WLAN is an open data communication system for wireless access to the Internet and Intetranet networks. It also allows LAN connection to a LAN in a building or a residential area, or campus. WLAN deployments include the deployment of WLAN components, protocol structures, and so on. 1. Introduction In the context of globalization, the explosion of demand for high-speed data transmission and the need to diversify the types of services provided such as Internet access, e-mail, e-commerce, file transfer, etc. has promoted the development of local area network (WLAN) solutions. The purpose of the WLAN is to provide an alternative solution to the customer side of solutions such as xDSL, Ethernet, GPRS, 3G, etc. WLAN is part of a mobile room solution that allows users Connect LANs from public areas such as hotels, airports and even possibly on transportation. In Vietnam, WLAN has been deployed for the first time at Horison hotel under the project "Internet surfing in Hanoi" with the cooperation of companies such as VDC, Cisco System, Pertlink. In addition, Hanoi Information Technology Company (HanoiCTT) has officially deployed this technology in online training. The laptops are interconnected through a Cisco Aironet 350 network card and access point at 1 to 11 Mbps. This technology allows users to use the Internet at a much greater speed than traditional indirect access. WLAN is an open data communication system for wireless access to the Internet and Intetranet networks. It also allows LAN connection to a LAN in a building or a residential area, or campus. A WLAN system can be integrated with a wide area network. The bit rate achieved in the WLAN should be supported by proper transmission from the backbone network. The main WLAN standard is IEEE 802.11b and IEEE 802.11a is for higher bit rates. HiperLAN2 is intended to include both the IEEE 802.11a standard and operate on the 5 GHz band. This standard will become mainstream in the market in 2003, 2004. WLAN deployment includes the deployment of WLAN components, protocol structures, WLAN model types, WLAN usage issues, as well as methods that can improve the quality of WLAN performance. 2. WLAN components WLAN components include wireless network interface cards, access points, and remote radio bridges. 2.1 Wireless network interface cards Wireless network interface cards do not differ much from adaptive adapters used in wired LANs. Like adaptive card adapters, wireless network interfaces exchange information with the network operating system through a dedicated controller, thus allowing applications to use wireless networks for data transmission. Unlike card adapters, these cards do not need any cables connecting them to the network and allow repositioning the network nodes without changing the network cable or changing the connection to the hub. . 2.2 Radio Access Points Access points create coverage areas that connect mobile nodes to a wired LAN infrastructure. This makes the WLAN transform into an extension of the wired network. Because access points allow for extended coverage, WLANs are very stable and additional access points can be deployed in either a building or university campus to create large wireless access areas. large. These access points not only provide information exchange with wired networks but also filter traffic and perform standard bridge functions. Due to the asymmetric unbalance between wireless and wire information, an access point with appropriate buffers and memory resources is needed. Buffers are also primarily used to store data packets at the access point when a mobile node tries to move away from coverage or when a mobile node is operating in low power mode. Access points communicate with each other over a wired network to manage mobile nodes. An access point does not require access control from multiple mobile nodes (meaning it can operate with a distributed random access protocol such as CSMA.) However, a centrally managed multi-access protocol Common access point network interfaces to the access point include 10Base2, 10BaseT, cable modem and ADSL modem, ISDN. 2.3 Remote radio bridge Remote radio bridges are similar to access points except where they are used for external channels. Depending on the distance and operating area, external antennas may be required. These bridges are designed to connect networks together, especially in buildings and away from dozens of kilometers. They provide a quick and inexpensive alternative to cable installations or leased lines, and are often used when traditional wire connections are difficult to implement in practice (for example, through rivers, entangled terrain, private areas, highways). 3. WLAN protocol architecture WLANs differ from traditional wired networks mainly in the physical layer and in the access control layer (MAC) layer of the Open System Interconnection Reference Model (OSI). These different sections provide two approaches to providing physical interface points for WLANs. If the physical interface point is in the logical channel control (LLC) layer, this approach requires that the client controllers provide higher level software as the network operating system. Such an interface allows mobile nodes to exchange information directly with each other through wireless network interface cards. Another logical interface point is the MAC layer and usually applies an access point. Therefore the access point performs bridging and does not perform routing. Although the MAC interface requires a wired connection, it allows any network operating system or any controller to work with WLAN. Such an interface allows an extensible wireless LAN to be provided by providing access to a new wireless network device. The protocol architecture of typical WLAN network interfaces is shown in Figure 1. The lower layers of the radio interface card are usually made by "Firmware" firmware and run on embedded processors. The higher layers of network protocol disks are provided by the operating system and application programs. A network controller allows the operating system to exchange information with the lower-level firmware embedded in the wireless network interface card. In addition it performs the standard LLC functions. For Windows operating systems, controllers generally comply with some versions of Network Controller (NDIS) specifications. Unix, Linux and Apple Powerbook-based controllers are also available. Figure 1. Protocol structure of WLAN components 4. WLAN profiles WLANs typically have two types of network configurations. It is a stand-alone configuration or basic configuration as described in Figure 2. The stand-alone configuration provides contiguous connectivity in which the mobile nodes exchange information directly with each other through the adapters. Online. Such configurations are ideal in commercial conferences or in setting up temporary work groups. However, they may have shortcomings in terms of limited coverage. An access point can extend the distance between two independent WLANs when it acts as a repeater, increasing the distance between two mobile nodes. Figure 2. WLAN configuration Basic WLANs allow mobile nodes to be connected to a wired network (Figure 2b). Transmission from wireless information to wireline information through an access point. Designing a WLAN can be relatively simple if network information and its management are in the same area. A central access point that can control and distribute access to disputed nodes provides access to the backbone network, assigns addresses and priorities, monitors network traffic , management of outbound packets and maintenance of network configuration tracking. However, a centralized multi-access protocol does not allow one node to transmit directly to another node and is in the same zone as the access point (Figure 2b). In this case, each packet must be transmitted twice (from the root node and then the access point) before it reaches the destination node, which in turn reduces the transmission efficiency and increases the transmission delay. In addition, a strategically located access point minimizes power generation and efficiently addresses hidden node issues. Since some WLANs use distributed multi-access protocols such as CSMA, it is possible for nodes in the base network to communicate directly with each other (Figure 3). However, some base WLANs require packet transfers to the access point even if CSMA is used. The access point then forwards the packets to the correct destination. 5. Issues related to WLAN usage These are issues with hidden nodes, power monitors, radio interference sources, and signal transmission impediments. Most of these problems are associated with wireless LANs. Hide button One of the disadvantages of the large signal power fluctuation in WLANs is the existence of hidden nodes where some of these nodes are located in the receiver area but are not transmitting. For example, in Figure 3a, the nodes A and C are in the receive range of node B. But the nodes A and C are not in the interval. If nodes A and C simultaneously transmit to node B, node B will suffer a collision and will not be able to receive any transmission. Both A and C will not know about this collision. The carrier response is ineffective in this hidden node situation because a power node blocks other nodes in its vicinity than in the destination node area. This reduces the quality of carrier-sensing protocols because the duration of unprotected collisions extends the entire length of the packet. With conventional unmatched carrier frequency the protection is much shorter, usually within the first few bits of the packet. Hidden buttons will not be a problem if the radio coverage area is well isolated. Because collisions are less common in spread spectrum systems than in narrow systems, the existence of hidden nodes can not cause many problems for WLAN DSSS and FHSS. In contrast, hidden nodes can be beneficial to both systems because unused multi-packet carrier transmissions with different time-varying versions of an interrupt or hop code can be used. Figure 3. Hide button in WLAN Figure 3b shows how hidden button collisions can occur in the base WLAN. In this case, the access point is subject to a collision caused by a transmission overlap between two nodes D and E. One major problem here is that D and E nodes can not exchange information when the access point is not configured as A repeater to forward packets of information between nodes in the coverage area. A centralized, multi-access protocol (by the access point coordinator) solves the hidden node problem for basic LANs. Nodes can not be transmitted if the access point does not issue explicit permission commands. However, a protocol collision can still occur when two neighboring access points simultaneously transmit to one node in the overlapping area. This situation can be reduced if the neighboring access points coordinate the transmission over the wireline network or operate over non-blocking frequency channels. 5.2 Power monitoring Due to large changes in signal attenuation, it is necessary to have capacity monitoring. This capability allows the radio receiver to successfully isolate higher-intensity signals even when multiple nodes are present at the same time. This is because the receivers can track the strongest signal if the power of the next strongest signal drops to 1.5 to 3 dB. Distance is a major factor determining the received signal power. The two nodes A and C are attempting to exchange information with node B. Both nodes are within the coverage of node B. However, because node A is closer to node B, the signal from node A may be larger. Much more than the signal power obtained from node C if both nodes overlap. This increases the balance problem because the farmost node is always discriminated against and it is possible that node C can never exchange information with node B. In other words, the effect of monitoring This can reduce the probability of conflict (including hidden button collisions) and thus improve the quality of the WLAN. In spread spectrum systems, the tracking process enables the decoder to successfully encode a pseudorandom code or frequency hopping pattern even if there are multiple overlapping signals with the same code or frequency hopping pattern. Generally, power monitoring does not occur in FHSS systems if there are multiple transmitters that do not share the same hopping code and the frequency channels are not synchronized simultaneously. However, most WLANs operate with a common frequency hopping code and the frequency channels are synchronized. For DSSS systems, CDMA power control becomes more urgent as multiple users are often opposed. The IEEE 802.11 standard mandates the use of power control for both DSSS and FHSS transmissions with a power rating less than 100 mW. Although such controls allow for efficient use of resources, it is difficult to maintain in high fading and mobile environments. 5.3 Radio interference sources For WLANs operating on the 2.4 GHz radio spectrum, microwave ovens can be an important source of interference. Microwave ovens have capacities up to 750W at 150 pulses per second and have a radius of operation of about 10 m. So for a data rate of 2 Mbit / s the maximum packet length must be less than 20,000 bits or 2,500 octets. Radiation emissions range from 2.4 GHz to 2.45 GHz and stay stable at short intervals of 2.45 GHz. Even though blocks are blocked, most of the power can still interfere with WLAN transmission. Other interference sources in the 2.4 GHz frequency range include photocopiers, anti-theft devices, elevator motors and medical devices. 5.4 Signal barriers spread For radio signals, how far the signals can travel depends very much on the building materials of walls, walls and other objects (Table 1). Table 1. Radio transmissions and their effects 6. WLAN enhancements This section will review some of the ways to improve WLAN quality. Specifically, it covers techniques such as increasing the capacity of the network with multi-frequency channels, extending coverage by reducing data rates, filtering outgoing traffic, and providing mobility via roaming. Improve network congestion by balancing the load and securing network access. 6.1 Build multi-channel configuration Figure 4. Multi-channel operation Multichannel configurations can be very effective in environments with high radio nodes operating in the same vicinity. If a particular WLAN coverage area has more nodes and additional data is needed, a second access point operating at a different frequency will be added, thus doubling the available data. Multi-channel operation also allows access points serving high-speed node access and can only be applied to wireless LANs. Thanks to the configuration of different access points with different frequency channels, the transmissions within a radio coverage area are separated. This will reduce the interference and frequency of information delay of the nodes. For a system that uses a single channel the nodes in the shadow area (Figure 4) divide the common environment. This means that if one button is in the play area, all other buttons are delayed. By assigning each access point to a different channel, the congestion in the area is reduced by the outgoing traffic to the two access points. Independent networks do not support multi-channel operation. Multi-channel operation can also be applied to the radio bridge (Figure 5). When another frequency channel is used for the bridge, it will not interfere with the operation of the normal access point. This allows for widening of the distance without the need for a tangent line. Some WLANs require an access point to serve as a wireless bridge, while other WLANs need outdoor antennas. Figure 5. Multichannel wireless bridge to widen the gap 6.2 Multi-channel operation for WLAN 2.4 GHz, WLAN DSSS 2.4 GHz In the 2.4 GHz ISM the entire communication channel for WLAN DSSS can be divided into different carrier frequencies. The number of carrier frequencies can be selected. The number of carrier frequencies is as follows: North America 11; entire Europe 13; France 4; Japan 1. When the DSSS signal is spread over a wide band, the preferred carrier frequency difference is between neighboring access points of at least 30 MHz. Meaning in the US and Europe, up to three carriers can be used in the same area. Table 2 shows 13 multi-channel DSSS assignments that can be based on 13 different carrier frequencies. The largest carrier frequency deviation will reduce near-field noise and improve quality compared to smaller frequency-separated networks. Table 2: Multi-Channel Stabilization for WLAN DSSS 2.4 GHz WLAN FHSS 2.4 GHz Since the frequency channels in the frequency hop sample occupy the entire 2.4 GHz ISM frequency band, the channel allocation method used in the DSSS can not be applied directly to FHSS systems. FHSS WLANs achieve multi-channel operation by implementing separate channels on different frequency hopping patterns. 6.3 Decrease the data rate (Fall back) Most WLANs have the advantage of small coverage and good wave conditions to increase the data rate. While transmitting at low speeds is often more reliable and allows for wider coverage, people sometimes prefer higher throughput. To balance the speed and coverage of wireless network interface cards, the most common data rates are available. After a few errors, the interface card will drop to a lower speed. 6.4 Filtering network traffic One of the ways to optimize WLAN quality is to avoid redundant broadcasts over radio channels. This excess can be: Network messages are converted by wired network devices (such as servers), but they are not related to wireless terminals. - Broadcast / multicast broadcasts do not have a specific address to radio terminals - Error messages generated by corrupted or misconfigured devices (devices in closed loop circuits) Excessive flow filtering saves bandwidth of radio channels for mobile nodes. By using the following functions of the access point bridge can achieve that: - Filter protocols to reject wired network protocols connecting to wireless networks - Filter the exchange traffic between two unspecified nodes - Enables tree extensions to resolve closed network faults - Filter thresholds to limit the number of messages 6.5 Coverage and roaming A major requirement for WLANs is the ability to monitor the location of the mobile nodes and devices. The portable device moves from one location to another but is only used in one fixed area. The mobile nodes actually access the LAN while on the move. The user's mobility requires a roaming function that allows the mobile node to move between different physical locations in a LAN environment without losing the connection. For continuous roaming, each location is served by a service access point and access point coverage must overlap. A mobile node checks the signal-to-noise ratio (SNR) as it moves and when it needs to scan access points can be used and then automatically connects to the desired access point to maintain. continuous access (Figure 6). When the SNR drops below the predefined threshold, the node searches for a nearby access point with better SNR. Figure 6 WLAN roaming If an access point is detected, the mobile node transmits a roaming request to the access point and the access point forwards the request to the old access point (Figure 7). The old access point will release the active connection's control and move it to the new access point. Roaming completes when the mobile node is notified. This procedure is similar to cellular roaming, except that roaming on packet WLANs is easier because transition from one coverage to another can be accomplished through packet transmission. Mostly roaming must be done fast because of the data rates of the WLANs, which means that many packets are transmitted while roaming. That can cause too much retransmission due to lost or misplaced packets. The data rate after roaming depends very much on the speed at which the SNR is degraded. Most LANs can support mobile nodes with walking speeds (less than 10 km / h). Some WLANs can ensure continuous network connectivity without losing or repeating frames when the switch from one coverage area to another at 60km / h. For roaming support in multichannel configurations, mobile nodes can often automatically switch to frequency channels or automatically convert hopping patterns when roaming between access points. Independent networks do not support roaming. Figure 7. Roaming negotiation 6.6 Load balancing Load balancing enables WLANs to serve larger loads more efficiently. Each access point can monitor the traffic load in its coverage area and then try to balance the number of nodes that have been serviced by the traffic load in the neighboring access points. To achieve that, the access points must exchange traffic information over the backbone network. Most load balancing methods do not depend on signal strength, as it can complicate routing algorithms a lot. Usually roaming has priority over load balancing because a mobile node can be connected to an access point by the signal strength level before load balancing is performed. 6.7 Protection of wireless access Radio channels are vulnerable to eavesdropping, fraud, and unauthorized transmissions rather than wireline. Therefore, some of the following mechanisms will apply to avoid unauthorized access to WLANs: - Encrypt all data transmitted over the radio channel - Network lock for all nodes without proper network identification - Limit access to WLANs only to nodes in the list that are broadcast - Implement the password in the network operating system. Conclude WLAN applications can be found in most environments, such as industry, government, and residential areas. An issue of concern for radio transmitters is that unauthorized people can interfere with the outside world. Therefore, the access of the object must be protected. Radio emissions can also be an undesirable source of interference to other radio networks and need to be controlled. WLAN technologies vary from stand-alone networks (in accordance with temporary configurations) to base networks (providing fully distributed roaming roaming connections). Various techniques such as load balancing volume, power management and multi-channel operation to improve the quality of WLAN. References 1 Benny Bing, "High-Speed Wireless ATMs and LANs", Artech House, InC., Boston London, 2000. [2] Bob O'Hara and Al Petrick, "The IEEE 802.11 Handbook," IEEE, 1999. [3] Richard van Nee andRamjee Prasad, "OFDM for Wireless Multimedia Communications", Artech House, Boston London, 2000. [4] Gilbert Held (2001), "Data Over Wireless Networks, BluetoothTM, WAP, and Wireless LANs," McGraw-Hill. [5] IETF RFC 1171 (1990): "The Point-to-Point Protocol for the Transmission of Multi-Protocol Datagrams Over Point-to-Point Links." [6] IEEE 1394 (1995): "IEEE Standard for High Performance Serial Bus". [7] Mobile Ad hoc Networks (MANET) URL: http://www.ietf.org/html. + charters / manet-charter.html. (2000-05-28). Work in progress. Nguyen Quy Sy Lam Ve Da
