m=audio 0 RTP/AVP 98 a=control:trackID=5 a=3GPP-QoE-Metrics:{Corruption_Duration};rate=20 The session level QoE field indicates that the initial buffering and the rebuffering duration should be monitored and reported once at the end of the session. The video specific metrics (decoded bytes) will be reported every 15 seconds until 40 seconds of NPT time. Finally, audio specific metrics (corrup- tion duration) will be reported every 20 seconds during all the session. A QoE aware client that receives a SDP description with QoE metrics fields may continue the negotiation with a SETUP request that includes a 3GPP-QoE-Metrics. This header allows the client to propose QoE metrics modifications. The value of the header can contain session and media metrics separated with the session-level and media-level URLs: 3GPP-QoE-Metrics: url=‘‘rtsp://rtsp.um.es/movie.3gp/trackID=3’’; metrics={Decoded_Bytes};rate=10;range:npt=0-40, url=‘‘rtsp://rtps.um.es/movie.3gp’’; metrics={Rebuffering_Duration};rate=End The server can accept the modifications of the client, echoing them in the SETUP response, or the server can deny modifications, continuing the re-negotiation until the PLAY request. QoE metrics reports can be disabled by the server using SET_PARAMETER requests with the 3GPP-QoE-Metrics header containing ‘Off’. To send QoE metrics feedback, the client will issue SET_PARAMETER requests with the 3GPP-QoE-Feedback header: 3GPP-QoE-Feedback: url=‘‘rtsp://rtsp.um.es/movie.3gp/trackID=5’’; Corruption_Duration={200 1300} 4.8 Research Challenges and Opportunities As the user might have noticed, the work in control protocols for multimedia communications is far from complete. In particular, during the last few years with the advent of wireless and mobile networks, a lot of new modifications and enhancements are being engineered to fulfill the wide range of new requirements that these networks are bringing up. In addition, future 4G wireless networks consisting of IP core networks to which different wireless access technologies will be interconnected are posing even stronger requirements, given the hetero- geneous nature of those networks. For instance, it is expected that the terminal capabilities might be completely different among devices. New concepts like session roaming, session transfers, service dis- covery and many others require new control-plane functions, which in the majority of the cases are not fully available. As a terminal roams across different access technologies in 4G networks its network connectivity might vary strongly. This kind of heterogeneous and variable scenarios is posing additional require- ments on multimedia internetworking technologies. For instance, applications should be able to adapt their operating settings to the changes in the underlying network. All these changing events, which are not currently notified by existing control protocols, will need to be considered by (in many cases) extending existing control protocols or even designing new ones. Moreover, location-aware and user-aware services are expected to be delivered in those networks. This means that signaling and control protocols will require extensions in order to be able to convey contextual information to multimedia applications. The paradigm shifts from the concept of establishing 118 Multimedia Control Protocols for Wireless Networks a session to the concept of establishing a session automatically configuring the session parameters according to the user’s preferences, location, contextual information, network capabilities, etc. These new requirements are opening up a number of research opportunities and areas which include among others:  context-aware and personalized applications and services;  adaptive applications and services that can self-configure;  middleware architectures for context-aware applications;  enhanced highly-descriptive capability negotiation mechanisms;  semantic technologies to abstract and model contextual information. In conclusion, we have explained how existing multimedia control protocols work, and why IETF proposals have been considered as the ‘de facto’ standard for existing and future wireless and mobile networks. We have given a detailed description of the main protocols (SDP, RTSP and SIP). Finally, we have described the multimedia control plane of UMTS (IMS), giving examples that allow the reader to understand the basic principles and operations. For those readers interested in obtaining detailed specifications, a great deal of relevant literature is cited below. Acknowledgment The work of Pedro M. Ruiz was partially funded by the Spanish Ministry of Science and Technology by means of the Ramo ´ n and Cajal work programme. References [1] ITU-T Rec. H.320, Narrow-band Visual Telephone Systems and Terminal Equipment, 1990. [2] ITU-T Rec. H.323, Visual Telephone Systems and Terminal Equipment for Local Area Networks which Provide a Non-Guaranteed Quality of Service, November, 1996. [3] ITU-T Rec. T.120, Data Protocols for Multimedia Conferencing, July 1996. [4] H. Schulzrinne, S. Casner, R. Frederick and V. Jackobson, IETF Request For Comments, RFC-3550: RTP: A Transport Protocol for Real-Time Applications, July 2003. [5] ITU-T Rec. H.235, Security and encryption of H-Series (H.323 and other H.245-based) multimedia terminals, February, 1998. [6] ITU-T Rec. H.225.0, Call Signaling Protocols and Media Stream Packetization for Packet-based Multimedia Communication Systems, February, 1998. [7] ITU-T Rec. H.245, Control Protocol for Multimedia Communication, September, 1998. [8] M. R. Macedonia and D. P. Brutzman, MBone provides audio and video across the Internet, IEEE Computer, 27(4), 30–36, April 1994. [9] M. Handley and V. Jacobson, IETF Request For Comments, RFC-2327: SDP: Session Description Protocol, April, 1998. [10] N. Freed and N. Borenstein, IETF Request For Comments, RFC-2045: Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies, November, 1996. [11] M. Handley, C. Perkins, E. Whelan, IETF Request For Comments, RFC-2974: Session Announcement Protocol, October, 2000. [12] J. Rosenberg, H. Schulzrinne, G. Camarillo, A. Hohnston, J. Peterson, R. Sparks, M. Handley and E. Schooler, IETF Request For Comments RFC-3261, SIP Session Initiation Protocol, June, 2002. [13] H. Schulzrinne, A. Rao, R. Kanphier, M. Westerlund and A. Narasimhan, IETF Request for Comments RFC- 2326: Real Time Streaming Protocol (RTSP), April, 1998. [14] D. Mills, IETF Request for Comments RFC-1305, Network Time Protocol (version 3) specification and imple- mentation, March 1992. References 119 [15] C. Huitema, Request For Comments RFC-3605, Real Time Control Protocol (RTCP) attribute in Session Description Protocol (SDP), October, 2003. [16] S. Olson, G. Camarillo and A. B. Roach, IETF Request For Comments RFC-3266: Spport for IPv6 in Session Description Protocol (SDP), June 2002. [17] G. Camarillo, G. Eriksson, J. Holler, H. Schulzrine, IETF Request For Comments RFC-3388: Grouping of Media Lines in the Session Description Protocol (SDP), December, 2002. [18] IETF Multiparty MUltimedia SessIon Control (MMUSIC) Working Group. http://www. ietf.org/html.charters/ mmusic-charter.html. [19] IETF RFC 2616, Hypertext Transfer Protocol, R. Fielding, J. Gettys, J. Mogul, H. Frystyk, L. Masinter, P. Leach and T. Berners-Lee, June 1999. [20] IETF RFC 3016, RTP Payload Format for MPEG-4 Audio/Visual Streams, Y. Kikuchi, T. Nomura, S. Fukunaga, Y. Matsui and H. Kimata. November 2000. [21] IETF RFC 3640, RTP Payload Format for Transport of MPEG-4 Elementary Streams, J. van der Meer, D. Mackie, V. Swaminathan, D. Singer and P. Gentric, November 2003. [22] Sue B. Moon, Jim Kurose and Don Towsley, Packet audio playout delay adjustment: performance bounds and algorithms. In Multimedia Systems, 6, 1998, 17–28, Springer-Verlag. [23] J. Rosenberg and H. Schulzrinne, IETF RFC 3263, Session Initiation Protocol (SIP): Locating SIP Servers, June 2002. [24] J. Rosenberg and H. Schulzrinne, IETF RFC 3264, An Offer/Answer Model with the Session Description Protocol (SDP), June 2002. [25] Pedro M. Ruiz, Antonio F. Go ´ mez-Skarmeta, Pedro Martı ´ nez, Juan A. Sa ´ nchez and Emilio Garcı ´ a, Effective multimedia and multi-party communications on multicast MANET extensions to IP access networks, In Proc. 16th IEEE International Conference on Information Networking ICOIN-2003, Jeju Island, Korea, pp 870–879, February 2003. [26] E. Wedlund and H. Schulzrinne, Mobility support using SIP, In Proc. of 2nd ACM International Workshop on Wireless Mobile Multimedia, Seattle, WA, August 1999. [27] 3GPP TS 23.002, Network Architecture, v6.2.0, September 2003. [28] 3GPP TS 23.228, IP Multimedia Subsystem (IMS) v6.5.0, March 2004. [29] P. Calhoun, L. Loughney, M. E. Guttman, G. Zorn and V. Jacobsen, Diameter Base Protocol, IETF-RFC 3588, September 2003. [30] D. Durham, J. Boyle, R. Cohen, S. Herzog, R. Rajon and A. Sastry, The COPS (Common Open Policy Service) Protocol, IETF-RFC 2748, January 2000. [31] J. Loughney, Diameter Command Codes for Thrid Generation Partnership Project (3GPP) Release 5, IETF-RFC 3589, September 2003. [32] ITU-T, Technical Recommendation H.248.1, Media Gateway Control Protocol, May 2002. [33] 3GPP TS 24.228, IP Multimedia Subsystem (IMS) Stage 3. [34] 3GPP TR26.234, Technical Specification Group Services and Aspects; Transparent end-to-end PSS; Protocols and codecs (Rel 6.1.0, 09-2004). [35] IETF RFC 3556, Session Description Protocol (SDP) Bandwidth Modifiers for RTP Control Protocol (RTCP) Bandwidth, S. Casner, July 2003. [36] IETF Internet Draft draft-ietf-avt-rtcp-feedback-11.txt, Extended RTP Profile for RTCP-based Feedback (RTP/ AVPF), Joerg Ott, Stephan Wenger, Noriyuki Sato, Carsten Burmeister, Jose ´ Rey, expires February 2005. 120 Multimedia Control Protocols for Wireless Networks 5 Multimedia Wireless Local Area Networks Sai Shankar N 5.1 Introduction Wireless networking has made a significant impact with the invention of wide area cellular networks based on different standards, e.g. the Global System for Mobile communications (GSM), Advanced Mobile Phone System (AMPS), etc. They have been defined with the main purpose of supporting voice, though some also offer data communication services at very low speed (10 kbit/s). With the invention of the WLAN, data communication services in a residential and office environments have undergone significant changes. WLAN products based on the different flavors of 802.11 are available from a range of vendors. Depending on the transmission scheme, products may offer bandwidths ranging from about 1 Mbit/s up to 54 Mbit/s. There is a significant interest in transmission of multimedia over wireless networks. These range from the low rate video transmissions for the mobile phones to the high rate Audio/Video (AV) streaming from an Digital Video Disk (DVD) player to a flat panel television inside the home. Typically, supporting the AVapplications over the networks requires Quality of Service (QoS) supports such as bounded packet delivery latency and guaranteed throughput. While the QoS support in any network can be a challenging task, supporting QoS in wireless networks is even more challenging due to the limited bandwidth compared with the wired counterpart and error-prone wireless channel conditions. Thanks to the emerging broadband WLAN technologies, it is becoming possible to support the QoS in the indoor environment. In this chapter, we introduce and review two distinct broadband WLAN standards, namely, IEEE 802.11e and ETSI BRAN HiperLAN/2, especially, in the context of QoS support for the AV applications. 5.1.1 ETSI’s HiperLAN HiperLAN/1 is a standard for a WLAN defined by the ETSI [1]. HiperLAN supports the ad hoc topology along with the multihop routing capability to forward packets from a source to a destination that cannot communicate directly [1]. The HiperLAN/1 MAC protocol explicitly supports a quality of service (QoS) for packet delivery that is provided via two mechanisms: the user priority and the frame lifetime. Emerging Wireless Multimedia: Services and Technologies Edited by A. Salkintzis and N. Passas # 2005 John Wiley & Sons, Ltd HiperLAN/2 is a European 5 GHz WLAN standard developed within ETSI BRAN. The standardiza- tion effort started in Spring 1997, and addressed specifications on both the physical (PHY) layer, the data link control (DLC) layer and different convergence layers as interfaces to various higher layers including the Ethernet, IEEE 1394, and Asynchronous Transfer Mode (ATM). HiperLAN/2 was designed to give wireless access to the Internet and future multimedia at speeds of up to 54 Mbit/s for residential and corporate users. The Mobile Terminals (MT) communicate with the Access Points (AP) as well as directly with each other to transfer information. An MT communicates with only one AP to which it is associated. The APs ensure that the radio network is automatically configured by using dynamic frequency selection, thus removing the need for manual frequency planning. HiperLAN/2 has a very high transmission rate of 54 Mbit/s. It uses Orthogonal Frequency Digital Multiplexing (OFDM) to transmit the analog signals. OFDM is very efficient in time-dispersive environments, where the transmitted radio signals are reflected from many points, leading to different propagation times before they eventually reach the receiver. Above the physical layer, the Medium Access Control (MAC) layer implements a dynamic time-division duplex (TDD) for most efficient utilization of radio resources. The following are the essential features of HiperLAN/2.  Connection-oriented. In a HiperLAN/2 network, data transmission is connection oriented. In order to accomplish this, the MT and the AP must establish a connection prior to the transmission using signalling functions of the HiperLAN/2 control plane. Connections are Time Division Multiplexed (TDM) over the air interface. There are two types of connections: point-to-point and point- to-multipoint. Point-to-point connections are bidirectional whereas point-to-multipoint is unidirectional and is in the direction towards the MT. In addition, there is also a dedicated broadcast channel through which traffic reaches all terminals transmitted from AP.  Quality-of-Service (QoS) support. The connection-oriented nature of HiperLAN/2 makes it straight- forward to implement support for QoS. Each connection can be assigned a specific QoS, for instance in terms of bandwidth, delay, jitter, bit error rate, etc. It is also possible to use a more simplistic approach, where each connection can be assigned a priority level relative to other connections. This QoS support in combination with the high transmission rate facilitates the simultaneous transmission of many different types of data streams.  Dynamic Frequency Selection. In a HiperLAN/2 network, there is no need for manual frequency planning as in cellular networks like GSM. The APs in the HiperLAN/2, have a built-in support for automatically selecting an appropriate radio channel for transmission within each AP’s coverage area. An AP scans all the channels to determine if there are neighboring APs and chooses an appropriate channel that minimizes interference.  Security support. The HiperLAN/2 network has support for both authentication and encryption. With authentication, both the AP and the MT can authenticate each other to ensure authorized access to the network. Authentication relies on a supporting function, such as a directory service, that is not in the scope of HiperLAN/2. The user traffic is encrypted on established connections to prevent eaves-dropping.  Mobility support. The MT tries to associate with the AP that has the best radio signal. When the MT moves, it may detect that there is an alternative AP with better radio transmission performance than the associated AP. The MT then initiates a hand over to this AP. All established connections from this MT will be moved to this new AP. During handover, some packet loss may occur. If an MT moves out of radio coverage for a certain time, the MT may loose its association to the HiperLAN/2 network resulting in the release of all connections.  Last Mile Access. The HiperLAN/2 protocol stack has a flexible architecture for easy adaptation and integration with a variety of fixed networks. A HiperLAN/2 network can, for instance, be used as the last hop wireless segment of a switched Ethernet, but it may also be used in other configurations, e.g. as an access network to third generation cellular networks. 122 Multimedia Wireless Local Area Networks  Power save. In HiperLAN/2, the MT may request the AP for entering into sleep mode. The MT sends a request to the AP about its intention to enter a low power state for a specific period. At the expiration of the negotiated sleep period, the MT searches for the presence of any wake up indication from the AP. In the absence of the wake up indication the MT reverts back to its low power state for the next sleep period. An AP will defer any pending data to the MT until the corresponding sleep period expires. Different sleep periods are supported to allow for either short latency requirement or low power requirement. 5.1.2 IEEE 802.11 In recent years, IEEE 802.11 WLAN [5] has emerged as a prevailing technology for the indoor broadband wireless access for the mobile/portable devices. Today, IEEE 802.11 can be considered as a wireless version of Ethernet by virtue of supporting a best-effort service (not guaranteeing any service level to users/applications). IEEE 802.11b is an extension to the original 802.11 to support up to 11 Mbps at 2.4 GHz, is the most popular WLAN technology in the market. The other extensions, called IEEE 802.11a and IEEE 802.11g, support up to 54 Mbps at 5 GHz and 2.4 Ghz respectively. IEEE 802.11 today is known as the wireless Ethernet, and is becoming very popular to replace and/or complement the popular Ethernet in many environments including corporate, public, and home. The mandatory part of the original 802.11-99 MAC is called the Distributed Coordination Function (DCF), which is based on Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA). However, as with Ethernet, the current 802.11 is not suitable to support QoS. Since early 2000, the IEEE 802.11 Working Group (WG) has been working on another extension to enhance the MAC to support QoS; the extension is to be called IEEE 802.11e. The overview of 802.11e in this paper is based on the draft specification [7]. The new standard is scheduled to be finalized by the end of 2004. The new MAC protocol of the upcoming 802.11e is called the Hybrid Coordination Function (HCF). The HCF is called ‘hybrid’ as it combines a contention channel access mechanism, referred to as Enhanced Distributed Channel Access (EDCA), and a polling-based channel access mechanism, referred to as HCF Controlled Channel Access (HCCA), each of which operates simultaneously and continuously within the Basic Service Set (BSS) a . This is different from the legacy 802.11-1999 standard [5], which specifies two coordination functions, one mandatory, the Distributed Coordination Function (DCF) and one optional, the Point Coordination Function (PCF). These two operate disjointedly during alternating subsets of the beacon interval. All of today’s products in the market only implement the mandatory DCF, which is based on Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA). The two access mechanisms of HCF provide two distinct levels of QoS, namely, prioritized QoS and parameterized QoS. EDCA is an enhanced version of the legacy DCF MAC and it is used to provide the prioritized QoS service. With EDCA a single MAC can have multiple queues that work independently, in parallel, for different priorities. Frames with different priorities are transmitted using different CSMA/CA contention parameters. With the EDCA, a station cannot transmit a frame that extends beyond the EDCA Transmission Opportunity (TXOP) limit. A TXOP is defined as a period of time during which the STA can send multiple frames. HCCA is used to provide a parameterized QoS service. With HCCA, there is a negotiation of QoS requirements between a Station (STA) and the Hybrid Coordinator (HC). Once a stream for an STA is established, the HC allocates TXOPs via polling to the STA, in order to guarantee its QoS requirements. The HC enjoys free access to the medium during both the Contention Free Period (CFP) and the Contention Period (CP) b , in order to (1) send polls to allocate TXOPs and (2) send downlink parameterized traffic. HCCA guarantees that the QoS requirements are met once a stream has been a A BSS is composed of an Access Point (AP) and multiple stations (STA) associated with the AP. b During the CP, the HC uses the highest EDCA priority and its access to the medium is guaranteed once it becomes idle. Introduction 123 admitted into the network, while EDCA only provides a QoS priority differentiation via a random distributed access mechanism. 5.2 Overview of Physical Layers of HiperLAN/2 and IEEE 802.11a The transmission format on the physical layer consists of a preamble part and a data part. The channel spacing is 20 MHz, which allows high bit rates per channel. The physical layer for both the IEEE 802.11a and HiperLAN/2 is based on Orthogonal Frequency Division Multiplexing (OFDM). OFDM uses 52 subcarriers per channel, where 48 subcarriers carry actual data and 4 subcarriers are pilots that facilitate phase tracking for coherent demodulation. The duration of the guard interval is equal to 800 ns, which is sufficient to enable good performance on channels with delay spread of up to 250 ns. An optional shorter guard interval of 400 ns may be used in small indoor environments. OFDM is used to combat frequency selective fading and to randomize the burst errors caused by a wide band fading channel. The PHY layer modes with different coding and modulation schemes are shown in Table 5.1. The MAC selects any of the available rates for transmitting its data based on the channel condition. This algorithm is called link adaptation and it is not specified by the standard as to how it should be performed, thus enabling product differentiation between different vendors. Data for transmission is supplied to the PHY layer in the form of an input Protocol Data Unit (PDU) train or Physical Layer Convergence Procedure (PLCP) Protocol Data Unit (PPDU) frame. This is then passed to a scrambler that prevents long runs of 1s and 0s in the input data. Although both 802.11a and HiperLAN/2 scramble the data with a length 127 pseudorandom sequence, the initialization of the scrambler is different. The scrambled data is then passed to a convolutional encoder. The encoder consists of a 1/2 rate mother code and subsequent puncturing. The puncturing schemes facilitate the use of code rates 1/2, 3/4, 9/16 (HiperLAN/2 only), and 2/3 (802.11a only). In the case of 16-Quadrature Amplitude Modulation (QAM), HiperLAN/2 uses rate 9/16 instead of rate 1/2 in order to ensure an integer number of OFDM symbols per PDU train. The rate 2/3 is used only for the case of 64-QAM in 802.11a. Note that there is no equivalent mode for HiperLAN/2. HiperLAN/2 also uses additional puncturing in order to keep an integer number of OFDM symbols with 54-byte PDUs. The coded data is interleaved in order to prevent error bursts from being input to the convolutional decoding process in the receiver. The interleaved data is subsequently mapped to data symbols according to either a Binary Phase Shift Keying (BPSK), Quadrature PSK (QPSK), 16-QAM, or 64-QAM constellation. OFDM modulation is implemented by means of an inverse Fast Fourier Transform (FFT). 48 data symbols and four pilots are transmitted in parallel in the form of one OFDM symbol. Numerical values for the OFDM parameters are given in Table 5.2. In order to prevent Inter Symbol Interference (ISI) and Inter Carrier Table 5.1 Different modulation schemes of IEEE 802.11a and HiperLAN/2 physical layer Mode Coding Bit rate Coded bits/ Coded bits/ Data bits/ scheme Modulation rate, R (Mb/s) subcarrier OFDM Symbol OFDM Symbol 1 BPSK 1/2 6 1 48 24 2 BPSK 3/4 9 1 48 36 3 QPSK 1/2 12 2 96 48 4 QPSK 3/4 18 2 96 72 5 16 QAM (H/2 only) 9/16 27 4 192 108 5 16 QAM (IEEE only) 1/2 24 4 192 96 6 16 QAM 3/4 36 4 192 144 7 64 QAM 3/4 54 6 288 216 8 64 QAM (IEEE only) 2/3 48 6 288 192 124 Multimedia Wireless Local Area Networks Interference (ICI) due to delay spread, a guard interval is implemented by means of a cyclic extension. Thus, each OFDM symbol is preceded by a periodic extension of the symbol itself. The total OFDM symbol duration is T total ¼ T g þ T, where T g represents the guard interval and T the useful OFDM symbol duration. When the guard interval is longer than the excess delay of the radio channel, ISI is eliminated. The OFDM receiver basically performs the reverse operations of the transmitter. However, the receiver is also required to perform Automatic Gain Control (AGC), time and frequency synchronization, and channel estimation. Training sequences are provided in the preamble for the specific purpose of supporting these functions. Two OFDM symbols are provided in the preamble in order to support the channel estimation process. A prior knowledge of the transmitted preamble signal facilitates the generation of a vector defining the channel estimate, commonly referred to as the Channel State Information (CSI). The channel estimation preamble is formed such that the two symbols effectively provide a single guard interval of length 1.6 ms. This format makes it particularly robust to ISI. By averaging over two OFDM symbols, the distorting effects of noise on the channel estimation process can also be reduced. HiperLAN/2 and 802.11a use different training sequences in the preamble. The training symbols used for channel estimation are the same, but the sequences provided for time and frequency synchronization are different. Decoding of the convolutional code is typically implemented by means of a Viterbi decoder. The physical layer modes (PHY modes) are specified in Table 5.1. 5.3 Overview of HiperLAN/1 The PHY layer of HiperLAN/1 uses 200 MHz at 5.15–5.35 GHz. This band is divided into five channels with channel spacing of 40 MHz in the European Union and six channels of 33 MHz spacing in the USA. The transmission power can go up to 1 W. The modulation scheme is single carrier Gaussian Minimum Shift Keying (GMSK) that can support up to 23 Mbps. Decision Feedback Equalizer (DFE) is employed at the receiver because of the high data rate and it consumes more power [3]. 5.3.1 MAC Protocol of HiperLAN/1 The HiperLAN/1 channel access mechanism is based on channel sensing and a contention resolution scheme called Elimination Yield – Non-preemptive Priority Multiple Access (EY-NPMA). In this scheme, channel status is sensed by each node that has a data frame to transmit. If the channel is sensed idle for at least 1700 bit-periods, then the channel is considered free, and the node is allowed to start transmission of the data frame immediately. Each data frame transmission must be explicitly acknowledged by an acknowledgement (ACK) transmission from the destination node. If the channel is sensed busy, a channel access with synchronization has to precede before frame transmission. Synchronization is performed to the end of the current transmission interval according to the EY-NPMA scheme. The channel access cycle consists of three phases: the prioritization phase, the contention phase and the transmission phase. Figure 5.1 shows the channel access using EY-NPMA. The aim of the Table 5.2 Overhead calculation for HiperLAN transmission Channel OFDM symbols BCH þ Preamble BCH 5 þ 4 ¼ 9 FCH min 6 ACH 3 RCH þ Preamble RCH 3 þ 4 ¼ 7 Uplink overhead 4 þ l 9 BpS m Overview of HiperLAN/1 125 prioritization phase is to allow only nodes with the highest channel access priority frame, among the contending ones, to participate in the next phase. In HiperLAN/1 a priority level h is assigned to each frame. Priority level 0 represents the highest priority. The prioritization phase consists of at most H prioritization slots, each 256 bit-periods long. Each node that has a frame with priority level h senses the channel for the first h prioritization slots. If the channel is idle during this interval, then the node transmits a burst in the ðh þ 1Þth slot and it is admitted to the contention phase, otherwise it stops contending and waits for the next channel access cycle. The contention phase starts immediately after the transmission of the prioritization burst, and it further consists of two phases: the elimination phase and the yield phase. The elimination phase consists of at most n elimination slots, each 256 bit-periods long, followed by a 256 bit-periods long elimination survival verification slot. Starting from the first elimination slot, each node transmits a burst for a number B (0 B n) of subsequent elimination slots, according to the truncated geometric probability distribution function: PrðB ¼ bÞ¼ ð1 ÀqÞq b : 0 b < n q n : i ¼ q: & After the end of the burst transmission, each node senses the channel for the duration of the elimination survival verification slot. If the channel is sensed idle, the node is admitted to the yield phase, otherwise it drops itself from contention and waits for the next channel access cycle. The yield phase starts immediately after the end of the elimination survival verification interval and consists of at most m yield slots, each 64 bit-periods long. Each node listens to the channel for a number D (0 D m) of yield slots before beginning transmission. D is an rv with truncated geometric distribution as follows: PrðD ¼ dÞ¼ ð1 ÀpÞp d : 0 d < m p m : d ¼ m: & If the channel is sensed idle during the yield listening interval, the node is allowed to begin the transmission phase, otherwise the node loses contention and waits for the next channel access cycle. The operation parameter settings, according to HiperLAN, are reported in Table 5.1. Elimination and yield phases are complementary to each other. Elimination phase drastically reduces the number of nodes taking part to the channel access cycle, and this result is remarkably achieved almost independently of number of nodes [2]. The yield phase reduces the number of nodes allowed to transmit possibly to one. In EY-NPMA at least one node will always be allowed to transmit. Real time traffic transmission is supported in HiperLAN by dynamically varying the CAM priority depending upon the user priority and the Cycle Synch. Interval Prioritization Phase Elimination Burst Yield Phase Acknowledgment Contention Phase PD PA PD - Priority Detection PA - Priority Assertion Elimination Survival Interval Data Transmission Figure 5.1 EY-NPMA MAC Protocol of HiperLAN/1. 126 Multimedia Wireless Local Area Networks packet residual lifetime as reported in Table 5.2. The user priority is an attribute that is assigned to each packet according to the type of traffic carried and it determines the maximum CAM priority value the packet may eventually reach. The residual packet lifetime is the time interval within which the transmission of the packet must occur before the packet has to be discarded. Figure 5.2 shows the aggregate throughput achieved by HiperLAN/2 vs. the number of data nodes. The throughput increases as the number of sources increase and stabilizes beyond a certain point. 5.4 Overview of HiperLAN/2 Figure 5.3 shows the protocol reference model for the HiperLAN/2 radio. The protocol stack is divided into a control plane part and a user plane. The user plane includes functions for transmission of traffic over established connections, and the control plane includes functions for the control of connection establishment, release and supervision. The HiperLAN/2 protocol has three basic layers: the Physical layer (PHY), the Data Link Control layer (DLC) and the Convergence layer (CL). The hiperLAN consists of an Access Point (AP) and the Mobile Terminals (MTs) that are associated with the AP. The Figure 5.2 Throughput as a function of number of MTs [2]. Figure 5.3 HiperLAN2 MAC architecture and protocol. Overview of HiperLAN/2 127 [...]... size This is shown in Figure 5.17 [27] Frame size 100 200 40 0 600 800 1000 1200 140 0 1600 1800 2000 2200 23 04 TXOP limit for 5 frames with Normal ACK (ms) 2.765 3.129 3.856 4. 583 5.310 6.038 6.765 7 .49 2 8.220 8. 947 9.6 74 10 .40 1 10.780 TXOP limit for 5 frames with No ACK (ms) 1 .47 5 1.839 2.566 3.293 4. 020 4. 748 5 .47 5 6.202 6.930 7.657 8.3 84 9.111 9 .49 0 Figure 5.17 TXOP duration for transmission of five frames... (55) 4 4 Service Start Time Minimum Data Rate 2 TS Info 2 4 Nominal Maximum MSDU Size MSDU Size 4 Mean Data Rate 4 Peak Data Rate 4 4 4 Minimum Service Interval Maximum Service Interval Inactivity Interval Suspension Interval 4 Maximum Burst Size 4 4 2 2 Delay Bound Minimum PHY Rate Surplus Bandwidth Allowance Medium Time Figure 5.11 TSPEC element as defined in IEEE 802.11e IEEE 802.11e HCF 141 Periods... add more and more background traffic, the performance of DCF will deteriorate considerably, whereas the performance of EDCA will not Table 5.5 Comparison of maximum number of VoIP calls that can be admitted by analysis and simulation [29] DCF PER (%) 0 10 30 Analysis 45 42 40 EDCA Simulation 45 45 42 Analysis 35 33 11 HCCA Simulation Analysis 35 34 13 65 58 46 Simulation 65 58 45 Multimedia Wireless. .. Networks 152 Table 5.6 Performance of DCF and EDCA in the presence of background traffic PER (%) 0 10 30 DCF EDCA 41 40 24 14 11 4 HCCA 65 58 45 Next, we consider the simulation scenario in the presence of background traffic We have 1 HDTV source and four heavy FTP sources as background traffic The HDTV source requires a bandwidth of 20 Mbps and the packet sizes of both FTP and HDTV are fixed at 1500 bytes In... represents Beacon and CFP represents the contention free priod) 136 Multimedia Wireless Local Area Networks 5.6 Overview of IEEE 802.11 Standardization As already explained IEEE 802.11 is an industry standard set of specifications for WLANs developed by the Institute of Electrical and Electronics Engineers (IEEE) IEEE 802.11 defines the physical layer and media access control (MAC) sub-layer for wireless communications... Task Group J is to enhance the 802.11 standard and amendments, to add channel selection for 4. 9 GHz and 5 GHz in Japan, to conform to the Japanese rules on operational mode, operational rate, radiated power, spurious emissions and channel sense 802.11k The IEEE 802.11 standard for wireless LANs enables inter-operability between different vendors’ access points and switches, but it does not let WLAN...128 Multimedia Wireless Local Area Networks AP acts like a central controller and coordinates the data and control information transmission over the wireless channel from all the MTs 5 .4. 1 Data Link Layer The Data Link Control (DLC) layer includes user plane functions, such as both medium access and transmission, as well as control plane functions such as connection handling Thus, the... starting point and length of the FCCH and the Random CHannel (RCH), wake-up indicator, and identifiers for identifying both the HiperLAN/2 network and the AP The FCCH, transmitted by AP, carries the information about the structure of the ongoing frame, and contains an exact description of how resources have been allocated within the current MAC frame in the Down Link (DL), the Up Link (UL)-phase and for the... U-PDU and all U-PDUs that have lower sequence numbers and that haven’t been acknowledged The result is that the transmission in DLC allows for holes (missing data) while retaining the DLC connection active It is up to higher layers, if need be, to recover from missing data 5 .4. 4 Association Control Function (ACF) 5 .4. 4.1 Association This starts with the MT listening to the BCH from different APs and. .. mechanism of the IEEE 802.11e MAC, called the HCCA, adopts a poll and response protocol to control the access to the wireless medium and eliminate contention among wireless STAs It makes use of the PIFS to seize and maintain control of the medium Once the HC has control of the medium, it starts to deliver parameterized downlink traffic to STAs and issue QoS contention-free polls (QoS CF-Polls) frames to . 1 48 24 2 BPSK 3 /4 9 1 48 36 3 QPSK 1/2 12 2 96 48 4 QPSK 3 /4 18 2 96 72 5 16 QAM (H/2 only) 9/16 27 4 192 108 5 16 QAM (IEEE only) 1/2 24 4 192 96 6 16 QAM 3 /4 36 4 192 144 7 64 QAM 3 /4 54 6. and the frame lifetime. Emerging Wireless Multimedia: Services and Technologies Edited by A. Salkintzis and N. Passas # 2005 John Wiley & Sons, Ltd HiperLAN/2 is a European 5 GHz WLAN standard. Ethernet, IEEE 13 94, and Asynchronous Transfer Mode (ATM). HiperLAN/2 was designed to give wireless access to the Internet and future multimedia at speeds of up to 54 Mbit/s for residential and corporate