Aportaciones recientes al estudio del pensamiento moral

Several systems support broadband wireless communications and mobile user access. These are the Multichannel Multipoint Distribution System (MMDS) and the Local Mul- tipoint Distribution System (LMDS), also called Local Multipoint Communication System (LMCS) or Microwave Video Distribution System (MVDS).

The MMDS systems work at frequencies lower than 5 GHz in large coverage areas with cell radius of up to 40 km. MMDS systems can be used for transmission of video

and broadcast services in rural areas. Because of the large cell size, MMDS systems do not perform well for bidirectional communication that integrates a return channel.

The LMDS systems work with higher frequencies where a larger frequency spectrum is available than that in the MMDS systems. The coverage for LMDS systems involves smaller cells of up to 5 km radius and requires repeaters to be placed in a Line Of Sight (LOS) configuration. This local coverage with a large available bandwidth makes LMDS systems suitable for interactive multimedia services distribution.

Broadband wireless access is based on the Two-Layer Network (TLN) concept in which subscribers are grouped into microcells, which are embedded into a macrocell. The microcells coverage uses local repeaters operating at 5.8 GHz fed by a BS through 40 GHz links. OFDM modulation is used to allow the reception with plug-free receivers located inside the buildings. A 40 GHz band fixed receiver provides a rooftop antenna in LOS with the transmitting antenna. This LMDS system provides an integrated wireless return channel.

The LMDS architecture uses co-sited BS equipment. The indoor digital equipment connects to the network infrastructure, and the outdoor microwave equipment mounted on the rooftop is housed at the same location. The Radio Frequency (RF) planning uses multiple sector microwave systems, where the cell site coverage is divided into 4, 8, 12, 16, or 24 sectors.

The user accesses the network through Hybrid Fiber Radio (HFR), Radio To The Building (RTTB) and Radio To The Curb (RTTC). In HFR, a Radio Frequency Unit (RFU) carries out signal down conversion from RF frequency to the intermediate frequency. The signal feeds the Radio Termination (RT) of each user through a bus link. In RTTB architecture the signal feeds the user Network Termination (NT) through point-to-point cable links. In RTTC the RFU is placed in a common outdoor unit and is shared among several buildings.

In high-population cities, LMDS systems can be used as LOS propagation channels at high frequencies. LOS operation is inherently inflexible even for low mobility services. On the other hand, the available bandwidth for LMDS frequencies exceeds 1 GHz, making it a very desirable transmission method. The frequency bands assigned to MMDS and LMDS are included in the frequency bands allocated for fixed services. The exception is the 40.5–42.5-GHz band allocated for MVDS systems. The 28-GHz channel is not generally open in several countries. This is why the 40-GHz technology is considered. However, the baseband system is designed to be compatible with interchangeable RF system (5/17/28/40 GHz).

LMDS is a stand-alone system providing wireless multimedia and Internet services, and it can be used as the support infrastructure for other wireless multimedia services, for example, UMTS, wireless LAN, and Broadband Radio Access Network (BRAN), which provide a high-speed digital connection to the user.

Sukuvaaraet al. proposed a two-layer 40-GHz LMDS system providing wireless inter- active cellular television and multimedia network. The first layer, a macrocell, uses 40-GHz wireless connection between the BS and the sub–base station, which can be a frequency and/or protocol conversion point called alocal repeater. The second layer, a microcell, operates at 5.8 GHz. The user can connect a multimedia PC (Personal Com- puter) to a local repeater access point at 5.8 GHz or directly to the BS at 40 GHz. The

WIDEBAND WIRELESS LOCAL ACCESS 41

5.8 GHz connection can be used cost effectively within cities and high-density population areas, and the 40 GHz connection can be used in rural areas. The macrocell size can be up to 5 km. The microcell size is from 50 to 500 meters depending on services and location. A 40-GHz transceiver unit serves dozens of microcell users. The microcell architecture prevents LOS indoor propagation, supports nomadic terminals, and is cost effective. 3.2.3 Media Access Control (MAC) protocols for wideband wireless local access Wireless LANs provide wideband wireless local access and offer intercommunication capabilities to mobile applications. This technology is supported by 802.11 standard developed by the IEEE 802 LAN standards organization. Wireless LANs are also pro- vided by High Performance Radio LAN (HIPERLAN) Type 1 defined by the European Telecommunications Standards Institute (ETSI) RES-10 Group.

IEEE 802.11 uses data rates up to 11 Mb s−1 _{and defines two network topologies. The}

infrastructure-based topology allows Mobile Terminals (MTs) to communicate with the backbone network through an access point. Inad hoc topology, MTs communicate with each other without connectivity to the wired backbone network. HIPERLAN uses data rate 23.5 Mb s−1 _{and thead hoc topology.}

QoS guarantees are achieved through infrastructure topology, and a priority scheme in the Point Coordination Function (PCF) in the IEEE 802.11. HIPERLAN defines a channel access priority scheme based on the lifetime of packets to achieve QoS.

Wireless Asynchronous Transfer Mode (WATM) standardization involves Wireless ATM Group (WAG) of the ATM Forum and the BRAN project of ETSI. These efforts involve developing a technology for wideband wireless local access that includes ATM features in the radio interface, thus combining support of user mobility with statistical multiplexing and QoS guarantee provided by wired ATM networks. The goal is to reduce complexity of interworking between the wireless access network and the wired ATM backbone and to attain a higher level of integration.

3.2.4 IEEE 802.11

The IEEE 802.11 MAC (Media Access Control) protocol provides asynchronous and synchronous (contention-free) services, which are provided on top of physical layers and for different data rates. The asynchronous service is mandatory, and the synchronous service is optional.

The asynchronous service is provided by the Distributed Coordination Function (DCF), which implements the basic access method of the IEEE 802.11 MAC protocol also known as Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) protocol. The implementation of DCF is mandatory.

Contention-free service is provided by the PCF, which implements a polling access method. A point coordinator cyclically polls wireless stations, allowing them to transmit. The PCF relies on the asynchronous service provided by the DCF. The implementation of the PCF is not mandatory.

Basic access mechanism illustrated in Figure 3.3 explains that in DCF a station must sense the medium before initiating transmission of a packet. If the medium is sensed to

Packet arrival Frame transmission

Elapsed backoff time Residual backoff time

Frame Frame Frame Frame Station 1 Station 2 Station 3 Station 4 Station 5

DIFS DIFS DIFS DIFS Frame

Figure 3.3 Basic access mechanism.

be idle for a time interval greater than a Distributed Interframe Space (DIFS), the station transmits the packet. Otherwise, the transmission is deferred and the backoff process is started. The station computes a random time interval, the backoff interval, uniformly distributed between zero and a maximum called the Contention Window (CW). This backoff interval is then used to initiate the backoff timer, which is decremented only when the medium is idle, and it is frozen when another station is transmitting. Every time the medium becomes idle, the station waits for a DIFS and then periodically decrements the backoff timer. The decrementing period is the slot time corresponding to the maximum round trip delay between two stations controlled by the same access point.

When the backoff timer expires, the station can access the medium. If more than one station starts transmission simultaneously, a collision occurs. In a wireless environment, collision detection is not possible. A positive acknowledgement ACK shown in Figure 3.4 is used to notify the sending station that the transmitted frame was successfully received. The transmission of the ACK is initiated at a time interval equal to the Short Interframe Space (SIFS) after the end of reception of the previous frame. The SIFS is shorter than DIFS; thus the receiving station does not need to sense the medium before transmitting the ACK.

If the ACK is not received, the station assumes that the transmitted frame was not successfully received, and it schedules a retransmission and enters the backoff process

Frame

ACK

SIFS Source station

Destination station

WIDEBAND WIRELESS LOCAL ACCESS 43

again. After each unsuccessful transmission attempt, the CW is doubled until a predefined maximum (CWmax) is reached. This reduces the probability of collisions. After a successful

or unsuccessful frame transmission, the station must execute a new backoff process if there are frames queued for transmission.

The hidden station problem occurs when a station successfully receives frames from two different stations that cannot receive signals from each other. This may cause a station to sense the medium being idle even if the other station is transmitting. This results in a collision at the receiving station. The IEEE 802.11 MAC protocol includes an optional mechanism based on the exchange of two short control frames, as shown in Figure 3.5, to solve the hidden station problem. A Request To Send (RTS) frame is sent by a potential transmitter to the receiver. A Clear To Send (CTS) frame is sent by the receiver in response to the received RTS frame. If the CTS frame is not received within a predefined time interval, the RTS frame is retransmitted by executing the backoff algorithm. After a successful exchange of RTS and CTS frames, the data frame is sent by the transmitter after waiting for a SIFS.

A duration field in RTS and CTS frames specifies the time interval necessary to com- pletely transmit the data frame and the related ACK. This information is used by the stations that hear either the transmitter or the receiver to update their Net Allocation Vector (NAV), a timer that is continuously decremented regardless of the status of the medium. The stations that hear either the transmitter or the receiver refrain from trans- mitting until their NAV expires, and the probability of a collision occurring because of a hidden station is reduced. The RTS/CTS mechanism introduces an overhead that may be significant for short data frames. When RTS/CTS mechanism is enabled, collisions can occur only during the transmission of the RTS frame, which is shorter than the data frame. This reduces the time of collision and wasted bandwidth.

The effectiveness of the RTS/CTS mechanism depends on the length of the data frame to be protected. The RTS/CTS mechanism improves the performance when data frame sizes are larger than the size of the RTS frame, which is the RTS threshold. The RTS/CTS mechanism is enabled for data frame sizes over the threshold and is disabled for data frame sizes under the threshold.

To support time-bounded services the IEEE 802.11 standard defines the PCF to allow a single station in each cell to have a priority access to the medium. This is implemented by using the PCF Interframe Space (PIFS) and a beacon frame that notifies all the other

RTS Source station (3)

Destination station (2) Stations close to the source (4) Stations close to destination (1)

CTS

NAV NAV

SIFS SIFS SIFS ACK Frame

stations in the cell not to initiate transmissions for the length of the Contention-Free Period (CFP). When all the stations are silenced, the PCF station allows a given station to have contention-free access by using an optional polling frame sent by the PCF station. The length of the CFP can vary within each CFP repetition interval, depending on the system load.