GPRS Attach

GPRS functionality in an MS can be activated
either when the MS itself is powered on, or perhaps when the browser is
activated. Whatever the reason for the initiation of GPRS functionality
within the MS, the MS must attach to the GPRS network, so that the GPRS
network (and specifically the serving SGSN) knows that the MS is available
for packet traffic. In the terms used in GPRS specifications, the MS
moves from an idle state (not attached to the GPRS network) to a ready
state (attached to the GPRS network and in a position to initiate a PDP
context). When in the ready state, the MS can send and receive packets.
Also, a standby state is available, which the MS enters after a time-out in
the ready state. If, for example, the MS attaches to the GPRS network but
does not initiate a session, then it will remain attached to the network, but
move to a standby state after a time-out.
Figure 5-5 shows the simple case of a Class-C MS performing a GPRS
attach. In this figure, we have included a great deal of the air interface signaling.
Many of the air interface messages shown in the figure are applicable
to any access to or from the MS, whether or not that access is just for
signaling of the transfer of user packets. For the sake of brevity, they will
not be repeated in every subsequent scenario we describe.

A GPRS Attach is somewhat similar in functionality to a location update
in GSM. The process begins with a packet channel request from the MS. In
the request, the MS indicates the purpose of the request, such as a page
response, a Mobility Management (MM) procedure, or two-phase access,
which would be used in the case of transferring user data. In the scenario
of Figure 5-5, a MM procedure is indicated. The network responds with a
packet uplink assignment, which allocates a specific timeslot or timeslots to the MS for the message that the MS wants to send. The network includes a
TFI to be used by the mobile, a USF value for the mobile on the timeslot(s)
assigned (in the case of dynamic allocation), and an indication of the number
of RLC blocks granted to the MS for the TBF in question.

The MS proceeds to send the attach request in one or more radio blocks
to the network on the assigned resources. The MS can send no more than
the number of blocks that have been allocated by the network. In the case
of MM messages, the assigned resources will typically be sufficient for the
MS to send the necessary data. If not, as might be the case when the MS
wants to send user packet data, then the MS can request additional
resources through a Packet Resource Request message.

Upon receipt of the attach request at the BSS, the BSS uses the PACCH
to acknowledge the receipt. In case the MS has sent all of the information
it wants to send, which would be the case in our example, then this is indicated
in the transmission from the MS to the network. In this case, the
acknowledgement from the network is a final acknowledgement, which is
indicated in the acknowledgement message itself. This causes the MS to
send a Packet Control Acknowledgement message back to the network and
release the assigned resources.

Meanwhile, the BSS forwards the attach request to an SGSN.The SGSN
may choose to invoke security procedures, in which case it fetches triplets
from the HLR. Note, however, that a slight difference can be seen in GPRS
regarding authentication and ciphering. Specifically, ciphering in GPRS
takes place between the MS and the SGSN such that the whole link from
MS to SGSN is encrypted. In standard GSM, only the air interface is
encrypted. The authentication and ciphering is initiated by the issuance
from the SGSN of the authentication and ciphering request to the MS via
the BSS.

The BSS first sends a Packet Downlink Assignment message to the MS.
This message can be sent either on the PCCCH or the PACCH. Which one
is chosen depends upon whether the MS currently has an uplink PDTCH.
If it does, then the PACCH is used. The Packet Downlink Assignment
instructs the MS to use a given resource in the downlink—including the
timeslot(s) to be used and a downlink TFI value. The BSS subsequently forwards
the Authentication and Ciphering request as received from the
SGSN.

Upon receipt of the request, the MS acknowledges the downlink message
and then requests uplink resources so that it can respond. Thus, it sends
another Packet Channel Request, much like the one it sent initially. Once
again, the network assigns resources to the MS, which the MS uses to send its authentication and ciphering response to the network. That response is
forwarded from the BSS to the SGSN. The BSS also sends an acknowledgment
to the MS, and the MS confirms receipt of the acknowledgement, just
as it did for the acknowledgement associated with the initial attach
request.

Once the MS is authenticated by the SGSN, the SGSN performs a GPRS
Update Location towards the HLR. This is similar to a GSM location update,
including the download of subscriber information from the HLR to the
SGSN. Once the Update Location is accepted by the HLR, the SGSN sends
the message Attach Accept to the MS. As for other messages, the BSS first
assigns resources so that the MS can receive the message. Similarly, once the
MS receives the message, it requests resources in the uplink so that it can
respond with an Attach Complete message. The BSS acknowledges receipt
of the RLC data containing the Attach Complete and forwards the message
to the SGSN. The MS confirms receipt of the acknowledgement.

Note that throughout the procedure just described, the MS requests
access to resources for each message that it sends towards the network.
This is typical of the manner in which GPRS manages resources and is one
of the main reasons why GPRS enables multiple users to share limited
resources. Of course, in our example, only signaling is occurring, which consumes
very little RF capacity (very few radio blocks). In the case of a packet
data transfer, many more data blocks would be transmitted for a given TBF.
Not every block needs to be acknowledged, however. In fact, GPRS enables
both acknowledged and unacknowledged operations. In the case of an
acknowledged operation, acknowledgements are sent only periodically, with
each acknowledgement indicating all the correctly received RLC blocks up
to an indicated block sequence number.