Connection management for broadband mobile sa tel I it e systems

B. Fan, R. Tafazolli and B.G. Evans

Abstract: A novel radio connection management scheme for QoS-provisioning transport of ATM traffic over a broadband satellite link is presented. The new scheme effectively manages the air interface connections via separation of connection types, establishng QoS-based connections and deploying an efficient communication mapping and lateral scheme. The scheme optimises in respect of cell loss rate for individual services and minimises bandwidth. Evaluation of the scheme is presented for a GEO multimedia satellite system.

1 Introduction

Next generation broadband satellite systems, such as Astrolink, SPACEWAY and SkyBridge, all anticipate the use of ATM-based technology over satellite links [l]. All such systems face a challenge in air interface designs that enable satisfactory QoS for different service types due to long propagation delays, degraded satellite channels and user mobility [2].

Most services are expected to be connection oriented and hence the connection management scheme (CMS), which contains connection set-up, maintenance and release, is of paramount importance in network connectivity.

The CMS has to contend with user mobility as well as a range of service types in allocation and management of resources and has to deliver high utilisation of expensive satellite channels. To achieve a successful CMS scheme, four major factors need to be taken into account:

(i) the physical channel effects on performance [3]; (ii) the MAC (medium access control) protocol [4]; (iii) resource allocation; and (iv) the LLC (logic link protocol).

In previous studies the above four factors have usually been studed separately. However, the requirement for effective ATM traffic transportation over satellites can only be achieved by studying them concurrently. Based on this premise, in the remainder of this paper, we introduce a novel method to include and relate all of the aforemen- tioned key parameters in a connection management design.

2 Design approach

In order to set up connections across a satellite network to provide traffic transportation for different services we propose a new CMS concept supported at the air interface

0 IEE, 2003 IEE Proceedings online no. 20030354 doi: 10.1049/ip-com:20030354 Paper first received 6th February 2001 and in revised form 19th August 2002 B. Fan was with the University of Surrey and is now with T-Mobile Technology Service Department, Imperial Place, Maxwell Road, Borehamwood, Herts WD6 lEA, UK R. Tafazolli and B.G. Evans are with the Centre for Communication Systems Research, University of Surrey, Guildford GU2 7XH, UK

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and logically defined via three parameters. These three parameters are connection configuration, connection mode and connection reliability. Th~s methodology enables the network to flexibly accommodate, with minimal provision of satellite channel resource, various external traffic types by dynamically tailoring its bandwidth into the different types of connection according to service type and QoS require- ments of the active users.

Based on the connection definition at the air interface, the traffic and performance objective parameters alerted by the individual service, a connection mapping algorithm at the network side is then required to map the external services onto a suitable air interface connection. The task of the mapping algorithm is to determine the three aforemen- tioned parameters that define a connection. The metrics of CLR (cell loss ratio) and maximum cell transfer delay (CTD) are used to determine the suitable radio connection type for an ATM service class. Once a connection is determined for a particular service, signalling procedures have to be designed to assist the set-up, release and maintenance of such a connection.

The three major components are shown in Fig. 1. Discussions on the design of efficient connection control signalling have been presented in [SI.

3 Connection definition

The novel concept of an ATM-satellite connection at the air interface is defined via three parameters: connection configuration, connection mode and connection reliability.

ATM-satellite connection = (configuration, connection mode, reliability)

This definition separates connection types to accommodate, with minimal provision of satellite channel resource, various extemal ATM traffic types with different QoS requirements. Connection diversification can keep unused bandwidth on connections to a minimum by shaping connections into dlfferent categories according to their service types and quality requirements. Each set of three parameters has been defined to include the following elements:

Connection conzguration = Cpoint-to-point bidirectional, point-to-point unidirectional, point-to-multipoint unidirec- tional) : The connection configurations are distinguished by the number of calling parties involved. Three types of connection configurations are provided to accommodate a

IEE Proc.-Commun., Vol. 150, No. 4, August 2003

ATM services 9

buffer

r...-L---.

i QoS .._._ _...

satellite connection mapping

fixed ATM satellite network .... ... .. ......................... .. ..... . ...

_ _ _ _ 1 .__.

.___._.... _ f l l 1-1 network 0

Af 4 channel w

1 SAT 1 CL> signalling control procedures

Fig. 1 System model

wide range of services, i.e. Internet service, broadcast service and video-conferencing.

Connection mode = (permanent, semi-permanent, mixed mode) : According to different resource-request methods and resource-holding times, a connection is categorised as permanent, semi-permanent or mixed mode. A permanent connection, provided with in-band resource request (IBRR), holds the resources assigned during the connection set-up throughout the duration of the communication. A semi-permanent connection relies on out-of-band resource requests (OBRR) to reserve resources that are assigned on demand for a fixed time interval only [6]. In a mixed mode connection, a user can utilise both IBRR and OBRR to adjust their temporary resource demand.

Connection reliability = (reliable, relaxed reliable) A connection can be labelled a reliable or relaxed reliable transmission (RRT) connection to meet the CLR and CTD requirements. Reliable connections use traditional ARQ protocols and RRT connections use modified ARQ protocols, which can perform relaxed reliable transporta- tion of ATM traffic by discarding cells that no longer have significance for the application. As a special case in which no service guarantee is provided by the network to the user, the relaxed reliable mode can operate in unreliable transmission (UT) mode. A similar study of ARQ with packet discarding, selective-reject-with-discarding (SR/D) ARQ, has been discussed in [I. With RRT, packet transfer can be accelerated to meet specific CTD and cell delay variation (CDV) requirements.

4 Connection mapping scheme

The task of connection mapping is to determine the three parameters in the ATM-satellite connection. The perfor- mance parameters of CLR and maximum CTD at the radio interface are used to determine the applicable radio connection type for each ATM service.

4.1 Mapping the connection mode Mapping the connection mode involves two major tasks: to determine the resource request method and the amount of resource to be allocated. The mapping considers both QoS requirements and channel impairments. The concept of effective bandwidth [8], which is the bandwidth needed for an ATM service to guarantee its required QoS on the ATM-satellite link, is adopted in ths study. In the following, we discuss the mapping method for different ATM service categories. The symbols Brq, Befl, Btg are used to denote, respectively, the required user bandwidth at the ATM layer, effective bandwidth and the temporary bandwidth reserved through OBRR.

CBR services: For CBR services that generate traffic with a constant bit rate, a permanent connection is the most suitable mode. To counteract the fading influence of a

fading satellite channel and to ensure real-time QoS, the effective bandwidth rather than the CBR peak cell rate needs to be allocated by the network. Assuming a large window size and accurate prediction of the channel packet error rate (PER), the Be# of a single CBR source can be computed using a simple queueing model as shown in Fig. 2, In this model, a buffer with an average input cell arrival rate 2, and average output cell rate L1 is considered. If we assume that the cell loss probability due to buffer overflow is PB and the cell flow rate into the channel is, on average, AI, we then have

21 = A, - 2,PB (1)

Fig. 2 CBR service effective bandwidth Bef calculation model

Assuming that the PER in the mobile channel is E and the link capacity is C, the effective bandwidth B d can be regarded as the assigned link capacity C that allows a cell loss ratio of CLRreq, and Benis determined as;

2, x (1 - CLR,) 1 - - E

Beff =

where CLR, denotes the required cell loss ratio for the service.

Rt- VBR services: The mixed connection mode applies to real-time variable bit rate (Rt-VBR). As Rt-VBR services are real time and the cells have to be transmitted withm the required maximum CTD time, effective bandwidth should be allocated on a permanent basis to guarantee the QoS. Meanwhile the out-of-band resource request method used in the semi-permanent connection mode is also provided to solve serious cell loss problems due to serious channel fading or peak rate traffic bursts. An approximate approach to calculate the effective bandwidth is presented in [9] for a single on-off VBR source. As derived in [9], the following equation is used to calculate the effective bandwidth for the traffic transmission on a channel with low bit error rate:

(3)

where

IEE Proc.-Commm. Vol. 150, No. 4, August 2003 299

II, A,(&), p1012) and CLRo, respectively, denote the terminal buffer size, packet rate at state off (on) of the VBR model, transition rate from state off to on (on to off) of the VBR model, and cell loss ratio due to buffer overflow.

Nrt-VBR, ABR, UBR services: Nrt-VBR, ABR, UBR services are all non-real-time services. Because they are CTD- and CDV-tolerant service types, allocation of effective bandwidth is not necessary for these services, and the semi-permanent connection mode can be applied to ABR and UBR services to improve channel utilisation. The resource assigned to Nrt-VBR services consists of two parts, the ATM layer requested cell rate and the bandwidth dynamically reserved by OBRR. So the Nrt-VBR services are best mapped onto a mixed connection mode.

4.2 Mapping the connection reliability For some real-time services, cells that are delayed in the network by more than the specified CTD are considered to be of limited or zero value to the application [lo]. If reliable traffic transmission cannot meet the required CTD when the effective bandwidth is provided, discarding these cells at the data link layer will improve CTD performance. This type of transmission is named ‘relaxed reliable transmission’ in this study.

Relaxed reliable transmission has to ensure that the total cell loss ratio CLRT resulting from buffer overflow and the non-ideal physical channel meets the required CLR,,,. Let CLRo denote the cell loss ratio due to the buffer overflow and CLRc denote the cell loss ratio due to the non-ideal physical channel, then CLRT can be given as [9],

CLRT = CLRo + (1 - CLRo)CLR, Let Timer denote the retransmission timer of the ARQ protocol and MuxCTD denote the maximum cell transfer delay required, then N, the maximum transmission number of each packet at the data link layer before the maximum cell transfer delay is reached, would be given as

(4)

Since each packet can only be transmitted N times, the packet loss ratio in the channel CLR, is,

CLR, =EN ( 5 )

CLRT 5 CLRi-eq (6)

(7)

To ensure the required CLR is achieved, CLRT should satisfy the inequality,

that is

CLRo + (1 - CLRO)E~ 5 CLR,

As indicated above, if we can choose a buffer sue for which CLRo satisfies (7), then the relaxed reliable mode can be used. For the case that a large buffer size can be provided, for most values of E, CLRo = 0, then we have

Given that (7) can be satisfied, cell-loss-tolerant services CBR and Rt-VBR, can use relaxed reliable transmission to improve CTD performance.

The Nrt-VBR and ABR services require a low cell loss ratio and have no strict requirements on CTD and CDV, so the reliable transmission modes can be adopted. For UBR services, because the network does not commit guaranteed service quality, a relaxed reliable transmission node can be utilised when the system is congested. The connection mapping scheme is summarised in Table 1.

5 Services required from MAC and LLC layer

A specific connection type with its three parameters for an individual service will be determined by the network at the connection set-up phase. Once the connection type is decided, the MAC and LLC protocols need to work cooperatively with the proposed connection management scheme.

5.1 MAC Protocol The MAC scheme should be able to perform the OBRR and/or IBRR according to the connection mode of the service decided by the network. In order to provide the OBRR, the MAC protocol needs to have reservation minislots that are used for out-of-band reservation pur- poses. The typical reservation-based MAC protocol shown in Fig. 3 provides this mechanism. In this protocol, the RRC (resource request channel) forms a specially designed minislot channel to carry the resource reservation requests. To provide the IBRR, each data packet transmitted on the traffic channels must carry a small message in order to send its in-band resource reservation request.

A table is made available at the MAC layer, at both the mobile terminal and network side, in order to retain the status of the user’s connection mode and the corresponding channel allocation information. An example is shown in Table 2. The connection mode information in this table indicates whether the OBRR, IBRR or both are applicable to the user’s connection. When the connection mode is changed, the table should be modified accordingly. The semi-permanent resource request generating method indi- cates whether an OBRR resource request needs to be generated upon instruction from the higher layers or generated automatically every time that new packets arrive and when no semi-permanent channel resource is available to transmit them.

5.2 ARQ Protocol The proposed CMS requires the data link layer to perform relaxed reliable transmission (RRT) in order to improve the poor performance of packet transfer delay and delay variations. We have therefore modified the SR ARQ protocol to include this mechanism. The modified protocol, called the ‘reliability-dependent selective repeat (RDSR) ARQ protocol’, can partially discard packets that are not delivered to the receiving end within a required maximum CTD time.

Table 1: Connection mapping scheme

CBR Rt-VBR Nrt-VBR ABR UBR

Connection reliability reliable or relaxed reliable reliable or relaxed reliable reliable reliable relaxed reliable

Connection mode permanent mixed mixed semi-permanent semi-permanent

Resource allocation Be, &i+Bta Bra+Bta Bra BtQ

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... ...

assuming that

ET 2 20, (10) where Tis the transmission interval between two transmis- sion timeslots. Then tintmi can be estimated as

t i n t m i = EaugRT (11) where caUg is the average packet error rate in the fading channel. Provided that the required CLR can be main- tained, RRT can be implemented for some real-time services that tolerate certain cell loss in order to meet the required CTD by discarding cells.

6 Performance assessment

USC ..' TCH TCH TCH ... TCH ... ...

... ...

TDM downlink

Fig. 3 Typical reservation MAC protocol

DSC ... ... CSC TCH ... TCH _ ._ . _ _

Table 2: Connection mode and resource allocation record

A

1

G i n t e w a , A

Connection identifier

T R T ~ = n*RT - Ds tn-I, s -

Connection Semi-permanent resource request mode generating method

I A A -

Permanently allocated channel details

0 : I I E , ' t= 0

Semi-permanently allocated channel details

t

t

The differences in protocol between RRT and conven- tional reliable transmission (CRT) of ARQ lie in the operation of the packet receiving process. The timeout value of DT varies from packet to packet. It is derived based on parameters such as maximum cell transfer delay (MaxCTD), retransmission timeout (RT), window sue W and single propagation delay 0,. Let tintma/ denote the time interval between times when packet (n-1) and packet (n) are transmitted for the first time, n denote the number of retransmissions of packet (n- 1) before it is correctly received, t,-l,s denote the storage time of packet (n-1) at the receiver after it is correctly received, and tnow denote the instant in time when the packet (n-1) is to be delivered to the network layer and the discarding timer for packet (n) to be set up. Then the transmission events of two consecutive packets can easily be understood from Fig. 4. Clearly, the DT timeout value, DT, for packet n will be

(9)

In the above equation, all the parameters except t,ntema/ are known to the receiver. The tinteruai can be estimated

DT, = Marc723 + tint-, - n * RT - D, - tn-1,,

h o w DTn - \

A fading channel model due to Zorzi [I I] was used in the simulation model and the operation of a modified finite buffer ARQ protocol investigated. The transmission performance using both reliable and relaxed reliable transmission mode was evaluated and compared to verify that relaxed reliable transmission can provide satisfactory CLR performance for individual users in the presence of channel fading conditions.

The round-trip propagation delay considered was 500 ms, representative of a GEO satellite. The considered voice traffic generated packets at 32 kbit/s and since voice packets are required to be delivered in real time, the maximum transmission delay, MaxCTD, was taken to be 1450ms in relaxed transmission mode. Partial voice packets that have not been transmitted within the required MaxCTD were discarded. MaxCTD is regarded to be infinite in a reliable transmission. The retransmission timeout at the data link layer was set to 750ms for a GEO satellite system.

Figures %3 present the CLR and CTD performance for voice traffic transmitted with an assigned Beflin a flat fading channel using reliable and relaxed transmission modes.

The results in Fig. 5 show that the cell ratio discarded in relaxed transmission mode has a polynomial relationship with the fading margin. When the fading margin is less than 16 dB, the number of discarded cells is large and sensitive to the fading margin. However, these results demonstrate that relaxed reliable transmission can maintain CLR at the required level as for the reliable transmission mode.

From the CTD delay performance shown in Figs. 6 and 7 we observe that the effective bandwidth computed approximately from (2) can provide a guaranteed CTD delay Performance for both transmission modes, whle meeting the required CLR,. The performance degrades when the fading margin is less than 14dB. However, a performance is obtained that is close to the required CTD of 1450ms. In Fig. 8, for CLR, = 0.01, the approximated

i MaxCTD

packet n is transmitted out at sender at this point

Fig. 4 Two consecutive packet transmission events on the time axis

discarding timer for packet n expires

at this point

IEE Proc.-Commun, Vol. 150, No. 4, August 2003 301

0.008 O . O 1 O I \ ff- CLRreq=O.O1

0.002 4 \ 0

In i - a, D a,

c

L

c

E c - - D

c x E 5 E

8 12 16 20 24 28 32 fading margin, dB

Fig. 5 fading margin Buffer size 172.8 kbit/s

CLR of the relaxed reliable transmission mode against

E 0.48 c

\ I --t reliable transmission -8- relaxed reliable transmission a '

20 22 24 26 28 30 32 fading margin, dB

Fig. 6 CTD against fading margin (CLR,,=O.OOOl)

-A- reliable transmission +relaxed reliable transmission

c -

1.4 - E 8

12 16 20 24 28 32 fading margin, dB

Fig. 7 CTD against fading margin (CLR,,= 0.00.5)

Befl cannot meet the required CTD, even for a fading margin of 31 dB. The reason for this is that the approximation of the effective bandwidth given by (2) is optimised to the performance objective of the CLR and not to the maximum CTD.

2.4 -

2.2 -

2.0 -

1.8 -

-A- reliable transmission -e- relaxed reliable transmission

1 . 6 1 I I I I I I I I I

9 11 13 15 17 19 21 23 25 27 29 31 fading margin, dB

Fig. 8 CTD against fding margin (CLR,,=O.Ol)

The poor CTD performance at low fading margins can be improved by increasing the allocated band- width, however, this is limited and may require addition FEC.

It is also observed from Fig. 8 that the relaxed reliable transmission mode can provide a much improved CTD performance over the reliable transmission mode at high PER and can maintain delay performance closer to the required delay level than in the case of the reliable transmission mode.

The effects of an inaccurately predicted channel PER on the performance of the CLR and CTD are shown in Figs. 9 and 10. The approximated B e , according to (2), is 51.72 kbit/s. Figures 9 and 10 show that the performances of CLR and CTD are critically dependent on the predlcted PER value. However, increasing the buffer size at the transmitter can solve this sensitivity problem. It was found that the CTD performance benefits more from increased bandwidth than increased buffer sue, because the CTD performance is mainly decided by the allocated bandwidth, channel fading condition and round-trip delay.

The simulation results demonstrate that the effective bandwidth approximation obtained using (2) can provide a guaranteed CLR for voice transmission over the satellite link given a buffer size larger than 89.6 kbit. It also provides

0.5 1 t reliable transmission -8- relaxed reliable transmission

0.4 I ' -

.O 0.3 - E 1 U) - 2 - E 0.2 - - -

m I . u w I I I I I

-0.006 -0.004 -0.002 0 0.002 0.004 0.006 0.008 0.010

real channel PER-predicted average channel PER (fixed at 0.07)

Fig. 9 real channel PER CLR,, = 5 x lop3; buf€er size 90 kbit/s; PERAv channel = 0.707

CLR against offset of the predicted channel PER from the

302 IEE Proc-Commun.. Vol. 150, No. 4, August 2003

2.4 -

2.1 - i 5 1.8 -

U)

U L c U al

m

4 0.9 - ?

E 0.6 - -G+ relaxed reliable transmission

0’3 0 1 -0.08 -0.04 0 0.04 0.08 0.12 0.16

real channel PER-predicted channel PER (fixed at 0.07)

Fig. 10 real channel PER CLR,, = 5 x

CTD against offset of the predicted channel PER from the

buffer size 90 kbit/s; PERAv channel = 0.707

a CTD performance that is close to the required MaxCTD. When a high PER occurs in the mobile channel, the CTD performance may be further improved by using the relaxed reliable transmission mode. The effective bandwidth approximation given in (2) offers the advantage of close approximation to the accurate effective bandwidth, which can guarantee both CLR and CTD for any channel condition.

7 Conclusions

The advantage of the proposed scheme is that it can provide efficient handling of the transportation of ATM traffic over

satellite links with the flexibility of managing user-required QoS, bandwidth and connection types for different applications. The proposed radio connection management scheme together with the optimised MAC and ARQ provides a framework of interworking protocols for ATM over satehte links. It can also be applied in systems (e.g. IP networks) that involve the integration of terrestrial and mobile satellite protocols.

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