QoS Evolution in 3GPP Mobile Networks

: The objective of this study is threefold: (1) to review the evolution of mobile networks in 3GPP standards; (2) to present the evolution of the Quality of Service (QoS) of the 3GPP mobile family, from GSM to LTE; (3) to present the QoS frameworks, characteristics and attributes of each technology. In the last two decades, 3GPP mobile networks have been evolving from GSM to LTE providing end-users with improved QoS. However, all these networks are still in use up to the present time. Users moving from a coverage area served by one technology to another area covered by a different system would have to perform vertical handovers impacting their established QoS and affecting the performance of their applications. All the QoS mechanisms as well as their theoretical performance as a whole are reviewed. However, in commercial networks, it is proposed that QoS attributes and data measurements be continuously performed in order for operators to set more realistic users’ expectations and provide more practical information for application developments.


INTRODUCTION
One of the most important evolutions, over the last three decades, is the tremendous developments, deployments and growths of mobile and wireless networks, particularly, 3GPP mobile networks family.Moreover, this tremendous growth has great impacts on applications developments that have been developed taking into consideration the mobility and location as well as the offered QoS by the latest technology deployed.However, as the users move within the various coverage areas, vertical handoff, between the different technologies, will always occur creating some challenges to the applications on one hand and affecting users' experiences and expectations on the other hand.Additionally, users might be communicating with each other using different technologies and releases, setting some major challenges to migrate these technologies together on one side and within the 4G heterogeneous network on the other side (Stankiewicz and Jajszczyk, 2011;Sarraf and Ousta, 2008;Sarraf et al., 2012).QoS attributes set the performance thresholds such as maximum bit rate error, delay variation tolerance, maximum transfer delay, etc.The support of Quality of Service permits different traffic flows to be treated with the appropriate attributes (delay, jitter, bit error rate) required by the application.
In this study, we present the evolution of the QoS of the 3 GPP mobile families from GSM to LTE, presenting as well the QoS frameworks, characteristics and attributes of each technology.
History and evolution of the 3GPP family: Since the establishment of the Global System of Mobile communication (GSM) in 1992, the 3GPP mobile networks family has gone through a tremendous evolution, from GSM to Long Term Evolution (LTE).The history of evolution of 3GPP mobile family is illustrated in Table 1 and 2, respectively.

METHODOLOGY
QoS in 2G mobile networks: GSM: When 2G mobile networks were introduced in the first half of 1990s, the main focus was on providing voice services.However, data services based on circuitswitched technologies were introduced with phase 2 in 1994.Initially, GSM, which was the 2 nd Generation (2G) mobile systems, provided symmetric data services with bandwidth of 9.6 kbps, while 14.4 kbps user data rate was introduced with R96 in 1997.However, as Circuit Switched Data (CSD) services were based on TDMA with circuit-switched technology similar to voice, they were charged based on time, thus they were: • Quite expensive • Inefficiently using the radio resources High speed circuit switched data-HSCSD: In an attempt to increase the bandwidth, High Speed Circuit Switched Data (HSCSD) services were introduced in release R96 along with the 14.4 kbps user rate.They consist of combining multiple timeslots of the TDMA frame, up to 8, thus using multiple traffic channels for data communications.However, due to system limitations on the A and E interfaces, only up to 4 timeslots of the 14.4 kbps of TCH can be combined, providing up to 57.6 kbps of data communication (3GPP-TS.22.034, 2008).However, HSCSD is still using circuit switched technology.Due to many limitations including technologies for phone developments that could not support 4 simultaneous timeslots on both the uplink and the downlink on the one hand and the high charges of the service and the inefficient radio resource utilization on the other hand, had resulted in very limited use of this technology.HSCSD was replaced by the General Packet Radio Service (GPRS).HSCSD data rates are presented in Table 3.

General packet radio service-GPRS:
In order to resolve the inefficiency with Circuit Switched Data (CSD), General Packet Radio Service (GPRS) was introduced with the release R97 in 1998.By introducing new nodes such as Serving GPRS Support Node (SGSN), Gateway GPRS.
Support Node (GGSN) and Packet Control Unit (PCU) in the BSC, as well as new interfaces, GPRS introduced the packet data service to GSM based networks.Based on R97, GPRS typically provides a bandwidth of 40 kbps in the downlink and 14 kbps in the uplink per one user by combining multiple timeslots of the GSM TDMA frame into one bearer.Enhancements were introduced in releases R98 and R99 to add new coding schemes theoretically providing speeds up to 171.2 kbps.Table 4 shows GPRS data rates.

Enhanced data for GSM evolution-EDGE:
To increase the data transmission rate and to improve   • 144 kbits/sec for satellite and rural outdoor • 384 kbits/sec for urban outdoor • 2048 kbits/sec for indoor and low range outdoor QoS in HSPA: WCDMA networks were further upgraded with R5, R6 and R7 to support High Speed Packet Access (HSPA).High Speed Downlink (DL) Packet Access (HSDPA) was introduced in R6, whiles an Enhanced Uplink (UL), also referred to as High Speed Uplink Packet Access (HSUPA) and was introduced in R6.
With the enhancements of the schedulers, multiple codes, new transport channel called the High-Speed Downlink Shared Channel (HS-DSCH) that uses both QPSK and QAM16 and channelization, data rates have been further increased to reach 14.4 Mbps, with a peak user data rate of 13.4 Mbps (on MAC level).Similarly, HSUPA can also achieve a data rate up to 5.76 Mbps (Siomina and Yuan, 2008;3GPP-TS.25.306, 2011a).To further increase bitrates, HSPA+, with new functions such as QAM64 on the DL and QAM16 on the UL as well as Multiple Input Multiple Output (MIMO) in the DL only, was introduced with R7.The maximum data rate on the DL is 21.1 Mbps and on the UL is 11.5 Mbps (3GPP-TS.25.306, 2011b).
However, in R7 MIMO cannot be used in combination with QAM64, but it is possible to use it in R8.Additionally, R8 has also introduced the Dual Cell-HSDPA, also referred to as Dual Carrier-HSDPA, DC-HSDPA; where carrier aggregation of two adjacent 5 MHz bands covering the same area.The combination of MIMO and Dual Cell-HSDPA would increase the data rate to a maximum of 28 Mbps with QAM16 and 42 Mbps with QAM64 (3GPP-TS.25.306, 2011a).By combining 64QAM, MIMO-2×2 and Dual-Cell, the data rate has further increased to reach a peak of 84.4 Mbps with R9.Similarly, data rate on the UL has also increased to reach 22.9 Mbps with R9 using 16QAM and dual cell (3GPP-TS.25.306, 2012).Uplink and downlink data rates are presented in Table 7 and 8,  respectively.QoS in LTE: LTE, which was introduced with release R8 is supposed to provide high throughput and low latency in a way to support richer quality of experience for users and the ability to provide sophisticated services and applications such as Video-VoIP, highdefinition video streaming, mobile gaming and peer-topeer file exchange (Yahiya, 2011).In LTE, QoS is provided between UE and PDN Gateway and is applied to the Evolved Packet System (EPS) bearer-Radio bearer, S1 bearer and S5/S8 bearer-as illustrated in Fig. 3. EPS bearer is further divided into two types of bearers: • Guaranteed Bit Rate (GBR) • Non-Guaranteed Bit Rate (non-GBR) referred to as the default bearer (3GPP-TS.36.300, 2010) A bearer is assigned a value referred to as a QoS Class Identifier (QCI), which refers to a set QoS attributes (3GPP-TS.25.306, 2011b).The mapping of LTE standardized QCI to QoS objectives is presented in Table 9.In addition to QCI, additional QoS attributes are associated with a LTE bearer including: • Allocation and Retention Priority (ARP) • Maximum Bit Rate (MBR) • Guaranteed Bit Rate (GBR) • Aggregate MBR (AMBR)  The traffic generated by a particular application can be differentiated into separate Service Data Flows (SDFs) and mapped to a particular bearer, based on specific parameters provisioned either in the PCRF or defined by the application layer signaling (Alasti et al., 2010).SDFs mapped to the same bearer are treated similarly from the QoS standpoint.LTE is based on new access technique called Orthogonal Frequency Division Multiple Access (OFDMA) on the downlink and Single Carrier Frequency Division Multiple Access (SC-FDMA) on the uplink.With a bandwidth consisting of 1.4, 3, 5, 10, 15 and 20 MHz, MIMO up to 4x4 and high modulation (e.g., 64QAM), data rates can be as high as 326.4 Mbps on the downlink and reach 86.4 Mbps in the uplink (3GPP-TS.36.211, 2013;3GPP-TS.36.212, 2013;3GPP-TS.36.300, 2013) Peek data rates, for both downlink and uplink, are presented in Table 10 and 11, respectively.

DISCUSSION
Over the past 2 decades, 3 GPP mobile networks have been evolving from GSM in 1992 to LTE Advanced in 2011.Consequently, the QoS frameworks as well as their attributes have also evolved.Peek data rates have increased from 9.6 kbps offered with GSM in 1992 to as high as 1 Gbps with LTE Advanced in 2011.Figure 4 shows the evolution of peak data rates for 3 GPP networks.
However, from our industrial experience, these QoS attributes, including the peek data rates, are theoretical and might not be obtained all the time in commercial networks.
Additionally, as users move within the coverage areas, they might switch to lower data rates, either due to lower radio quality or due to a vertical handover to another technology.Thus, measurements of real QoS attributes, including vertical handovers, in commercial networks should be performed in order to: • Provide realistic figures for customers • Improve the QoS performances of the networks Additionally, mobile applications should take into consideration the real QoS attributes, rather than the theoretical ones.They also should be designed to properly work with the continuous movements between these different technologies and their associated QoS attributes.

CONCLUSION
In this study we discussed the evolutions of QoS in 3GPP standards showing the various classes and their attributes and frameworks.Furthermore, this study outlined the advantages and drawbacks of each of these technologies and presented the different radio access technologies indicating their digital modulation schemes.Finally, the data rates of the 3GPP standards were theoretically compared.

Table 1 :
History of 3GPP mobile family

Table 4 :
GPRS data rates (kbps) contains a set of new features.With the introduction of 16 QAM and 32QAM modulation combined with higher symbol rate (Level B) and turbo coding in the downlink, latency improvements and the use diversity at the mobile station, the data rate would theoretically reach 1.9 Mbps in the downlink.The primary motivation behind EDGE evolution is to ensure the future competitiveness of the dominant second-

Table 6 :
UMTS QoS classes and parameters The specifications of GPRS and EDGE did not include any QoS framework.This has been introduced in 2000 with the introduction of UMTS in R99.Frameworks for QoS including an end-to-end QoS involving GPRS have been introduced in this release(3GPP-TR.23.802, 2007; 3GPP- TS.23.107, 2004).The main purpose in3GPP- TS.23.107 (2004)is to specify the list of attributes applicable to UMTS Bearer Service and Radio Bearer Service, as well as to describe the Quality of Service architecture to be used in UMTS networks.The(3GPP- TS.23.207, 2004)describes a the interaction between the TE/MT Local Bearer Service, the GPRS Bearer Service and the external Bearer Service and how these together provide Quality of Service for the End-to-End Service.Such architecture is depicted in Fig.2.

Table 7 :
HSUPA peak data rates

Table 8 :
HSDPA peak data rates MHz, UMTS provides 0.2 bit/sec/Hz, a maximum data rate of 2 Mbps, RTT at the user plane of 50 msec and call setup time of 2 sec.However, UMTS bearer services have different QoS parameters for maximum transfer delay, delay variation and bit error rate; thus the theoretical offered data rate targets are: