Introduction

3GPP-standardized Long Term Evolution (LTE) technology was introduced with the intention of being used as a capacity-improved data access technology only.

With the first commercial launches of 4G mobile networks in the year 2010, based on LTE, it is important to note the change of the implementation concept for the delivery of mobile voice and SMS services. While these services have (in 2G and 3G networks) relied on circuit-switching technologies, an all-IP network-based architecture is taking place in 4G, with the central role of the IP Multimedia Subsystem (IMS).

3G to 4G Evolution

FIGURE 1 (source: www.3gpp.org):

Releases of the 3GPP Access, Core Network and Services

By introducing the 3GPP Release 10, LTE will be able to reach peak data rates of 1Gbps/500Mbps (downlink/uplink). The bandwidth necessary for mobile voice is a very small part of the total available bandwidth.

VoLTE Reference Architecture

Unlike circuit-switching technology, VoLTE has no dedicated voice channels. Voice is transported within the prioritized mobile IP traffic, or, alternatively, could still rely on circuit-switching technology. The 4G architecture has changed, when compared to 3G.

FIGURE 2:

Voice and data bearer paths in Universal Mobile Telecommunication System (UMTS) and LTE network architectures

When comparing the voice and data bearer paths in 3G and 4G network architectures, VoLTE uses a few new nodes:

Home Subscriber Server (HSS) performs AAA and subscriber database functionality, eliminating the need for a separate Radius server

The Policy and Charging Rules Function (PCRF) server is an important element, taking care of the direct control of resources (quality of service) according to user profile

Mobility Management Entity (MME) has a role similar to that of the Serving GPRS Support Node (SGSN) in the 3G packet core, except that it only carries the signalling path

Serving GW (SGW) has inherited the SGSN's bearer role in 4G, transporting large amounts of traffic

Packet Data Network Gateway (PDN GW) has replaced the Gateway GPRS Support Node (GGSN) in the 3G packet core, thus having the Network Access Server (NAS) role

eNodeB serves in 4G instead of NodeB and Radio Network Controller (RNC) in 3G

The Role of the All-IP Network (AIPN)

The all-IP network (AIPN, 3GPP TR 22.978 specification) is divided into the Core Network (CN), called the Evolved Packet Core (EPC) in a non-radio-related System Architecture Evolution (SAE), and the radio access network E-UTRAN (evolved UMTS terrestrial radio access). The EPC is an efficient network element, capable of delivering mobile Internet services over a variety of access technologies (2G/3G/4G, WiMAX, WiFi etc.), and performing the inter-RAT (radio access technology) handover to provide service continuity. A bearer is an IP packet flow with the designated quality of service (QoS) between the gateway and the User Equipment (UE), performing optimized Robust Header Compression (RoHC), of IP, UDP, RTP and TCP headers, considered for the wireless links (having a high packet loss rate). The UE and the network are required to support both IPv4 and IPv6. This concept has changed since evolved NodeB (eNodeB), which is directly connected to the IP cloud. Similarly to 3G is the GPRS tunneling protocol (GTP) tunnel between eNodeB, SGW and Packet Data Network Gateway PGW.

FIGURE 3:

Protocol stack in LTE and IMS architecture

While 2.5G networks were based on the best-effort service, 3G created basic QoS capabilities. In 4G LTE all-IP network architecture, the QoS concept has a fundamental importance, since it determines resources and traffic scenarios. A single value (1–9) of QoS Class Identifier (QCI) is mapped to a number of the QoS parameters per IP packet, related to the bearer: resource type (GBR [Guaranteed Bit Rate] or non-GBR), priority (relative), packet delay, packet error loss rate. GBR are real-time services, and non-GBR are non-real-time. Alternatively, QoS could be user terminal-initiated and network-initiated.

Network-initiated QoS could be negotiated to be assigned different bearers:

DEFAULT BEARER is obtained during the attach process, and ensures continuous IP connectivity.

DEDICATED BEARER is obtained on demand, when it is required to carry GBR for certain delay-sensitive services, such as voice. The same bearer is to be used also in case of multiple concurrent voice sessions (as in call waiting, conference supplementary services).

FIGURE 4 (source: www.3gpp.org):

Default and dedicated bearers

Voice is given the first QCI value with the lowest delay, and almost the highest priority (only signalling has higher priority). Session Initiation Protocol (SIP) is used as a control protocol for multimedia sessions (audio, video etc.) on IMS.

FIGURE 5 (source: www.3GPP.org):

QCI classification

The Role of the IP Multimedia Subsystem (IMS)

The IP Multimedia Subsystem is a conceptual framework providing IP multimedia services to mobile subscribers. The platform was first introduced in 3GPP Release 5, with the goal of providing the service of a single provider anywhere.

Instead of using only the E.164 numbering plan in 3G, according to the ITU-T standard, addressing in 4G is based on the SIP URI. The user can be identified either as an alphanumerical or an MSISDN SIP URI (examples below).

ALPHANUMERICAL: nil-user@nil.com

MSISDN: SIP: +38614746500@nil.com; user=phone

The negotiated terminal capabilities (codecs, ports …) are contained in the Session Description Protocol (SDP) within SIP. The UE must support the Adaptive Multi-Rate (AMR) speech codec.

FIGURE 6 (source: www.cisco.com):

IMS - Referential diagram

The central part of the IMS is the Call Session Control Function (CSCF), further separated into three elements:

P-CSCF - Proxy is the first point to reach, analyzing messages and establishing IPSec toward the UE

I-CSCF- Interrogation interrogates the HSS database and determines the correct S-CSCF per user

S-CSCF - Serving unit is the central SIP server, doing SIP registration

To create a complete migration to the all-IP network, the UE may also support SMS Over IP.

In the network design with IMS only serving for telephony, it is highly recommended to use the default bearer for SIP, and consequently IMS-specific APN. Alternatively, another design might be considered in which a multipurpose APN (combined dedicated bearer) is used for several diferent services.

Implications for the Backbone Infrastructure

The increase of the total available radio-access bandwidth capacity for the 4G user affects the MPLS backhaul traffic capacity. The expected necessary capacity improvements consider the extensions such as 1Gbps to 10Gbps, or 10Gbps to 40Gbps, with the traffic engineering required. A special virtual routing and forwarding (VRF) creation for LTE is not recommended, and QoS is based on traffic-class identification in QCI for backhaul and the Evolved Packet Core.

In the case of 2G/3G/4G coexistence and migration to the single high-capacity backhaul Carrier Ethernet Transport (CET) with the IP Radio Access Network (IP RAN), it is consequently possible to optimize the transmission cost by using Pseudowire triple-E (PWE3) over MPLS, and to encapsulate legacy time-division multiplexing (TDM) bearer links used for 2G/3G, and overcome transport issues in the legacy core. It is desirable also to offer an offload of the user's traffic and the backbone transport, by using the generic access offload (i.e., WLAN), and by distinguishing the traffic and choosing among the available access networks, applying the preferred routing per APN, per IP flow.

Conclusion

LTE and System Architecture Evolution (SAE) will be the unified 4G wireless network, carrying voice among the other data services, over the efficient all-IP network. The major suppliers and most of the operators have already agreed to use IMS as the major platform for the voice control within the LTE network, leading to the possibility of seamless convergence of the different wireless technologies on the same UE, and providing optimized access to the same voice service over the IMS-based control. The direct impact might be improved reachability and cost reduction of the 4G mobile voice service, effectively meeting the demands of future wireless broadband growth.

In many initial deployments, however, the IMS-based VoIP-capable radio coverage will not be sufficiently geographically extended, and therefore a variety of the circuit-switched radio-access voice coverage should be made available, to create a complementary failover.