This is part 3 of my mini-series on the latest version of 3GPP TS 23.401 which describes how the LTE/SAE core network manages user mobility and routes data. In part 1, I've taken a look at the flexibility in terms of load balancing and network node distribution and part 2 featured a look at connection and mobility management. Part 3 now focuses at how the MME (Mobility Management Entity) keeps track of users while they are moving and helps handing over connections from one base station to the other.
As described in previous parts an LTE mobile always has an IP address assigned while it is switched on. To conserve battery and to reduce signaling there are two basic activity states are: While data is exchanged the mobile is seen as connected. If no data is transferred for some time, the network moves the connection to idle, which means the mobile has no physical connection over the air interface during that time. Data can still be sent and received but the air interface connection needs to be re-established first. For applications this is transparent, they will just notice some delay.
Mobility While Being Idle
Let's look at the idle state (to be exact RRC idle and ECM idle) first because that is the most simple from the network point of view. Here, the mobile is free to roam from one cell to another and only contact the network if it suddenly finds itself with a cell that is outside the group of cells to which its former cell belonged to. Such groups are referred to as Tracking Areas (TA) and the action performed when changing to a new cell in a different TA is referred to as a Tracking Area Update (TAU). For a mobile it is simple to detect a tracking area change because each cell broadcasts its Tracking Area ID as part of its general cell information. In case data arrives from the Internet for that user while the device is in idle state, the network has to search the mobile first and sends a 'paging' message via all cells belonging to the TA. The mobile then re-establishes the air interface connection and implicitly reports it's current location to the network.
Now let's have some fun with a couple of further abbreviations, because they are really cute. In GPRS and UMTS the mobile's temporary id was the Packet Temporary Mobile Identity, or the P-TMSI. This id is changed on a frequent basis and used instead of the IMSI (The International Mobile Subscriber Identification) in most air interface messages for security reasons. In LTE, the P-TMSI is now called the Globally Unique Temporary ID, or the GUTI. Some of the digits in the GUTI identify the Mobility Management Entity the mobile was last registered with and they are referred to as the Globally Unique MME Identifier, or the GUMMEI.
When contacting the network, the mobile sends the GUTI to the base station which then uses the parameter to identify the MME to which it will send the request to re-establish the communication session. It's also possible to roam between different radio technologies. If the mobile has reselected from a UMTS cell to an LTE cell, a TAU is made and since the mobile does not have a GUTI, the P-TMSI is sent instead. This way, the newly assigned MME can contact the 3G SGSN to request the subscribers current profile (IP address, PDP contexts, etc.). The same mechanisms apply when the mobile reselects from an LTE cell to a UMTS or GPRS cell. In this case the GUTI is sent in the P-TMSI parameter and the procedure is reffered to as Routeing Area Update (RAU) instead of TAU.
Handover between Two LTE Cells
3GPP TS 23.401 also describes the possible scenarios for handovers, i.e. the handing over of an active radio network connection. Unlike the cell reselection in idle state above, which is controlled by the mobile device, handovers are controlled by the network. Due to the flat network architecture of LTE, the handover is directly initiated by the base station and not, like in previous network architectures, by a higher network element. If implemented, two base stations can organize a handover between the two of them (over the X2 interface) and only report the successful execution to the MME afterwards. The MME then either just acknowledges the handover and the serving gateway is informed to redirect the downlink data traffic to the new cell. In case it makes sense from a network topology or traffic point of view, the MME can at this point also assign a new serving gateway. Note that in practice, the X2 interface is not a physical interface, i.e. base stations are only logically connected with each other over IP and not via a direct physical link.
If there is no direct interface between two base stations, or the base stations do not yet have that functionality implemented, the current base station can also ask the MME to coordinate the handover. This is called an S1 handover, due to the name of the interface between the base station on the MME. Again, there are a number of different variants such as with or without a change of the Serving Gateway.
Inter Radio Access Technology (Inter-RAT) Handovers
And last but not least, there is also the possibility to perform a handover from and to a different radio network architecture, i.e. from/to a GPRS or UMTS network. Three different variants exist:
- To/from an SGSN that is aware how a LTE core network works and has the S4 interface implemented.
- In case Direct Tunnel is used in a UMTS network, the S4 and S12 interfaces are used for the handover.
- And for networks without upgraded SGSNs there is a backwards compatible variant. In this case the MME acts like an SGSN for the older network elements. This is probably the way how handovers to and from LTE will be made at first. This variant is described in Annex-D of the specification, which kind of marks it as a temporary solution.
It all sounds very complicated with lots of options and it probably is. It makes one appreciate network operators who have done their homework and have optimized their networks for seamless nationwide handovers without dropped calls and lost IP addresses. Yes, they do exist.
Nevertheless, to me this all looks a lot more straight forward compared to how things are done in UMTS. Here, the RNC complicates matters and in practice not all network elements can communicate with all others. In LTE/SAE, the removal of the RNC and using IP as the routing protocol for all network activities instead of ATM makes things a lot simpler. Those who don't believe me should have a look at the situation that requires a UMTS SRNS relocation for both the circuit switched and packet switched side simultaneously and how the messaging looks like…
For those who would like to know more about Inter-RAT handovers with CDMA networks I can warmly recommend the several hundred pages of specification in an additional document, 3GPP TS 23.402. Another tribute to CDMA that is also described there is the use alternative use of PMIP (Proxy Mobile IP) instead of the 3GPP GTP (GPRS Tunneling Protocol) on the Interface between the MME and the PDN-Gateway (to the Internet). Long live the options!
There we go, we are almost through with the main SAE features. Remains the Idle State Signaling Reduction (ISR) feature, but I keep that for part 4.