, the reception conditions experienced by the mobile.

The most comment GPRS/EDGE mobile station class today seems to be multislot class 10. Mobiles of this class can use 4 timeslots in downlink direction and 2 timeslots in uplink direction with a maximum number of 5 simultaneous timeslots. Depending on the amount of data to be transfered in the uplink the network will automatically configure the ongoing data stream for either 3+2 or 4+1 operation.

Some high end mobile, usually also supporting UMTS also support GPRS/EDGE multislot class 32. According to 3GPP TS 45.002 (Release 6), Table B.2, mobile stations of this class support 5 timeslots in downlink and 3 timeslots in uplink with a maximum number of 6 simultaneously used timeslots. If data traffic is concentrated in downlink direction the network will configure the connection for 5+1 operation. When more data is transferred in the uplink the network can at any time change the constellation to 4+2 or 3+3. Under the best reception conditions, i.e. when the best EDGE modulation and coding scheme can be used, 5 timeslots can carry a bandwidth of 5*59.2 kbit/s = 296 kbit/s. In uplink direction, 3 timeslots can carry a bandwidth of 3*59.2 kbit/s = 177.6 kbit/s.

While the numbers are quite impressive it should not be forgotten that a single carrier has 8 timeslots of which two are reserved for signaling on the first carrier. Using 6 timeslots for a single mobile means almost the complete carrier is used for the data transfer of a single mobile. Under realistic conditions it’s unlikely a cell has that much free capacity to offer next to voice calls.

Deep Inside the Network: T-Mobile starts using GPRS NOM-1

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.