Quality-of-Service-Oriented Media Access Control ... - Semantic Scholar

Proceedings of the 36th Hawaii International Conference on System Sciences - 2003

Quality-of-Service-Oriented Media Access Control for Advanced Mobile Multimedia Satellite Systems Petia Todorova†, Alexander Markhasin‡ 1 FhG Fokus, Kaiserin-August-Alle 31 Berlin, Germany, 2 Siberian State University of Telecommunications 86, Kirov Str., Novosibirsk 630102, Russia •

Abstract Satellite networks offer a number of desirable characteristics including wide area coverage, unique broadcast capabilities, the ability to communicate with hand-held devices, and low cost-per-minute access time, among others. Low Earth Orbit (LEO) satellites are expected to support multimedia traffic and to provide their users with the appropriate Quality of Service (QoS). However, the limited bandwidth of the satellite channel, satellite rotation around the Earth and mobility of endusers makes QoS provisioning a challenging task The main coontribution of this work is to investigate the efficiency of various known MAC protocols in a predefined satellite network environment by using a unified analysis and methodology. Our principle objective is to define a variable bandwidth MAC protocol improving satellite network uplink performances and to develop a novel technology of adaptive long-delay adaptive distributed MAC control of QoS.

1. Introduction Due to various economic and technical constrains, terrestrial mobile networks provide communication services with a limited geographic coverage. Recently, in response to increasing demand for a truly global coverage needed by Personal Communication Services (PCS) a new generation of mobile satellite networks has been proposed [1,2]. This new generation of mobile satellite systems are well suited to provide anytime-anywhere communication services and are expected to play a key role in the broadband integrated services networks (BISDN) of the future, as illustrated in Figure 1. These satellite systems are specifically designed to handle bursty Internet and multimedia traffic and to offer low cost access to end-users equipped with commodity hand-held devices [1]. By design, mobile satellite networks offer a number of advantages over their terrestrial counterparts, including: • A much wider coverage area, • Low cost-per-minute access time, • Significantly faster set-up time, • Inherent broadcast capabilities, enabling to reach millions of end-users simultaneously,

An innate ability to communicate with low-end mobile stations. Most of the early satellite systems were based on geostationary (GEO) units deployed at altitudes of about 36,000km. As a rule, these early systems involved little on-board processing power and only limited communication capabilities, functioning, in bent-pipe mode, essentially forwarding calls from one Earth station to the next. In addition, due to the high altitude at which they were deployed, GEO satellites were unsuitable for handling real-time traffic. Not surprisingly, the satellite systems recently proposed by the International Telecommunication Union (ITU) within the framework of the International Mobile Telecommunications after the year 2000 (IMT-2000) are based on non-GEO satellites, as they are expected to provide, among others, low propagation and transmission delays [1,2]. In this respect, Low Earth Orbiting (LEO) satellite constellations deployed at altitudes ranging from 500km to 2,000km are particularly attractive [1-4]. LEO satellite networks are expected to support realtime interactive multimedia traffic and must be able, therefore, to provide their users with Quality-of-Service (QoS) guarantees including bandwidth, delay, jitter, call dropping probability, to name a few. While providing many advantages, satellite systems also present protocol designers with daunting challenges. Indeed, the main characteristics of satellite systems - limited bandwidth on the uplink channel, non-negligible propagation delays, and power limitations - necessitate a careful design and implementation of appropriate media access control (MAC) techniques for bandwidth assignment, particularly in applications with real-time multimedia content [3,4]. MAC protocols have been developed for different environments and applications, ranging from the wellknown wireline networks, to wireless networks, to mobile satellite networks [1-4]. A survey of MAC protocols for wireless ATM networks is given in [3,5]; a comprehensive survey of MAC protocols for satellite networks is offered in [6,7]. One important point is in order: because of the propagation differences between terrestrial and satellite links, MAC protocols developed for terrestrial networks: fixed or mobile cannot be directly used for satellite networks since the performance differences could be significant. In turn, this implies that the design of efficient MAC protocols for satellite

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networks must take the basic characteristics and limitations of satellite networks into account. Since the performance of the MAC protocol impacts significantly the task of QoS provisioning, the typical way to proceed is for a predefined satellite system environment to choose the appropriate MAC scheme that maximizes the utilization of the uplink channel, while meeting, at the same time, the negotiated QoS obligations. One of the important ongoing research topics in mobile multimedia satellite systems is how to integrate in an efficient manner traffic streams with various QoS requirements, in the face of the stringent bandwidth and power limitations of satellite communication channels [1,4]. Given that, as a rule, the downlink channel operates in broadcast mode allowing user traffic to be naturally multiplexed, most of the current research focuses on designing efficient multi-access protocols for the uplink channel. It is well documented that the traditional access methods including Frequency Division Multiple Access (FDMA), Time Division Multiple Access (TDMA) or combined FDMA/TDMA schemes are not suitable because they are prone to bandwidth wastage for many traffic patterns [1,7].

Figure1. Illustrating an advanced mobile satellite multimedia

2. A taxonomy of satellite MAC protocols MAC protocols constitute one of the key factors that determine the performance of satellite networks. They spell out specific ways in which: • an end-user establishes contact with the satellite network and hence gains access to its resources, and • the end-users compete for network resources – especially bandwidth.

For a comprehensive survey of MAC protocols for satellite networks we refer the reader to [6,7]. Fundamental architectural objectives in the design of MAC protocols for satellite networks are: • high channel throughput, • low transmission delays, • channel and protocol stability • low complexity of implementation and control. In addition to the basic function of controlling the access of several users to the shared satellite uplink, in ATM-based satellite networks, the MAC protocol must provide support for the ATM service classes: • Class-1 (Stringing class): CBR, rt-VBR; • Class-2 (Tolerant class): ABR, nrt-VBR; • Class-3 (Bi-level class): VBR and ABR high-speed data; • Class-4 (Unspecified class): UBR. The MAC protocol taxonomy proposed in [6] is based on the static or dynamic nature of the channel, the centralized or distributed control mechanism for channel assignments, and on the adaptive behavior of the control algorithm. According to this classification MAC protocols fall into one of the following categories: • Fixed assignment protocols, • Demand assignment protocols, • Random access protocols, • Hybrid random access/reservation protocols, and • Adaptive protocols. The fixed assignment and demand assignment protocols are contention-free, using either static allocation (fixed assignment) or dynamic allocation (demand assignment). The last three categories of protocols are contention-oriented, where collisions may take place on either the data channel (random access) or on the signaling channel (hybrid of random access and reservation). The general characteristic of most of the satellite MAC protocols proposed in the literature that they use fixed time length framing and some variant of TDMA, FDMA or CDMA (perhaps grafted on top of TDMA) within the frame. In turn, frames are ruled into equalsized slots. Slot assignment to connections can be performed by a centralized scheduling mechanism, by a distributed assignment protocol or by a combination thereof. Depending on the on-board capabilities assumed, the scheduler is either located on the ground or resident within the satellite. The scheduler is responsible for deciding the exact location, within the various frames, of reserved slots for a particular connection according to the pre-negotiated QoS parameters stipulated by the traffic contract. The problem is that the scheduler has to reserve and allocate resources to the connections before data transmission, based on the information available. The stations get the information on the allocated slots from the

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downlink broadcast channel with a certain delay. We shall refer to this as the scheduler problem. The functions of the satellite MAC protocols are also the functions of control of the MAC-sublayers of satellite broadband core networks and backbones. A very recent trend of the core networks and backbones architecture is to turn to IP/ATM integration. A very important problem is the creation of universal MAC protocols for allIP/ATM satellite broadband core, backbone, and access networks. A similar trend is that of carrying TCP over satellite networks. Since standard TCP cannot tell the difference between network congestion and network disconnection, recently it was suggested that the MAC layer plays a very important disambiguating role: it may signal disconnection's back to the source preventing it from taking inappropriate actions.

2.1. MAC protocols for satellite uplink access Because of significant differences in bandwidth availability and propagation delays between terrestrial and satellite links, MAC protocols developed for terrestrial networks cannot be directly used for satellite networks without a severe degradation in performance. In turn, this implies that the design of efficient MAC protocols for satellite networks must take into account the basic characteristics and limitations of satellite communications. Given that in satellite communications the downlink channel operates in broadcast mode allowing user traffic to be naturally multiplexed, most of the design of efficient MAC protocols for satellite networks focuses on the uplink channel. There have been a number of systems proposed for satellite uplink media access. Early in the development of communication networks, ALOHA was studied for both packet radio and satellite communications [1,6,7]. It is folklore that the traditional FDMA and TDMA schemes are prone to bandwidth wastage for many traffic patterns [1,7]. As a result, two approaches for the uplink MAC proposed in the literature are Multi-Frequency TDMA (MF-TDMA) a hybrid of FDMA and TDMA schemes and the code division multiple access (CDMA) scheme. In the case of MF-TDMA a bandwidth reservation scheme is required for all service categories except UBR. In this case, statistical multiplexing is provided by the uplink scheduler implemented as part of the MAC protocol in the satellite. Unlike MF-TDMA, CDMA schemes operate without the need of pre-allocated resources. Thus, statistical multiplexing can be implemented directly in the air interface. Power control management functions could be used for capacity allocation. In the Fixed Bandwidth Assignment (FBA) schemes [7,8], a ground terminal is allocated a fixed number of slots per frame for the duration of the connection. The

technique has the advantage of simplicity, but lacks flexibility and reconfigurability. The main disadvantage of this solution is that during idle periods, slots go unused. Thus, fixed allocation of channel bandwidth leads to inefficient use of transponder capacity. This is, in part, responsible for the well-known inefficiencies of both FDMA and TDMA. Because of the constant assignment of capacity, such techniques are clearly not suitable for VBR applications. In the Demand Assignment Multiple Access (DAMA) schemes, the capacity of the uplink channel is dynamically allocated on demand in response to requests issues by stations based on their queue occupancies. Thus, in principle, the time-varying bandwidth requirements of individual stations can be accommodated and no bandwidth will be wasted. Dynamic allocation using reservation (implicit or explicit) increases transmission throughput. Real-time VBR applications can use this scheme. DAMA does not presuppose any particular physical-layer transmission format and can be implemented using TDMA, FDMA, or CDMA. Typically, DAMA protocols consist of three phases: • A first phase dedicated to specifying bandwidth requests by the connections, • An arbitration phase performed by the satellite, and • A data transmission phase. The main disadvantage of this solution is the long delay experienced by the data waiting in the queues from time the stations send bandwidth requests to the satellites until the time the data is received by the destination. These protocols also have limited dynamic characteristics by QoS control and adaptation to bursty Internet and multimedia traffic. In Random Access (RA) schemes [6,9] all the slots of the frame are available to all the users. Each station can send its traffic over the randomly selected slot(s) without making any request. There is no attempt to coordinate the ready stations to avoid collision. In case of collision, data will be corrupted and will have to be retransmitted. However, the RA technique is simple to implement and adaptive to varying demand, but very inefficient in bandwidth utilization. The resulting delays are in most cases unacceptable for real-time traffic. Hybrid MAC schemes are specifically designed to borrow the good features of different techniques to improve QoS. Hybrid MAC protocols derive their efficiency from the fact that reservation periods are shorter than transmission periods. Some examples of Hybrid MAC schemes are: • Combined Random/Reservation schemes [10] combine DAMA with Random Access. The main advantage is the very good bandwidth utilization at high loads and low delay. The disadvantage of this technique is the necessity to monitor the Random Access part of the channel for possible collisions, leading to large processing time.

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2.3 MAC protocols "matrix" classification In this Section the idea of MAC “matrix” classification offered in [19] will be extended. This classification table is suitable well to the QoS-oriented long-delay MAC requirements. The MAC “matrix” classification is based on two fundamentally characteristics of the MAC protocol’s nature: on the media access regulation mechanisms – the strings of “matrix”, and on the media access time process – the columns of “matrix”. According to this classification the (5×4)-“matrix” of MAC protocols, which elements submit to the certain legitimacy's, can be defined [19]. Via “matrix” strings five media access regulation mechanisms can be defined as follows: • Centralized controlled access mechanism (CCA), • Distributed controlled access mechanism (DCA), • Carrier insensitive free access mechanism (IFA),

• •

Carrier sensitive free access mechanism (SFA), Hybrid access mechanism (HYA), i.e. combined free/controlled access mechanisms for signaling/data packets. Via “matrix” columns four media access media access time process can be defined as follows: • Stochastic continuous time process (CTP), • Stochastic define frame discrete time process (DTP), • Stochastic adaptive frame discrete time process (ATP), • Fixed (deterministic) discrete time process (FTP). The additional classification was entered above. According to MAC instructions processing classification was defined: • Serial MAC instructions processing (SIP), • Parallel MAC instructions processing (PIP), • Parallel-Conveyer MAC instructions processing (CIP), • Not-used MAC instructions dynamically processing (NIP). As an example for the NIP subclass one can show the completely free access methods used in ALOHA. Time process Stochastic access process Determiaccess to nistic medium Continu- Discrete process (Fixed) Access regulation ous Definite Adaptive access mechanism process frame frame process Centralized Non-slotted DAMA, Slotted S-TDMA, controlled Polling Reservation, Polling PAMA SPOTNET access Non-slotted RS-Token Superframe Distributed Token- Reservation Broadcast JTIDS controlled (SFR) Reservation, passing access MLMA Slotted Pure Multi-Slotted Carrier ALOHA insensitive ALOHA ALOHA Controlled access

In the Combined Fixed/Demand Assignment schemes [11] a fixed amount of uplink bandwidth is always guaranteed to the stations. The remaining bandwidth is assigned by DAMA. The efficiency of the technique depends on the amount of fixed bandwidth allocated to the stations. • In the Combined Free/Demand Assignment (CFDAMA) scheme, the reserved slots are assigned to the stations by DAMA based on their demands. The unused slots are distributed in a round-robin manner based on weighting algorithms [12,13]. • In the Adaptive Protocol the number of the contending stations is controlled to reduce the probability of collisions; moreover, the channel switches between random access and reservation mode depending on the traffic load. Adaptive protocols are an important area of research with many possible solutions having different advantages and disadvantages [14-17]. Somewhat surprisingly, in spite of the good deal of highly motivated effort, the design of efficient MAC protocols for satellite networks is still an open area [6,7,18]. As the above discussion suggests some of the existing MAC protocols for satellite communications are outright inefficient, while some others gain efficiency at the cost high computational overhead implementation complexity. One of the goals of the paper is to take a fresh look at MAC protocols for satellite networks. On the one hand we propose to investigate in a unified way the existing protocols and to evaluate them in terms of their sensitivity to various traffic and QoS parameters. Our second stated goal is to use the lessons gleaned by the sensitivity analysis of existing satellite MAC protocols to various parameters, to design novel, highly efficient MAC protocols.

Free (Random) access

•

Carrier sensitive

Hybrid access (controlled/free)

Slotted Non-slotted CSMA CSMA (v0 =1) ALOHA/ Polling

GPRS

Slotted CSMA, Ethernet Reserved ALOHA

Table 1: MAC protocol classification “matrix” with examples The comparing of the MAC protocols about QoSorienting abilities is shown in Table 2. The criterion of the QoS-orienting abilities of MAC protocols was assumed: • for statical QoS control – as its characteristics of the controllability, differentiation, and guarantee by the QoS control and bandwidth resources assignment; • for dynamical QoS control – as, adding to statical criteria, characteristics of the dynamical efficiency, stability, and hard-working by the QoS dynamical (“on-the-fly”) control and bandwidth resources assignment.

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Free (Random) Controlled access access

Time process Stochastic access process Determiaccess to nistic medium Continu- Discrete process (Fixed) Access regulation ous Adaptiveaccess mechanism process Definite frame frame process Centralized controlled High QoS controllability access Distributed High QoS guarantee controlled access Carrier insensitive LowQoS controllability Carrier sensitive

Hybrid access (controlled/free)

Non-guarantee QoS

Middle QoS controllability Low QoS guarantee

b) Dynamical barrier overcoming (Dynamical efficiency, stability, and hardworking of the DynQoS ) Time process Stochastic access process Determiaccess to nistic medium Continu- Discrete process (Fixed) Access regulation ous Adaptiveaccess mechanism process Definite frame frame process Centralized Bad Medium Good Bad controlled access Dyn. Dyn. Dyn. Dyn. Distributed QoS QoS QoS QoS controlled access Carrier insensitive Free (Random) Controlled access access

a) Statical controllability, differentiation, and guarantee QoS

Carrier sensitive

Does not ensure dynamical barrier overcoming

Hybrid access (controlled/free)

Smaller Middl- Larger Smaller ing Table 2: Comparing MAC protocols with QoS oriented abilities

3. A variable bandwidth MAC protocol for satellite uplink The main goal of this subsection is to discuss the details of a MAC protocol for the graceful integration of multimedia traffic on the satellite uplink channel. Our protocol is based on a generic proposal being specifically designed to be capable of embedding within its structure a number of protocol standards [20]. The exact structure of the uplink frame in our case is developed by the satellite controller and transmitted to each station sharing the frequency band through the downlink. Obviously, the round-trip delay will influence the time response to call data-rate requests, that is, the requests for a station’s resources providing in the current downlink frame corresponds to was requested in the previous uplink frame. The frame is subdivided into subframes which are further subdivided into slots as shown in Figure 2. To enable maximum channel efficiency, stations presently assigned to the channel are enabled to do control signaling and data transfer using the SA subframe. Each station has a beginning control slot followed by a series of data slots. Calls have been assigned slots through requests honored by the satellite as a result of previous frame requests incorporated in the first station’s control field. Each station is assigned a continuous series of slots in order to reduce the need for guard slots to a minimum.

After each station with active calls, a short subframe provides control slots. These slots are used by the stations that wish to communicate with the satellite but do not have any active calls. An example may be to establishment of a call reservation. Finally, a series of data slots are provided. Both control and data slots use random access so that contention can occur in either case. The slot characteristics depend upon the network protocol supported through the satellite. For example for ATM, data slot bit size would be nominally 53 bytes although additional bytes may be added to both up and down links in order to accommodate additional information needed for satellite control. Stations indicate their need for SA slots (or bytes) for their ongoing call(s) in the control slots or other information as, for example, call termination. We provide separate RA control slots to handle call set-up packets because we need to be able to make setting up a call easy and highly probable. Stations that want to establish a call will contend for a control slot. Random access control request slot size is designed so that data can be provided for communication with the satellite call control system. For example in call setup, the call’s characteristics would be supplied, including mean, maximum and minimum data rates, call priority, billing information and possibly the expected duration of the call. If some data needed for control is too large for a RA control slot, it could use a data slot or multiple control slots. The remaining data slots are open to contention by all stations wishing to send

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data packets on a best effort basis. Random access data slots size depends, as with

refer ence slot

SA subframe

station

SA RA subfram e bound r station

RA subframe

RA

RA

frame bound ary

frame bound ary

Station control block

Guard

refer ence slot

station #1

handoff

T

Guard

station #1

station #1

call requ ests

Figure 2: Frame structure showing slot assignment and utilization calls, upon the underlying characteristics of th network being supported. Since collisions are bounded to occure, RA subframe slot size may be limited so that effects of collision do not wasted in ordinate amount of capacity. For the case of the ATM, RA data slots would be a nominal 53 cells. Our MAC up-link is designed to support a wide variety of low-level ground communication protocols. For example, it readily supports the QoS found in ATM. In ATM various traffic types such as CBR, rt-VBR, nrtVBR, ABR and UBR are defined. Here, QoS is provided by resource allocation and sharing as well a preference to higher priority traffic when bandwidth resources are limited. For service with varying rates, the reservation system can indicate when data rate requirements change so the capacity can be reassigned. When resources are limited the satellite can assign resources amongst the stations so that higher priority traffic can be served at the expense of delay or deletion of lower priority traffic.

4. Long-delay adaptive distributed medium access control protocol (LAD-MAC) In case of distributed ATM Hyperbus architecture [21] a long - delay distributed MAC protocol is proposed offering a number of benefits to the users.

4.1 The distributed ATM hyperbus architecture The distributed ATM Hyperbus architecture builds a virtual bus between all the users being in sight of a given satellite. The satellite is merely a re-translator, so that users in different geographical locations can share the same virtual bus. Resources on this bus are controlled via tokens that are administered by a central per-bus entity (Dynamic Control Server – DCS). Tokens are allocate on basis of pseudo-random numbers. Each terminal receives a set of tokens dependent on the resource requirement (see Figure 3). This allocation is dynamic in a very short time frame, which allows easy adaptation of the terminal bandwidth to the actual requirements The DCS is used for: re-configuration, dynamic control of bandwidth, control of common universal time and QoS translation from IP to the Hyperbus for IP integration. The terminals itself are multifunctional in the sense, that they can implement one or more of the following functionality: • Plain broadband multimedia terminal, • LAN emulation, • WAN/MAN connection, • Distributed switching, • Distributed concentration, • Distributed routing. Owing to such features networks of different scales (LAN, MAN, WAN, GM PCS et. al.) and classes different according to technical characteristics, functions, and kinds of medium can be configured by from small numbers (till 3) of logically homogeneous components: 1) Universal Multifunctional ATM Network Adaptive Medium Access Controller (UMAC), 2) Universal Adaptive Medium Interface with Medium Type Cartridge (UAMI), 3) Universal Server of ATM Network Distributed Dynamic Control (DSC). The proposed ATM Hyperbus architecture provides a wide range of services with different traffic streams and different QoS characteristics.

4.2 Our novel MAC protocol The proposed protocol [22] is based on the development of the potential of recurrent quasi-random sequences (RS) in order to organize an effective distributed multi-access control to long-delay spacemedium. This technology provides a low-cost method for: • addressing of adaptive ATM TDMA time slots (one address bit aJ per one ATM cell slot, j = 0, 1,..., M, M is the RS period, M = 2n-1, n = 32÷64 , see [22], • synchronization and synphasing of adaptive ATM TDMA RS-periodical hyperframe, • RS-addressing of MAC commands,

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• • • • •

RS-addressing of ATM cells and ATM terminals, bandwidth distribution, Internal Unified System Time (IUST) maintenance, adaptive time division of channels (time slots), priority queuing. A combination of mechanisms of authority (RS-token) passing for MAC commands and broadcast reservation for information cells are used. The following conditions are assumed for the operation of the protocol: • Bit propagation delays of 1,000 ÷ 10,000,000 bit • Distance and bit rates in the range of 30,000km/50Mbit/sec to 3,000km/500Mbit/sec to 3000km/5Gbit/sec • Up to 1000 ÷ 5000 terminals • Throughput 0.90 ÷ 0.95.

better QoS guarantees for real-time as well and non-realtime traffic. Our future plans can be defined as follows: • the design of a universal all-IP/ATM-oriented MAC protocol for satellite broadband core, backbone, and access networks, • developing a novel technology for adaptive longdelay MAC and dynamic "on-the-fly" control of QoS, traffic parameters, and bandwidth resources, and • improving satellite network uplink performance by using dynamic bandwidth allocation schemes.

Acknowledgments The work was support in part, ny NATO grand PST.CLG979033.

References DistributedATM Hyperbus

UAMI

UAMI

UMAC

UAMI

UAMI

UMAC

UMAC

BroadbandMultimediaTerminals

UMAC DCS

External backbone

Figure 3: Distributed ATM Hyperbus Architecture

5. Conclusion and future work The first major objective of the work was to investigate the efficiency of various known MAC protocols in a predefined satellite network environment by using a unified analysis. The second major focus was to design of novel QoS-oriented MAC protocols for advanced multimedia mobile satellite networks offering

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