HSPA+ Session Establishment
15th August 2011
In order to establish a PS (Packet Switched) session, i.e. PDP (Packet Data Protocol) Context, the mobile must first acquire the system and Attach to the network. This process is the same as Release 5/6, however the SRB (Signalling Radio Bearer) may be sent over HSPA.
The initial access message to the network indicates that the mobile is switched on and requesting resources. Since the mobile has not yet been assigned a specific channel by the network it must use a common uplink channel, namely the RACH (Random Access Channel), which is sent on the PRACH (Physical Random Access Channel). This is a contention based channel so there is a chance that collisions may occur between users.
The mobile’s initial access message will be an “RRC Connection Request”. This originates from the RRC protocol, but will have been triggered from higher layers, e.g. CC (Call Control), MM (Mobility Management) etc.
After the mobile sends the initial access message it receives a reply from the network on the FACH (Forward Access Channel), which is in turn sent in the S-CCPCH (Secondary - Common Control Physical Channel). Alternatively, if Enhanced Cell_FACH is deployed, the messages may be sent using the HS-DSCH (High Speed - Downlink Shared Channel) with parameters derived from SIB5. The message to the mobile will be either an RRC Connection Setup or a RRC Connection Setup Reject message. The content of the RRC Connection Setup messages will describe the configuration of the SRB (Signalling Radio Bearer) along with providing a RNTI (Radio Network Temporary Identity). This message also requests Physical Channel Capability information from the mobile.
In the diagram the mobile has been allocated a DCH (Dedicated Channel) or E-DCH (Enhanced -Dedicated Channel) for the SRB. The mobile, on receiving the RRC Connect Setup message, will indicate its acceptance by sending the RRC Connection Setup Complete message back to the network on this channel.
Packet Switched Session
The mobile, once attached to the network, may trigger the establishment of the Packet Switched Session, i.e. PDP (Packet Data Protocol) Context. This procedure starts with the mobile establishing an RRC Connection.
Once this is in place, the Activate PDP Context Request message is sent to the SGSN (Serving GPRS Support Node) using the services of RRC and RANAP (Radio Access Network Application Part). This message includes:
- Requested PDP Address - this address information element indicates the type of address; typically this will be an IPv4 (Internet Protocol version 4) or IPv6 (Internet Protocol version 6) address. How the address should be allocated is also indicated. This may be static, subscribed or dynamic.
- Requested APN - the purpose of the APN (Access Point Name) is to identify the network or service to which the application wishes to connect. The APN is composed of two parts:
- Network Identifier - the Network Identifier is mandatory and takes the format of a FQDN (Fully Qualified Domain Name) and is used to identify the external network or service.
- Operator Identifier - the Operator Identifier defines in which PLMN GPRS backbone the GGSN is located. This part of the APN is optional.
- Requested QoS - the QoS (Quality of Service) parameter is used to define the QoS that should be applied to the data as it is transported across the GPRS network.
- Requested NSAPI - the NSAPI (Network Service Access Point Identifier) is used to address the protocol above the GPRS service that the data is intended for. This protocol will typically be IP.
- PDP Type - this parameter is used to indicate the type of data being carried. For example, IP or PPP (Point to Point Protocol).
Following the response from the GGSN, the SGSN establishes the RAB (Radio Access Bearer) towards the mobile. This is achieved using the RAB Establishment Request message which includes the RAB Identifier, QoS Profile and the transport layer addressing. The QoS requested for this RAB is based on the SGSN Negotiated QoS. The RNC, on receiving the RAB Establishment Request message, configures the Iub and air interface before informing the mobile of the new Radio Bearer. The RAB Establishment Response message indicates a positive response. The procedure is finally completed with the Activate PDP Context Accept message being sent to the mobile. This uses the Direct Transfer services of RANAP and RRC.
CS voice over HSPA+
Voice is still the most important service for service providers. Traditionally, voice in UMTS has been Circuit Switched and delivered on dedicated channels. Release 7 added the support of VoIP over HSPA, however the implementation of VoIP mobile networks is in some cases costly and “too early”. One addition to the standards was “CS voice over HSPA” as part of Release 8.
The main benefits for deploying CS voice over HSPA are:
- UE power consumption is reduced - this is due to the additional DTX/DRX capabilities.
- Improved spectral efficiency.
Voice Deployment Options
The diagram illustrates the three main deployment options for voice over a UMTS network. The first scenario is the current implementation, standard AMR (Adaptive Multi-Rate) speech utilizing RLC in TM (Transparent Mode) and being delivered by a DCH (dedicated Channel).
The second option illustrates CS voice over HSPA. In this case the voice is still using the CS Core Network and at the RNC is packetized. At this point additional functionality is required to compensate the jitter introduced by the HSPA air interface.
The final option is the move towards VoIP, as such it will require an IMS (IP Multimedia Service) type deployment.
The diagram illustrates a typical level of improvement when CS voice over HSPA is deployed. In addition, it also illustrates the capacity benefits when the codec rate is reduced. The benefits come from an optimized Layer 1, such that the downlink utilizes a Fractional DPCH with discontinuous reception and the uplink is based on DTX. In addition, features such as HS-SCCH-less operation can be used.
As illustrated the gain in spectral efficiency is estimated at between 50-100%.
Instant Expert HSPA+ - Part 3
In the next part of our instant expert series we will be looking at the two main HSPA+ throughput enhancements, namely MIMO and DC-HSDPA.