Etapa III: Evaluación del impacto

The Universal Mobile Telephone System (UMTS) is part of a family of propos-als adopted by the International Telecommunications Union [219] as its 3G stan-dard, the IMT-2000. The history of UMTS can be traced back to 1986, to the research activities supported by the European Commission’s collaborative RACE projects, such as CODIT, ATDMA and MONET. These projects first defined the term UMTS and investigated different and competing multiple access schemes for a new air interface and the network aspects of a future UMTS system. UMTS is now being developed in the Third Generation Partnership Project (3GPP) [206].

The UMTS packet domain has inherited the Core Network (CN) structure from the General Packet Radio Service (GPRS) network. The Packet Domain can be divided into two parts: the UMTS Terrestrial Radio Access Network (UTRAN) and the UMTS Core Network (CN). This division can be seen in Figure 4.3. The CN consists of the 3rd Generation Serving GPRS Support Nodes (3G-SGSNs) and the Gateway GPRS Support Nodes (GGSNs). These network nodes correspond directly to the GPRS network SGSN and GGSN network elements.

The 3G-SGSN handles the MS mobility management, authentication, and gathers charging information. It does not interpret or monitor the traffic flow that goes through it. Instead, it merely sees that the traffic is routed to the correct GGSN. The 3G-SGSN is connected to the UTRAN via the Iu interface [184] and to the GGSNs and to the other 3G-SGSNs via the Gn Interface [183].

The GGSN serves as a gateway between the UMTS Core Network and the outside networks. It takes care of Packet Data Address allocation for the MS, for example, the IP address, and it may filter the traffic sent to the MS.

The protocol used for the Gn interface is the GPRS Tunneling Protocol (GTP) [186]. The purpose of the GTP is to tunnel the user data from the GGSN over

4.3 Third Generation Networks 55

Figure 4.3: The UMTS Network Architecture.

the UMTS IP Backbone. These tunnels are called Packet Data Protocol (PDP) Contexts. The tunneling increases security, because it prevents the vital Core Network entities from being pointed directly from outside.

The Mobile Station (MS) is a combination of User Equipment (UE) and Mo-bile Terminal (MT). The UE is the UMTS device such as a UMTS telephone. The MT can be for instance a laptop computer or a PDA device.

The UTRAN consists of a set of Radio Network Controllers (RNCs) and one or more abstract entities currently called Node B. Node B is a logical node re-sponsible for radio transmission in one or more cells to and from the UE. The radio interface provides two modes of operation, a Frequency Division Duplex (FDD) mode that uses the Wideband CDMA multiple access scheme, and a Time Division Duplex mode using TD-CDMA. It is likely that the FDD mode may be used for macro- and micro cellular coverage and the TDD mode for picocellular deployment, including indoor and corporate office environments [34, 156]. The maximum theoretical user data bandwidth available on the radio link is 2 Mbps, although in the first phase of deployment, rates from 64 to 384 kbps may be avail-able. A more detailed description of UMTS and its history can be found in [156].

4.3.1 Quality of Service in UMTS

The signaling for QoS in UMTS is similar as in GPRS and is based on PDP Contexts. The QoS concepts are mainly used for packet-handling on the radio

Table 4.1: The UMTS attributes for each traffic class

Traffic class Conversational Streaming Interactive Background Maximum rate 2048 kbps 2048 kbps 2048 kbps 2048 kbps

Guaranteed rate 2048 kbps 2048 kbps – –

Maximum SDU 1500B 1500B 1500B 1500B

SDU format info Yes Yes – –

Source descriptor speech/ - speech/ - – –

Transfer delay 80 ms and up 250 ms and up – –

Delivery order yes/no yes/no yes/no yes/no

Deliver

erroneous SDU yes/no yes/no yes/no yes/no

SDU error ratio 0
1 to 10 ⁵ 0
1 to 10 ⁵ 0
001 to 10 ⁶ 0
001 to 10 ⁶ Residual bit

error ratio 0
05 to 10 ⁶ 0
05 to 10 ⁶ 0
004 to 10 ⁸ 0
004 to 10 ⁸

Traffic priorities – – Three levels –

link. Unlike in GPRS, UMTS can support several PDP Contexts per mobile and per IP address. When a mobile is initiating new flows, it can evaluate whether an existing context has resources for supporting the new flow, or it can request a new PDP context from the network.

The QoS and traffic characteristics of the radio bearers in UMTS are defined by several attributes describing the traffic characteristics and QoS [187]. The profiles, or traffic classes, incorporate different combinations and possible value ranges of the attributes. There are four traffic classes specified: Conversational, Streaming, Interactive and Background. When a service is requested the radio management selects the one that best corresponds to the requirements.

The first two classes are designed for real-time conversational and streaming services, respectively. The fundamental characteristics of the conversational class are low delay and low delay variations, and the streaming class is characterized by low delay variations. The internal payload format can also be specified. This allows higher spectrum efficiency and support Unequal Error Protection (UEP) and Unequal Error Detection (UED) mechanisms, where different parts of the payload are protected and detected differently [107]. This is especially important for codecs like the Adaptive Multi-Rate (AMR) codec [175].

The two last classes are designed for non-real time services. The interac-tive class is used when a request-response exchange, as for instance in HTTP, is requested, and background class when there are no requirements on delay, for example background file transfers. Within the interactive class, radio bearers are differentiated by a relative priority. Thus, different levels of QoS can be provided for interactive traffic.

The attributes per traffic class are summarized in Table 4.1. The traffic class roughly defines the type of application that the radio bearer is optimized for. It

4.3 Third Generation Networks 57 also defines the set of attributes that can be used for that specific traffic class.

By including the traffic class itself as an attribute, UMTS can make assumptions about the traffic source and optimize the transport for that traffic type.

The maximum bit rate is defined as the maximum number of bits delivered be-tween the mobile and the edge of the UMTS network within a period of time. The guaranteed bit rate defines a guaranteed bandwidth within a period of time. The guaranteed bit rate may be used to facilitate admission control based on available resources, and for resource allocation in UMTS.

The maximum SDU size or SDU format information defines the maximum radio SDU size or the exact payload format. This information can be used for packet scheduling, for example. In addition, the spectral efficiency and delay can be optimized for transparent transmission, if the exact sizes of the radio SDUs are known. Transparent transmission is here referring to transmission without adding any protocol information. Also, mechanisms like UEP and UED require that the internal payload format is known. The bearer can, thus, be less expensive if the application can specify the payload formats and packet sizes. An SDU corresponds to an IP packet.

The source statistic descriptor identifies if there is a characteristic pattern, for example, if the application data has a packet arrival pattern typical of speech. By identifying the characteristics of the source of submitted radio SDUs, the best admission control algorithm can be applied.

There are also six attributes describing the provided QoS. The transfer delay indicates the maximum delay for the 95th percentile of all delivered radio SDUs over the wireless link. Delay for a radio SDU is defined as the time from a request to transfer a radio SDU at one end point to its delivery at the other, including a possible re-transmission time. It is used to specify the delay tolerated by the application, which allows UTRAN to set transport formats and ARQ parameters.

The delivery order parameter indicates whether the UMTS bearer shall pro-vide in-sequence radio SDU delivery or not. Whether out-of-sequence radio SDUs are dropped or re-ordered depends on, for example, the specified SDU error ratio and Residual bit error ratio. Without strict in-sequence delivery various buffer sizes can be minimized.

The delivery of erroneous SDUs is used to decide whether error detection is needed or not, and indicates whether radio SDUs detected as erroneous should be delivered or discarded. The SDU error ratio indicates the allowed fraction of lost or erroneous radio SDUs. The residual bit error ratio indicates the allowed undetected bit error ratio in the delivered radio SDUs. If no error detection is requested, the residual bit error ratio indicates the total bit error ratio in the de-livered radio SDUs. These three parameters are used to configure radio interface protocols, algorithms and error detection coding [187].

The traffic handling priority gives an internal priority handling for the inter-active class. It specifies the relative importance for handling of all radio SDUs belonging to one specific interactive bearer compared to the radio SDUs of other interactive bearers. The traffic handling priority can be used, for example, for traffic scheduling and admission control.

Some of these attributes are actually general to other wireless and wired net-works, as well. Peak and guaranteed bit rates, and SDU size are well-known traffic descriptors. The source statistic descriptor is more UMTS specific, but still provides quite general information. Almost all of the wireless networks make use of basic wireless parameters like Transfer delay, SDU error ratio and Residual bit error ratio. Parameters similar to SDU format information, Delivery order, Deliv-ery of erroneous SDUs are found also in other wireless networks. As indicated above, a gain in service quality and spectrum efficiency is achieved when speci-fying the payload format, the exact packet sizes, and whether erroneous packets should be discarded or not. Traffic handling priority uses similar prioritization as, for example, is used in DiffServ [187]. A more thorough overview of the GPRS and the UMTS QoS approaches can be found in [187, 103].