As LTE technology becomes more widespread, concerns for the technology's interoperability and quality of service (QoS) is at the forefront.  As already evidenced, early phases of LTE network deployments will coexist with legacy technologies, which implies frequent inter-technology handovers between 3G systems such as UMTS, for example.  Managing data traffic in a network working interchangeably with UMTS or other legacy technologies does requires special attention to ensure that QoS and customer expectations are met, which are key to retaining customers and running a profitable network.

LTE INTEROPERABILITY

Two interfaces in the LTE network are provided for interworking. The S3 is the reference point, based on the legacy Gn interface. It lies between the SGSN and the MME where it enables user and bearer information exchanges for inter-3GPP access system mobility.  The S4 is the reference point, based on the older GTP-based Gn interface in UMTS, between the SGSN in the GPRS core network and the S-GW.

The preferred way to interwork UMTS with LTE is though a Serving GPRS Support Node (SGSN) upgraded to Release 8. This enhancement deploys the S3 and S4 interfaces that somewhat mimics the strict separation of user data flows from the control plane messages so evident in LTE.

Though the protocol stacks are incompatible with each other, LTE supports interworking with the legacy 3GPP and non-3GPP accesses. The intention is to provide LTE service continuity that is transparent to the access technology. Access independence is one of the requirements of the NGN visions. The idea assumes a generic approach, which decouples the NGN core network and its procedures as much as possible from the access technologies.

QUALITY OF SERVICE

QoS in LTE Networks
Since LTE and UMTS employ different QoS mechanisms, we need to be able to map between LTE's QCI parameters for EPS bearers and the four QoS categories and associated parameters of Pre-Release 8 PDP Contexts. The 3GPP recommendations provide rules for mapping QoS definitions between the systems.

The QoS parameter sets supported within the EPC concern themselves with how packets are handled as they enter, traverse and leave a network. Adding more bandwidth at the edge of a network may resolve some capacity or congestion problems, but it does not resolve jitter, nor can it fix traffic prioritization problems.

QoS in an all-IP Environment
QoS is the management of the data traffic in a network. Be it a LAN, WAN or wireless, packets are subjected to scrutiny and control. QoS is primarily a layer 3 Internet Protocol (IP) concept. It uses tools that have existed since the early days of IP plus some newer tools and protocols that are designed to aid in the provisioning of precisely defined and predictable data transfers in accordance with certain characteristics.

LTE and QoS
Each bearer (user data) path in LTE is assigned a set of QoS criteria. In the case a user may have services requiring different QoS criteria, additional bearer paths may be added. LTE identifies a set of QoS criteria with QoS Class Identities (QCIs). These are listed in Figure 2.

The critical QoS parameter for any EPS bearer is its QCI, which represents the QoS features an EPS bearer should be able to offer for a Service Data Flow (SDF). Each SDF is associated with exactly one QCI. Network operators may pre-configure all QCI characteristics in an eNB, for example, based on their actual characteristics. The parameters they choose to define these determine the allocation of bearer resources in the E-UTRAN.

QoS between LTE and other 3GPP systems
Since LTE and UMTS employ different QoS mechanisms, we need to be able to map between LTE's QCI parameters for EPS bearers and the four QoS categories and associated parameters of Pre-Release 8 PDP Contexts. The 3GPP recommendations provide rules for mapping QoS definitions between the systems.

EPS Bearers and SDFs
LTE sees any Service Data Flow (SDF) as belonging to one of nine QoS-based data flow specifications listed in Figure 2.

Several 3GPP releases have moved toward a sophisticated multimedia-based policy-oriented environment. LTE offers the promise of policy-based, fixed-mobile convergence for new real-time applications such as VoIP, push-to-talk dispatch, messaging enhanced with video clips and even two-way video telephony.

LTE PCC
Policy control is the mechanism that allows network operators control access to his network and services with as fine a resolution as desired. Operators can decide precisely what a subscriber can do, what applications he can use, which ones he cannot use, what kinds of content he can enjoy, and how a service is actually delivered. Policy control depends on rules formulated from the technical details of an SDF. The appropriate policy rules can join control, the user's services, the sense of what a user is willing to pay for a service, and the capabilities of an access network into one functioning unit. Policy control is frequently coupled with charging: Policy and Charging Control (PCC). This allows for the possibility of charging a user for his experience and not just for the number of bits delivered to his handset.

Keith Cobler is senior marketing manager at Tektronix Communications.