Wireless Future - Telenor

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Figure 4 Basic MAC frame structure 76 2 ms MAC Frame BC-Phase 3.2 DLC Layer The Data Link Control (DLC) layer is divided into three major entities [4]: • Medium Access Control (MAC) protocol [8]; • Error Control (EC) protocol [8]; • Radio Link Control (RLC) protocol [9]. 3.2.1 MAC Protocol One of the prime tasks of the MAC layer is to map the data and control information from the higher layers on to transport channels (transport channels are discussed in more detail in section 4.1). These transport channels are then used together with a preamble to construct PDU trains (PHY bursts) that are delivered to and received from the physical layer. This process is based on a centrally controlled TDMA/TDD scheme [8], where the AP (or central controller) controls all the resource allocations inside its cell. The MTs request resources to the AP which in turn decides which MTs are allowed to transmit. The MAC frame has a fixed duration of 2 ms and consists of five different phases (see Figure 4); Broadcast (BC), Down Link (DL), Direct Link (DiL), Up Link (UL) and Random Access (RA). The length of the individual phases can vary inside the MAC frame and is set by the AP depending on the traffic situation. • Broadcast (BC) phase The BC phase consists of three transport channels, broadcast (BCH), frame control (FCH) and access feedback (ACH) channels (see Figure 6). The broadcast channel (BCH, downlink only) contains control information that is sent in every MAC frame and reaches all the MTs. The BCH provides information about transmission power levels, starting point and length of the FCH and the RCH, wake-up indicator, and identifiers for identifying both the HIPERLAN/2 network and the AP. The frame control channel (FCH, downlink only) contains an exact description of how resources have been allocated (and thus granted) within the current MAC frame in the DL- and ULphase and for the RCH. The access feedback channel (ACH, downlink only) conveys information on previous access attempts made in the RCH. MAC Frame MAC Frame MAC Frame DL-Phase DiL-Phase UL-Phase RA-Phase • Downlink (DL) and Uplink (UL) phases The DL and UL phases carry user specific control information and user data. The user data uses the Long transport CHannels (LCHs, 54 bytes long) and the control information uses Short transport CHannels (SCHs, 9 bytes long) but can also use LCHs. The MTs can use UL phase to request resources for following frames. • Direct Link (DiL) phase The DiL phase carries user data traffic (LSCs and SCHs) between MTs without direct involvement of the AP. The AP is indirectly involved by receiving Resource Requests from MTs for these connections and transmitting Resource Grants. • Random Access phase (RA) The RA phase carries a number of Random Access Channels (RCHs), which are used by MTs who have no SCHs in the current MAC frame to request resources. The RCH is also used by non-associated MTs to initiate contact with the access point. 3.2.2 Error Control The EC is responsible for detection and recovery from transmission errors on the radio link and three types of modes are defined: 1 Acknowledged mode uses a selective (SR) ARQ to provide a reliable transmission and a discard mechanism is used for low latency. 2 Unacknowledged mode provides an unreliable, low latency transmission. No feedback channel is generated. 3 Repetition mode is used to repeat LCH PDUs in order to enhance reception. The transmitter can arbitrarily retransmit PDUs, and this mode is typically used to broadcast data. 3.2.3 Radio Link Control Protocol The Radio Link Control (RLC) sublayer is made up of three main control functions. These three entities comprise the DLC control plane for the exchange of signalling messages between the AP and MT [9]: 1 The DLC Connection Control (DCC) is responsible for setup and release of connections. 2 The Association Control Function (ACF) is responsible for the association procedure of an MT to an AP. The tasks of the association control are: association, encryption, authentication, and key exchange. The default encryption scheme is DES (56 bits), optionally triple DES or no encryption can be used. The key Telektronikk 1.2001
exchange is based on the Diffie Hellman procedure. 3 The Radio Resource Control (RRC) is responsible for the surveillance and efficient use of available frequency resources. Important functions of RRC are dynamic frequency selection (DFS), handover and power control. The DFS ensures that HIPERLAN/2 will operate in a plug-and-play manner and frequency planning will not be required. The decision to initiate a handover is primarily done by the MT (forced handover initiated by the AP is also possible) and three types of handover are supported. Sector handover if a sector antenna is deployed (Inter sector/Intra AP), radio handover if the AP consists of several transceivers (InterAPT/Intra AP) and network handover (Inter AP/Intra Network). The network handover involves higher layers, and signalling via the backbone may be needed. 3.3 Convergence Layer The convergence layer acts as a bridge between higher layer protocols and the DLC Layer. This involves adapting the service requests from the higher layers to that used by the DLC layer, and segmentation/re-assembly of packets to fit the fixed length of DLC packets. Currently, convergence layers for ATM, IEEE1394 and Ethernet have been standardised, and work on a UMTS CL is underway. The convergence layers can be split into two different types, cell based (ATM) [10] and packet based (Ethernet, IEEE1394) [11]. The structure of the packet based convergence layer is displayed in Figure 5, and consists of a common part (with the segmentation/reassembly) and a service specific part. 4 Performance of HIPERLAN/2 MAC protocol As mentioned in section 2 the HIPERLAN/2 system is intended to provide multimedia communication in a variety of deployment scenarios. It is therefore of major interest to investigate the overall performance in terms of throughput, delay and QoS support. In order to predict the overall performance, the characteristics of the individual layers need to be evaluated. Already, there is published work considering the performance of the HIPERLAN/2 system [12], [2]. In this section the performance of the MAC protocol for different settings will be simulated. In essence this is done by calculating the ratio between overhead and user data for the concerned MAC setting. The number of OFDM symbols in each phase of the MAC frame can be found from the following steps. Using the values from Table 3 the OFDM symbol interval t OFDM , can be calculated Telektronikk 1.2001 Service Specific Part t OFDM = 64 ⋅ T S + T G = 4 µs (1) where Ethernet Service Specific Common Part IEEE1394 Service Specific • • • Segmentation/Re-assembly Common part convergence sublayer T S = sampling rate = 20 MHz or 50 ns T G = guard time = 800 ns The number of OFDM per frame is then: NOFDM = tframe tOFDM (2) 4.1 PDU Train Structure It was mentioned in section 3.3 that the phases of the MAC frame were constructed using transport channels. These transport channels are used together with preambles to form Protocol Data Unit trains, or PDU trains, and represents the interface between the DLC protocol and the PHY layer. Three types of preambles are defined [8]: • Broadcast control channel preamble, P BCH , enables frame synchronisation, automatic gain control, frequency synchronisation and channel estimation. The length of P BCH is 16 µs or 4 OFDM symbols. • Downlink traffic preamble, P DL , is used for channel estimation only, and is 8 µs long or 2 OFDM symbols. • Uplink traffic preamble enables channel and frequency estimation. Two types of uplink preambles are defined P UL-S (short) = 12 µs or 3 OFDM symbols and P UL-L (long) = 16 µs or 4 OFDM symbols. The standard specifies 6 types of PDU trains [8]: 1 Broadcast PDU train; 2 FCH and ACH PDU train (multiple antenna elements only); = 200 µs 4 µs = 500 XXX Service Specific Figure 5 Structure of packet based convergence layer 77
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Telektronikk Volume 97 No. 1 - 2001
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Per Hjalmar Lehne (42) obtained his
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Stein Svaet (41) is Senior Research
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Pbit/day 6 150 125 100 75 50 25 0 F
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8 Video communication as a utility
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10 The idea of reconfigurable mobil
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12 Switched lobe Dynamically phased
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Figure 10 The All IP vision Figure
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16 The IP world has some problems t
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18 Abbreviations and acronyms less
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Jorge Pereira (40) obtained his Eng
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(Millions) Figure 1 Exponential gro
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24 Defining Generation based upon D
Page 28 and 29: Multi-mode Terminal by Software Rad
Page 30 and 31: distribution layer possible return
Page 32 and 33: 30 A Co-Ordinated Approach to 4G As
Page 34 and 35: Figure 2 Applications will grow fas
Page 36 and 37: Figure 5 Evolution from multi-mode
Page 38 and 39: Operator/Provider download librarie
Page 40 and 41: Radio network subsystem - service c
Page 42 and 43: Dr. Techn. Jørgen Bach Andersen (6
Page 44 and 45: Figure 3 Two arrays with M and N el
Page 46 and 47: Figure 5 Cumulative probability dis
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Page 50 and 51: 48 Conclusion The challenge of prov
Page 52 and 53: 50 models are described simply by a
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Page 62 and 63: Figure 2 Business case study Figure
Page 64 and 65: Figure 6 BRAIN QoS architecture 62
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Page 68 and 69: Figure 2 The Bluetooth protocol sta
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126 6 The ICEBERG project. (2001, M
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128 new door towards the upcoming f
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? Figure 1 The Mobile Service Platf
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Figure 2 Needs and Related Services
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Anne Lise Lillebø (52) is Adviser
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140 with other SDOs (Standards Deve
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142 Box 2 - ETNO Declaration We, th
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