Tài liệu Thực hiện chất lượng dịch vụ trong các mạng IP (P9) pptx

Adaptive applications have beenfound to make it possible to raise the utilization level of an accessnetwork [CZ02], and indeed the adaptive bit rate AMR codec that is used in the 3GPP sy

Case Study: Service

Quality Support in an

IP-based Cellular RAN

In this chapter, we shall study an IP-based Radio Access work (RAN) as an example of applying the technologies of thepreceding chapters In the framework of preceding chapters, an

Net-IP RAN can be considered to be a multi-service Internet accessdomain supporting endpoint mobility From the viewpoint of Diff-Serv, mobility is handled on the link layer The traffic engineeringframework of IETF is used to structure the example

In what follows, we shall first study the motivation for usingIP-based transport in cellular radio access network Next, IP RANtransport architecture is explained After that, an implementation

of service quality support in IP-based RAN is accounted for usingIETF’s traffic engineering framework

The technologies listed here are generic to use of DiffServ-basedtransport in IP based RAN DiffServ transport will be used incellular backbones as well Concentrating on IP-based RAN in thischapter has the advantage of providing a relatively well-definedcase study of a multi-service access network The same principlescan be applied in the backbone network, too, but QoS managementthere is easier due to higher aggregation level of flows

Implementing Service Quality in IP Networks Vilho R¨ais¨anen

 2003 John Wiley & Sons, Ltd ISBN: 0-470-84793-X

Further optimizations for service quality support in IP-basedRANs are possible, but are not discussed here The goal of thepresent chapter is to provide a case study showing how trafficengineering and other advanced IP technologies reviewed in pre-vious chapters can be put together to implement service quality

trans-Traditionally, Radio Access Networks of GSM, GPRS, and 3Gnetworks have been built using Time Division Multiplexing (TDM)links, based on an Erlang calculation for the peak hour traffic.This has been an adequate solution for networks in which the enduser traffic consists almost solely of circuit-switched voice tele-phony Interactive voice has strict delay requirements, wherebythe network needs to be dimensioned to accommodate bursty datatraffic demand during peak hours The dimensioning of telephonynetworks for aggregated speech traffic, taking into account Pois-son process-like arrival of connections in individual servers (basestations) is an established discipline within traditional telephony(cf., e.g., [McD00]) The problem with applying this approach to

a multi-service network domain is that it does not make best use

of the operator’s investment in transport capacity, especially cerning the variety of services that need to be supported by thirdgeneration mobile networks For this reason, new alternatives havebeen evaluated In a study made by the Mobile Wireless InternetForum (MWIF), using IP-based transport in the RAN of differentthird generation networks was found to be a viable option [IRT01].Data-type applications have been adopted in the mobile net-work, including browsing the Internet using Wireless Application

con-Protocol (WAP) With my GPRS phone, I can initiate a

connec-tion and start browsing the news titles from Financial Times or

Helsingin Sanomat, latter being the largest daily newspaper in

Fin-land, within a few seconds I have ample time to check for latestnews headlines during a 5-minute bus trip to work from home.Other traffic types, which are made possible by the 3GPP QoSframework for mobile networks, are streaming and data transfer,such as uploading or downloading of digital photographs Addingpictures to text using Multimedia Messaging Standard (MMS) ispossible already with the “2.5G” GPRS networks The set of sup-ported end user services for 3G will consist of not only speechand short message service (SMS), but to include also MMS, data,and real-time content As discussed in Chapter 3, the differences

in service quality requirements in a heterogeneous traffic mix can

be used in network dimensioning

An IP-based transport network in the RAN still has to be sioned to support delay-critical traffic during peak hours, in thesame way as with traditional telephony For the less urgent traffictypes, however, no fixed capacity needs to be reserved, but insteadthe multiplexing gain of packet switching can be leveraged toobtain high utilization level in the network The enabling technol-ogy here is DiffServ, which makes possible sharing a single capac-ity “pipe” by diverse class of applications with varying servicequality support requirements Implementation of service qualitywith IP and differentiated treatment is advantageous when com-pared to the traditional concept of reserving capacity for differentservices in the network The exact benefits from using DiffServ-based transport vary according to the precise combination of endusers’ services in the mobile network, as well as the access networktopology, but analyses indicate that savings can be up to tens ofpercent as compared to traditional network dimensioning Radioaccess network being a major factor in cellular network CAPEX,this fact translates to monetary savings for the operator

dimen-In general, a Radio Access Network transport based on IP bringswith it several benefits:

• Less transport capacity is needed in the RAN

• There are fewer protocol layers to be managed

• Same type of transport hardware can be used in RAN as inInternet backbone

Leveraging the benefits of IP in a best possible manner requiresfurther technologies, which are not within the scope of this chap-ter Discussion about some of the related issues can be found

in [IRT01]

An IP-based RAN also has the benefit of allowing the ral incorporation of other access technologies apart from cellularradio into a single multi-service, multi-access network In such

natu-a network, IP is the protocol tying different natu-access technologiestogether This concept, sometimes called the All-IP network, isillustrated in Figure 9.1 In addition to the WLAN access shown

in Figure 9.1, other possible access methods include ADSL/SDSLand wireline Ethernet

The usability of IP-based RAN is not limited to 3GPP networks,but can be used in other mobility networks as well Indeed,the analysis of MWIF referred to previously covers both 3GPPand 3GPP2 networks, the latter being CDMA2000 variant of theInternational Mobile Telecommunications 2000 (IMT2000) thirdgeneration mobile framework In the MWIF study, also IntServ hasbeen included as a potential IP service quality support technology

in the RAN

More generically, service quality support in mobile networkshas been discussed recently in [CZ02] In that scheme, bandwidthbrokers in radio network access network border routers performadmission control to SLAs in a DiffServ network, based on effec-tive bandwidth approximation Adaptive applications have beenfound to make it possible to raise the utilization level of an accessnetwork [CZ02], and indeed the adaptive bit rate AMR codec that

is used in the 3GPP systems is able to adjust to available bit rate.Irrespective of the radio access technology used, having a proper

Operator backbone

IP-based RAN

WLAN access

Public internet

Figure 9.1 A High-level view of an All-IP network

service quality support model is important for providing properservice quality support in a multi-service environment [GC02].

We shall next take a brief look at 3GPP IP RAN transport ture for WCDMA radio access The architecture will be extended

architec-to cover evolution versions of GPRS radio technologies as well, butthese are not covered here Before taking a look at architectures,let us make a short excursion to the world of generic transportarchitecture of a mobile network

9.2.1 PLMN transport architecture

A generic cellular network, called Public Land Mobile Network(PLMN) in GSM parlance, consists of three distinct transport parts:

• long-haul backbone;

• medium-capacity fibre transport;

• bandwidth-limited access links

The high-level transport architecture domains are shown inFigure 9.2

The long-haul backbone is typically a general-purpose transportnetwork having traffic engineering capabilities adequate for hightraffic aggregation levels The backbone connects the medium-capacity access part of the mobile network to mobility servers

GPRS roaming exchange

High-capacity backbone Medium-capacity

Mobility servers Cellular network

Other PLMN

Bandwidth-limited

access links

Figure 9.2 Transport domains in a GPRS/3GPP network

(including SGSN and GGSN), to other cellular networks via GPRSRoaming Exchange (GRE) network via SGSN, and to non-cellularInternet domains via GGSN As the name implies, GRE networkfacilitates roaming between operators by providing service qual-ity enhanced connectivity between mobility servers in differentPLMNs We shall return later in this chapter to the topic of servicequality support towards Internet domains.

Medium-capacity fibre transport network delivers the trafficbetween access links to the long-haul backbone (Node B isthe canonical name for base station in WCDMA.) The medium-capacity fibre transport can often be dimensioned with sufficientcapacity so that it will not be congested

Bandwidth-limited access links connect BTSs and Node B’s tofibre-based transport The link layer technology used in this part

of the transport architecture depends on the environment Typicaltechnologies include, leased fibre links and microwave links.The bandwidth-limited part of RAN transport can be hierar-chical in nature, resembling a tree with Node Bs as leaves, andDiffServ-capable routers making up the branches of the tree trunk(see Figure 9.3) For resiliency purposes, also loops may be used

As the name implies, the bandwidth-limited links are typically oflimited capacity, and interface to the medium-capacity fibre links

A single PLMN may encompass thousands or tens of thousands ofNode Bs, and an accordingly large number of narrow-bandwidthlinks in RAN branches The cost of entire RAN equipment makes

up the largest part of the cost of an entire PLMN, part of that

DiffServ router

Fibre transport

Figure 9.3 An illustration of RAN transport hierarchy

Note: Frame combiner not shown

being attributable to the transport used Even though the cost

of the actual transport may not be percentually the largest part

of the total cost, transport capacity can be limited by the ability of leased capacity or licences for the microwave transport.Thus, transport capacity savings of tens of per cents are madepossible multiplexing gain using differentiation-based transportinside RAN branches instead of per-aggregate capacity reserva-tions translates significant transport cost savings for the mobilenetwork operator

avail-9.2.2 IP RAN transport architecture

IP-based RAN is an evolution of the 3GPP mobile network tecture, extending the scope of use of IP transport (Figure 9.4) InGPRS and Release 99 3GPP architectures, the “core network” inter-faces between GGSN and SGSN (including GRE interface) havebeen based on IP In Release 4, a 3GPP standardization successor

archi-of Release 99, IP transport option up to Radio Network Controller(RNC) was made possible In the IP option of Release 4, the basicR99 architecture is not modified, the only change being the trans-port technology used beneath the GTP tunnel between the NodeB’s and RNC

An IP RAN is a logical next step in this evolution, integrating theIP-based transport more closely into the RAN architecture It turnsout that for overall efficiency, it is beneficial to redesign the radionetwork control layer, replacing a single RNC with distributedradio resource control architecture The explanation of the entire

IP RAN radio resource control architecture is not within the scope

of the book For the purposes of this chapter it is sufficient to knowthat from the viewpoint of mobile terminals, the 3GPP servicequality support control provides the same service quality support

GTP IP Link

GTP IP Link

GTP IP Link

GTP IP Link

Link User IP layer User IP layer

Terminal IP base transceiver

station & radio network gateway

Serving GPRS support node

Gateway GPRS support node

Figure 9.4 IP RAN architecture protocol stacks for user layer traffic

Note: The architecture has been simplified to make the role of ers clearer

bear-as the preceding 3GPP network variants An overview of the IPRAN protocol stacks on a logical level is shown in Figure 9.4.From the viewpoint of protocol stacks, the only difference in theactual stacks is that the GTP tunnel uses IP-based bearer up tobasis station Please note that in Figure 9.4, only user layer trafficprotocol stack is shown.

9.2.3 Handover traffic

Mobile networks based on Code Division Multiple Access (CDMA),

of which Wideband CDMA (WCDMA) used in 3GPP networks is

a subspecies, use the so-called soft handover for handling terminal

mobility This means that a terminal may communicate with themobile network through more than one base station (called “NodeB” in CDMA) at a time A mobile terminal does not have to detachfrom the previous Node B before commencing communication withthe next one This arrangement is necessary due to CDMA powercontrol Due to this, traffic to a particular mobile participating to softhandover is being transmitted via multiple Node Bs in both uplinkand downlink directions

A practical implementation of soft handover requires the function

of a frame selector function in the network, which – for uplink

direc-tion – receives radio frames from different soft handover “legs”,and combines the signals arrived via different routes into a singlePDU stream towards the core network (see Figure 9.5) The frameselector is also known as a macrodiversity combiner, with the term

“microdiversity” being reserved for mobility handled by ual Node Bs For downlink direction, the signal is split into differentpaths and frame combining is performed in the terminal In 3GPP

individ-Terminal Node B

Node B

Frame selector

Core network

Figure 9.5 Principle of soft handover

Note: Transport of radio frames is shown by dashed lines, and normal GTP tunnelled traffic by a solid line

Release 99 networks, frame combining for uplink and frame ting for downlink is done in RNCs.

split-The importance of the soft handover concept for the presentdiscussion is that in addition to 3GPP user layer and signallingtraffic, radio frames being part of soft handover traffic need to betransported in the links of bandwidth-limited RAN Soft handovertraffic has typically high forwarding priority, the frame-combiningalgorithm having limitations for the allowed delay difference onthe different paths

In addition to soft handover traffic, WCDMA networks in

gen-eral can also carry so-called drift traffic, consisting of non-processed

radio frames In the case of 3GPP R99 network, drift traffic can

be transported between a drift RNC and a serving RNC In this

case, the drift RNC forwards non-processed radio frames to theserving RNC Drift traffic, too, has high priority In what follows,generic references to handover traffic are made, covering both softhandover and drift traffic

9.2.4 Service mapping in IP RAN

The 3GPP QoS model, reviewed in Chapter 5, applies dently of the transport technology used in the RAN Thus, theuser layer QoS model used in IP RAN is the same as in Release

indepen-99 networks What is different between IP RAN and Rindepen-99 UTRAN

is the implementation of the transport part of the radio accessbearer The implementation of service quality support in IP RANtransport is described below on a generic level

As in 3GPP R99, each application flow is associated with a PDPcontext describing the negotiated QoS support for the flow TheQoS attributes of a PDP context have been described in more detail

in Chapter 5, and for the present purposes we are interested inthe QoS attributes of the PDP context which may affect IP RANtransport These include:

• Traffic class (conversational, streaming, interactive, or ground);

back-• Traffic Handling Priority (THP);

• Allocation/Retention Priority (ARP)

These QoS attributes are used in mapping the PDP context ciated with the application flow onto a Radio Bearer (RB) for the

PDP context Managed mapping

IP transport bearer

Radio access bearer

Figure 9.6 Service mapping for user layer traffic in IP RAN

radio interface, and a Radio Access Bearer (RAB) in RAN port Service quality support in the backbone transport can bebased on RAB used in the RAN (Figure 9.6) Thus, as in R99,the PDP context is a service quality abstraction layer between theapplication, and the different link-layer technologies (radio inter-face, IP transport)

trans-Seen from end-to-end viewpoint, service quality support in anAll-IP network for a terminal communicating with a host in theInternet includes the following steps:

1 Terminal maps application flow requirements into PDP contextQoS attributes

2 PDP context of appropriate type is initiated

a The QoS parameters of the PDP context map onto a suitableRAB and CN bearer

b RAB maps to specific WCDMA radio interface parameters

c RAB in the RAN maps onto suitable IP transport ters (PHB)

parame-d QoS treatment for packets belonging to the PDP context isbased on the RB service quality support in the radio interface,and DSCP marking in the RAN and backbone transport

3 Between GGSN and external network, possible QoS ing is based on PDP context QoS profile

interwork-a In uplink direction, minterwork-apping to QoS mechinterwork-anism beyondGGSN is based on PDP context properties

Table 9.1 Traffic types in IP RAN

priority

Tolerance to delay jitter

Drift traffic Radio frames Typically high Typically little

Control traffic No tunnelling Depends on traffic Not applicable Management traffic No tunnelling Depends on traffic Not applicable

b In downlink direction, a Traffic Flow Template (TFT)

is created within GGSN, mapping incoming flow intoappropriate PDP context

Mapping of traffic onto 3GPP traffic classes is controlled by theterminal, and mapping of traffic classes onto QoS support schemes

in radio interface and IP transport is controlled by the mobile work The latter mapping can be implemented in many differentways In what follows, the simple solution of predefined map-ping onto RAB and further from RAB to IP QoS parameters in thetransport is assumed

net-All told, IP RAN transport needs to be able to deal with thetraffic types listed in Table 9.1 It is useful to note that differentkinds of tunnelling are involved, including radio framing, GTPtunnelling, and no tunnelling at all

The scheme that has been accounted for above constitutes thebasic implementation of service quality mapping aspects in IPRAN Further enhancements are possible, but are not central tothe current discussion

9.3 TRAFFIC ENGINEERING IN ALL-IP RAN

In this chapter the technologies used in IP-based RAN transportare presented within the framework of IETF traffic engineering.The “full loop” of traffic engineering process in IP RAN includespolicy-based management for the configuration of the networkelements, as well as technologies for measuring service quality inthe network There is also a shorter timescale process involved,