Posts Tagged ‘direct tunneling’

Mwell…my dear dear journal, today I had to learn about Mobility over SAE. As we very well know, our naughty user (User Equipment) does not just stay in one single cell, but rather moves around between different antennas. As per TS 23.401 (I have studied the June edition), there are several “cases” of mobility, or handover, as the 3GPP guys call them.

What is to know about how these “cases” are delimited:

1. whether the UE only moves from one eNodeB to another (the rest of the EPS is the same) or other components (like MME and/or SGW) are also changing => X2-handover and S1-handover

2. whether or not the eNodeBs are connected each-other (when they are connected, the interface is called X2) => this results in 2 separate cases: Direct Tunneling (we have X2) and Indirect Tunneling (we don’t have X2)

3. whether or not the MME changes (is relocated, as per the TS) => no MME relocation and MME relocation scenarios

4. whether or not the SGW changes (is relocated, as per the TS) => no SGW relocation and SGW relocation scenarios

5. in each of these cases, what happens to the user-plane traffic in terms of the path it takes; the uplink usually goes directly through the new components of handover, but the downlink data is forwarded back and forth around those elements – in the diagrams attached I have represented the user-plane in dotted lines – hope you’d like it 😛

6. the user-plane flow problem appears only in the time interval that the handover is not completed, otherwise it is the usual; this is why there is only a downlink user-plane traffic described

So—let’s do this by the book.

TS 23.401, section 5.5.1.1.2 – X2-based handover with NO SGW relocation and NO MME relocation (implicit direct tunneling)

– UE moves from source eNB to target eNB, the X2 interface is present

– the downlink data flows this way: PGW -(via S5/S8)> SGW -(via S1-U)> source eNB -(via X2)> target eNB -(via LTE radio)> UE

55112

TS 23.401, section 5.5.1.1.3 – X2-based handover with SGW relocation and NO MME relocation (implicit direct tunneling)

– UE moves from source eNB to target eNB, the X2 interface is present

– the downlink data flows this way: PGW -(via S5/S8)> source SGW -(via S1-U)> target eNB -(via LTE radio)> UE

55113

* no MME relocation for X2-based handover :d

TS 23.401, section 5.5.1.2.2 – S1-based handover, NO SGW relocation and MME relocation + Direct Tunneling

– UE moves from source eNB to target eNB, the X2 interface is present

– the downlink data flows this way: PWG -(via S5/S8)> SGW -(via S1-U)> source eNB -(via X2)> target eNB -(via LTE radio)> UE

55122-dir

TS 23.401, section 5.5.1.2.2 – S1-based handover, NO SGW relocation and MME relocation + Indirect Tunneling

– UE moves from source eNB to target eNB, the X2 interface is NOT present

– the downlink data flows this way: PGW -(via S5/S8)> SGW -(via S1-U)> source eNB -(via S1-U)> SGW -(via S1-U)> target eNB -(via LTE radio)> UE

* this is the case when there are some downlink packets that have been forwarded from the SGW to the source eNB, BEFORE the handover is completed; this means that the source eNB (knowing there is a handover ongoing), resends/sends back these packets to the SGW they came from; the SGW, at this point, should be aware of the handover and buffer the packets until the handover is completed, then forward them via the appropriate S1-U to the target eNB

55122-indir

TS 23.401, section 5.5.1.2.3 – S1-based handover, SGW relocation and NO MME relocation + Indirect Tunneling

– UE moves from source eNB to target eNB, the X2 interface is NOT present

– the downlink data flows this way: PGW -(via S5/S8)> source SGW -(via S1-U)> source eNB -(via S1-U)> source SGW -(via…to check this up)> target SGW -(via S1-U)> target eNB -(via LTE radio)> UE

55123-indir-no-mme

TS 23.401, section 5.5.1.2.3 – S1-based handover, SGW relocation and MME relocation + Direct Tunneling

– UE moves from source eNB to target eNB, the X2 interface is present

– the downlink data flows this way: PGW -(via S5/S8)> source SGW -(via S1-U)> source eNB -(via X2)> target eNB -(via LTE radio)> UE

55123-dir

TS 23.401, section 5.5.1.2.3 – S1-based handover, SGW relocation and MME relocation + Indirect Tunneling

55123-indir

– UE moves from source eNB to target eNB, the X2 interface is NOT present

– the downlink data flows this way: PGW -(via S5/S8)> source SGW -(via S1-U)> source eNB -(via S1-U)> source SGW -(via…to check this up)> target SGW -(via S1-U)> target eNB -(via LTE radio)> UE

 

The signaling required for these handovers are described in TS 23.401 as a flow and in TS 29.274 at the IE level.

I will try to describe each flow (or at least the most significant ones) in future articles. Enjoy :p