WiMAX Handover

The WiMAX architecture extends the 802.16 standard and that also includes the mech-
anisms for handovers. While the 802.16 standard provides support for handover between
base stations WiMAX offer protocols for handover higher up in the network structure.
The WiMAX architecture shall support mechanisms such as intra/inter ASN handover,
roaming between NSPs, seamless handover at vehicular speed and micro/macro mobility.
This section will study the architecture and its handover procedures more thoroughly
with the focus on intra/inter ASN handovers.

Access Service Network
Inside an ASN network entity there are at least one ASN Gateway (ASN GW) and a
base station. The BS handles the connection to the MS while the ASN GW takes care
of the contact with the CSN. An ASN GW can be associated with one or more BSs and
a BS can have relations to one or more ASN GWs. This segmentation
of the ASN enables multi vendor systems where different vendors can produce different
parts of the ASN and they still function together.
Depending on which role a BS or ASN GW take on in a handover they get different
names. The BS in charge of the MS before the handover
is called the serving BS and the ASN GW the serving BS forwards the data to is the
serving ASN GW. The BS and ASN GW associated with the MS after the handover are
the target BS and target ASN GW respectively. The term anchoring ASN GW is used
when an ASN GW relays MS data to the serving ASN GW.

Anchoring
The anchoring ASN GW is the network’s or CSN’s attachment to the MS. Incoming
data will be sent to the anchoring ASN GW and the CSN does not need to know at
which ASN GW the MS’s current BS is located. The forwarding of data to the serving
ASN GW is performed by the anchoring ASN GW. This makes the mobility of the MS
transparent to the CSN and the need to change IP-address becomes less frequent. In
the case where the serving ASN GW is receiving the data directly from the network the
serving ASN GW is also the anchor. The anchoring ASN GW does not need to be any
of the serving or target ASN GWs.

ASN Reference Points
To identify the different interfaces used to communicate within an ASN, with the MS and
the rest of the network a number of reference points are introduced [19], see figure 5.1
on the facing page. These reference points define the set of protocols and procedures
needed in the communication. Most of the reference points are logical mappings but
when, as in the case of R1, the functional entities are in different physical devices the
reference point refers to a physical interface.
R1 and R3 are the reference points used in communication with entities outside of
the ASN while R6 and R8 are used inside an ASN. The R4 interface is used both inside
and outside of the ASN since it is the logical link between ASN GWs regardless of
whether they are within the same ASN or in different ASNs. R1 is the physical interface
between the MS and the serving BS and R3 is the logical link between ASN GW and
CSN. The communication among BSs is handled through R8 while the BS-ASN GW
interaction goes via R6.

Inter ASN Handover
An inter ASN handover is a handover between BSs not part of the same ASN, see
figure 5.4 on the next page. During an inter ASN handover ASN GWs in separate ASNs
need to coordinate their actions to make the handover smooth to the MS. There are two
possible ways of dealing with the data flow during an inter ASN handover, anchoring
and re-anchoring. The purpose of anchoring is to avoid an path update and hence a
redirection of the data path, where in the re-anchoring case an update will be performed.
The decision to anchor or re-anchor the data path is made by the target or an-
chor ASN GW and there are three different decision procedures with two possible out-
comes 5.3.4. Either both parties can decide that a re-anchoring is not needed or one
of the ASN GW decides that it wants a re-anchoring. If the target ASN GW wants a
re-anchoring the anchor ASN GW will follow that decision and vice versa. It is always
the target ASN GW who will make its decision first. What this decision is based upon is
implementation dependent and not included in the scope of the WiMAX documen.