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13 Wireless Local Area Networks ∗ Since the introduction of lightweight portable computers (laptops, notebooks), a great deal of attention has been focused on the development of wireless computer networks (Wireless Local Area Network, WLAN). Thanks to standardization in the field of local area networks, it is com- paratively easy to find systems that will still be upgradable even in a few years’ time. Around 70 % of all computers connected to networks are compli- ant with the IEEE 802.3 (Ethernet) and IEEE 802.5 (Token Ring) standards. Connection is normally over a permanent wireline link. The problems that can occur are the surfacing of mechanical defects (corrosion) after a few years and violations of rules on radiated interference. It is difficult to adapt these networks to cope with changing office conditions. Mobile network nodes are not possible. The obvious approach is to leave out the cable entirely. This idea is almost as old as the concept of the so-called ALOHA system, which used radio to connect terminals to their processing computers. The newer wireless LANs work with the most up-to-date radio technology. Data is encrypted and ex- tensive error-protection mechanisms are available. Integrity of data is also guaranteed. Just like wireline LANs, wireless LANs can be divided into different archi- tectures and performance categories. Many companies offer products for wire- less point-to-point connections, but only very few build LANs for multipoint communication. Today wireless networks use spread-spectrum, narrowband microwave or infrared signals for transmission (see Table 13.1). Because of legal regulations, networks using spread-spectrum and narrowband microwave cannot be operated in most countries unless special authorization has been given. Until now, wireless LANs have only had a very small share of the market. This is partly due to the higher costs per network node, but no doubt also because of the late standardization in this area. In spite of this, suppliers are projecting a growth in wireless networks over the next few years. Standards such as IEEE 802.11 or HIPERLAN/1 discussed below will help to increase user acceptance of wireless LANs. ∗ With the collaboration of Christian Plenge and Andreas Hettich Mobile Radio Networks: Networking and Protocols. Bernhard H. Walke Copyright©1999 John Wiley & Sons Ltd ISBNs: 0-471-97595-8 (Hardback); 0-470-84193-1 (Electronic) 656 13 Wireless Local Area Networks Table 13.1: Characteristics of different transmission techniques Spread spectrum Microwave Infrared Frequency [GHz] 1–6 18.825–19.205 30 000 Range [m] 30–250 10–50 25 Power [W] <1 0.025 — 1 2 3 4 5 6 7 Application Layer Presentation Layer Transport Layer Network Layer Data Link Control Layer Physical Layer Session Layer 2b 2a 1 LLC Logical Link Control MAC Media Access Control Physical Layer Higher Layers Figure 13.1: The IEEE 802/ISO 8802 standard 13.1 Standards The diversity of LAN systems in terms of cabling, transmission techniques, transmission speeds, access procedures and variations thereof necessitated standardization in order to facilitate their acceptance and enable different LANs to work together. Committee 802 of the Institute of Electrical and Electronics Engineers (IEEE) in the early 1980s had developed a standard for Local Area Networks (LANs) with speeds of up to 20 Mbit/s that offers security on the commu- nications side for manufacturers as well as for users and has largely been accepted. The standard mainly restricts itself to the lower two layers of the ISO/OSI reference model (see Figure 13.1). A separation is made between Logical Link Control (LLC) and Medium-Access Control (MAC). The LLC layer upwardly offers all systems a standard interface for establishing logical connections. The MAC sublayer supports protocols such as token ring, token bus, CSMA/CD (Ethernet). In Western Europe the standards for wireless radio LANs are specified by ETSI. The technical group RES 10 (Radio Equipment and Systems) at ETSI has developed HIPERLAN/1, the European standard for wireless LANs [3]. 13.2 Technical Characteristics of HIPERLAN/1 657 Table 13.2: Mid-frequencies of HIPERLAN/1 Channel No. Mid-frequency [MHz] Channel No. Mid-frequency [MHz] 0 5 176.4680 3 5 247.0562 1 5 199.9974 4 5 270.5856 2 5 223.5268 The frequency bands at 5.2 GHz and 17.1 GHz are being reserved throughout Europe for this WLAN. HIPERLAN Type 1 (HIPERLAN/1) describes a wireless LAN for com- puter–computer/terminal communication. ETSI BRAN (Broadband Radio Access Network, combining RES 10 and TM 4) also has workgroups that are developing specifications for wireless ATM systems (Asynchronous Transfer Mode) under the designation of HIPERLAN Type 2 (Wireless-ATM LAN), HIPERACCESS (Wireless-ATM Remote Access) and HIPERLINK (Wireless- ATM Interconnect). Results of their efforts are expected in 1998/99 (see Section 12.1.5). In parallel to the European HIPERLAN/1, the IEEE in the USA has spec- ified 802.11 as an additional standard for WLANs [5]. Both standards are equipped with an IEEE 802.2 or ISO 8802 compat- ible interface [6], allowing them to replace the wired transmission systems described above. Because of the restriction of the radio medium (e.g., ra- dio range), both standards must contain functions for the management and maintenance of the radio network that far exceed the normal tasks of the MAC sublayer. These are described along with the technical characteristics of HIPERLAN/1 and IEEE 802.11 in the following sections. 13.2 Technical Characteristics of HIPERLAN/1 HIPERLAN/1 can be used as a universally accepted broadband and flexible ad hoc LAN (see Section 13.3.1) and as such can be connected to other LANs. Up to five frequency channels in the 5.15–5.30 GHz range are being provided for HIPERLAN/1; see Table 13.2. One channel provides a bit rate of 23.5294 Mbit/s for user and control data. The data rate available to the user is reduced to 10–20 Mbit/s because of the overhead added by the protocols of the different sublayers and the channel-access procedure. At maximum 1 W transmitter power the range of a HIPERLAN/1 node should be around 50 m indoors. Three transmit and receive classes with varying power and sensitivity are specified in the standard. A GMSK modulation method with a bandwidth time product of 0.3 is used on the radio channel. 658 13 Wireless Local Area Networks Synchronous applications as well as those with real-time requirements are supported. The transmission time is not a critical factor with asynchronous traffic, e.g., with electronic mail or file transfer. The standard contains mechanisms for encryption of sensitive data before it is sent. HIPERLAN/1 terminals have to be small so they can be used in portable computers. Plans are underway to make them available the size of a PCM- CIA card (Personal Computer Memory Card Interface Association) with the dimensions 85×54×10.5 mm (excluding antenna system). Since HIPERLAN/1 systems support applications for battery-operated sys- tems, they must offer low power consumption of a few hundred mW. HIPER- LAN/1 offers an energy-saving mode. HIPERLAN/1 networks should support the mobility of terminals. HIPER- LAN/1 stations are therefore designed to be able to exchange information with other stations at up to a speed of 10 m/s, which corresponds to 36 km/h, or up to a rotational speed of 360➦/s. 13.3 Network Environments for HIPERLAN/1 The ETR 069 technical report [4] produced by the ETSI RES 10 commit- tee defined the services and possibilities planned for HIPERLAN/1. Some of the applications that will benefit from new solutions and an overview of the HIPERLAN/1 network topologies are presented in the following [2]. HIPER- LAN/1 appears without the “/1” below. 13.3.1 HIPERLAN Applications Wireless offices A WLAN is a better option than a fixed network in listed buildings or in environments where constructional changes are so fre- quent that cabling cannot be installed, e.g., film and photographic stu- dios. Furthermore, it should be possible, for example, to use portable computers in different locations and connect them easily to a network. Ad hoc networks Ad hoc networks are radio networks without any kind of permanent communications infrastructure. A group of users can form its own closed complex. This means that at conferences, conventions, and large functions, or in the event of accidents or catastrophes, computers can communicate with each other without having been cabled together beforehand. Each user carries his part of the network with him in the form of his computer with a radio-LAN connection. Medicine Within a radio-LAN, doctors would be able to have direct and interactive access to remote data such as X-rays when visiting their patients. This would make the work of doctors easier and produce better and faster diagnosis for patients. 13.3 Network Environments for HIPERLAN/1 659 CN CN CN CN CN CN CN CN . . B 1 B 2 B n-1 B n A n-1 A n A 2 A 1 Figure 13.2: Independent HIPERLANs Industrial applications More and more work in industry is being automated. In many cases the controlling computers are in a central facility man- aging a large number of machines that are restricted to use in their locations. A wireless connection to the network would allow these ma- chines (e.g., industrial robots or unmanned vehicles) more freedom of movement and the possibility of being used with more flexibility. Main- tenance personnel would use laptops to retrieve the data needed for diagnostic purposes. 13.3.2 Network Topologies A HIPERLAN is not organized centrally, and instead has a completely dis- tributed architecture with a dynamic allocation of network and network node identifiers. Each station (node) is differentiated from the other stations by a unique node identifier (NID). A number of stations are combined into a network with a shared HIPERLAN identifier (HID). This network forms a HIPERLAN. In contrast to a wireline network, HIPERLANs using the same radio chan- nel cannot be separated from one another. Overlapping can occur. Another problem with radio channels is the range restriction. Mobile HIPERLAN nodes and unfavourable propagation characteristics can cause the fragmenta- tion of a network. The following network topologies exist as a result of the channel character- istics and the applications presented: Independent HIPERLANs Two HIPERLANs, A and B, are considered to be independent of each other if no member from HIPERLAN A is located in the transmission range of a member from network B (see Figure 13.2). Even if the same frequencies are used for transmission in both networks, it is assumed that subnetworks A and B do not share the communica- tion medium and therefore also do not cause any interference in each other’s network. HIPERLAN A could be an ad hoc network set up for 660 13 Wireless Local Area Networks CN CN CN CN CN CN CNCN A n B 1 B 2 B n A 1 A 2 A n-1 B n-1 Figure 13.3: Overlapping HIPERLANs a conference of company X, and HIPERLAN B an LAN used in the factory of company Y. Overlapping HIPERLANs If the radio range of some of the stations in net- work A should overlap with some of the stations in network B then these members share the communications medium and its transmission capacity in the area where the overlapping occurs (see Figure 13.3). An overlapping of networks produces two effects: • The senders in the different HIPERLANs use the same frequency band, thereby increasing the occurrences of interference. As a re- sult, optimal use of the frequency band is no longer possible because not all the stations are able to receive from each other (hidden sta- tions) and therefore can cause interference. • A station receives data packets from several HIPERLANs with dif- ferent HIDs. All received data packets are evaluated, and only those with their own HIDs are accepted. As a result, there is a decrease in the maximum possible data transmission capacity and consequently also the data transmission rate in this area. These effects can be reduced if several frequency channels are introduced. With HIPERLAN, up to five frequency channels in the 5.15–5.30 GHz range are provided. Multihop networks In addition to their original role as transmitting and re- ceiving stations for their own terminals, some stations in a multihop network also perform the function of relay stations. This allows data to be transmitted over larger distances despite the restricted reachable range of the radio medium. In Figure 13.4 the relay stations (forwarders) 2, 4 and 6 are forwarding the traffic of node 1 to destination 7. Interworking Since most HIPERLAN applications already exist, it must be possible for HIPERLANs to be connected to the usual fixed networks 13.4 HIPERLAN Reference Model 661 CN CN CN CN CN CN CN A 7A 6A 3 A 4 A 5 A 2A 1 Figure 13.4: Communication in a multihop radio network CN CN CN CN CN CN CN CN CN CN CN CN CN CN CN CN CN . CN A n-1 A n A 2 A 1 . B1 B 2 B n B n-1 Figure 13.5: Connection to fixed network (see Figure 13.5). This affects the network layer, and is not part of the HIPERLAN/1 standard. 13.4 HIPERLAN Reference Model The HIPERLAN reference model defines the components needed to install a private radio subnetwork. It is based on the ISO/OSI reference model and consists of the Medium-Access Control (MAC) sublayer, the Channel-Access Control (CAC) sublayer and the physical layer; see Figure 13.6. The organizational part of access is described in the MAC sublayer, access to the radio channel takes place in the CAC sublayer. HIPERLAN provides an ISO-8802 standard interface. In accordance with the OSI service user/service provider model, each layer provides services for the layer above it. These services are offered at the 662 13 Wireless Local Area Networks Application Layer Presentation Layer Session Layer Transport Layer Network Layer Data Link Layer Physical Layer Medium-Access Control Sublayer Physical Layer Channel-Access Control Higher Layer Protocols Sublayer Figure 13.6: ISO/OSI and HIPERLAN reference models HCPDU HIPERLAN CAC Sublayer HC-Entity HC Entity HCSAP HCSAP HIPERLAN CAC Service HMPDU HIPERLAN MAC Sublayer HM-Entity HM Entity MSAP MSAP HIPERLAN MAC Service HIPERLAN Physical Layer Figure 13.7: Service model for MAC and CAC sublayers service access points between the layers, and are controlled by exchange of service primitives (SP). The individual sublayers are described in more detail in the following sec- tions. Figure 13.7 shows the HIPERLAN service model. 13.5 HIPERLAN-MAC Sublayer 13.5.1 Tasks of the MAC Sublayer The MAC sublayer provides the following functions to ensure smooth and reliable HIPERLAN operation. 13.5.1.1 MAC Address Mapping Because HIPERLAN stations share the radio channel, an overlapping of neigh- bouring HIPERLANs can occur that causes the radio ranges of a number of wireless networks to overlap in the same radio channel (see Figure 13.3). 13.5 HIPERLAN-MAC Sublayer 663 A restricted radio range, the mobility of the stations and unfavourable propagation conditions can cause the fragmentation of a HIPERLAN (see Figure 13.2), although A and B are separate subnetworks of the same HIPER- LAN. The standard defines internal address structures. A HIPERLAN address consists of a HIPERLAN name (HID) and a station identification (NID). The HID is used by the MAC protocol to differentiate between the MAC commu- nication of the individual HIPERLANs. If there is a dynamic allocation of the HID because of the possible overlapping of cells, it is less likely that mixed communication will occur. The MAC protocol reserves the use of special HIDs for communication between the stations of neighbouring HIPERLANs. It is easier for a user to identify his network using a name rather than the numerical HID. The name is also used by the lookup function for establishing which HIPERLANs are operating in the area. Since no clear guidelines and no administrative coordination exist for HIPERLAN identification, it is possible for a MAC communication to take place from non-distinguishable LANs. This situation is very unlikely, because of the MAC identification scheme used. The address mapping function converts IEEE-MAC addressing into HI- PERLAN addressing. 13.5.1.2 Communication Security A radio channel can be listened for in a neighbouring area. An encryption algorithm with the appropriate cryptographic management is therefore pro- vided in the MAC sublayer. This protects confidential data from unauthorized eavesdropping, and also guarantees communication security for the radio net- work. The HIPERLAN encryption scheme provides for a common set of keys, one of which is used in the encryption operation. Each key has a number that is transmitted with the encrypted data to the receiver. In addition, a common initialization vector for encryption and decryption is required and if necessary transmitted. The level of transmission security increases with the frequency of the change of key and initialization vectors. 13.5.1.3 Addressing of Service Access Points For compatibility with the ISO-MAC service definition the MAC service (HM service) uses 48-bit LAN-MAC addresses for identification of MAC service access points (MSAP) (see Figure 13.7). The standard recognizes separate addresses for individual MAC service access points and group addresses for contacting several MSAPs. An HM entity is linked to a single MSAP, through which it offers MAC services. It is also connected to a single HIPERLAN-CAC Service Access Point (HCSAP), over which it uses the HIPERLAN-CAC services. 664 13 Wireless Local Area Networks CN CN CN CN CN CN CN CN CN CN A 8 A 5A 4 A 6 A 7 A 9 A 10 A 1 A 2 A 3 Figure 13.8: Broadcast transmission CNCN CN CN CN CN CN CN CN CN A 9 A 10A 8 A 7A 6A 5A 4 A 1 A 2 A 3 Figure 13.9: Unicast transmission A single 48-bit LAN-MAC address is used as an MSAP address for ad- dressing the service access point, the HM entity and the users of HIPERLAN- MAC services. An address of the same length is used to identify a group of MSAPs and the users associated with it. 13.5.1.4 Forwarding The MAC protocol incorporates a multihop relaying facility that enables the transmission of data beyond the boundaries of a station’s sending area—in other words over several stations. An HM entity is either a forwarder or a non-forwarder. Only forwarders carry out the forwarding of MSDUs when re- quired. Point-to-point (unicast) as well as broadcast transmission (broadcast, multicast) is possible for the transmission of packets. Broadcast relaying is used to relay information to all HM entities or when the transmission route is not known. Each station that recognizes a route to the destination station forwards the data packet accordingly. To avoid data packets from being routed by several stations at the same time, the protocol ensures that only a limited number of stations can forward data. Figure 13.8 shows a broadcast transmission being sent by station 4. It is more efficient to forward packets that are addressed to a particular receiver if a unicast transmission is used (see Figure 13.9). A packet is then routed to its destination through successive hops in accordance with an (op- timal) route. Broadcast relaying must be used if the route is not known, as explained above. Each HM entity collects and manages routing information in its Routing Information Base (RIB). This information is continuously updated. The data in the RIB becomes obsolete and is discarded once it has passed its validity period. Because of this constant updating of routing information, it is even possible to specify quasi-optimal paths for the forwarding of packets in con- tinuously changing HIPERLANs. [...]... Mechanism (CAM) It is here that it is decided during di erent phases which station has access to the radio channel to enable it to send asynchronous or time-critical traffic over the physical layer to the receiver The CAC sublayer contains the functions described below 13.6.1.1 Selection of HIPERLAN Radio Channel Although the CAC protocol for multiple radio channel operation is defined, channel selection... their parameters are listed in Table 13.4 In addition to the quality of service parameters, the following parameters are also used: Source Address (SA) The individual sending address of the MSAP Destination Address (DA) Address of an individual MSAP or a group of MSAPs to which a packet is directed MAC Service Data Unit (MSDU) Data that is transported without modification by the HM service provider between... In a synchronized HCSDU transfer the transmissions directly follow each 13.6 HIPERLAN-CAC Sublayer Site A Layer N +1 Layer N Site B Layer N +1 HC_Status_indication Site A Layer N +1 HC_Sync_Indication HC_Sync_Indication t SYNC HC_Unitdata_Request 679 HC_Unitdata_request t SYNC HC_Unitdata_Indication Layer N Site B Layer N +1 tSYNC tSYNC HC_Status_Indication Figure 13.16: Sequence of primitives with... sublayer notifies the MAC sublayer accordingly through an HC Sync Indication when the new channel-access cycle (tSYNC ) begins This signals to the HCS user that the CAC sublayer can accept data for the following access cycle on the radio channel If the MAC sublayer has HCSDUs available for transmission, it must, as soon as it receives the HC Unitdata Indication message, immediately create an HC Unitdata Request... synchronize itself If the radio channel is Idle for a longer period of time, the HCS provider notifies the MAC sublayer accordingly through HC Free Indication The CAC sublayer thereby indicates that the HCS user can invoke the HCSDU transfer function at any time until further notice If there is a change to the channel status, the HCS user is notified through an HC Status Indication 680 13 Site A Layer... Area Networks Site B Layer N +1 HC_Free_Indication HC_Status_Indication Figure 13.18: Sequence of primitives for ready-to-transmit state when channel is idle Site A Layer N +1 Layer N Site A Layer N +1 Site B Layer N +1 Site B Layer N +1 HC_Free_Indication HC_Free_Indication HC_Unitdata_Request HC_Unitdata_Request HC_Status_Indication Layer N HC_Unitdata_Indication Figure 13.19: Sequence of primitives... longer prepared to carry out an HCSDU transmission The following parameters are used in addition to channel-access priority: Source address Individual address of the HCSAP sender Destination address Address of an individual HCSAP or a group of HCSAPs to whom a packet is directed CAC service data unit Data not requiring modification by the HC service provider that is transported between HCS users HIPERLAN... demodulation of a prescribed bit stream on the carrier of a radio connection • Establishment and maintenance of bit and packet synchronism between senders and receivers • Transmission or receipt of a prescribed number of bits over a special carrier frequency at a requested time • Addition and removal of synchronization sequence • Coding and decoding in accordance with a forward error-correction scheme... physical layer scans the radio channel at regular intervals, and notifies the CAC sublayer of the channel status through an HPh ChannelState Indication This process is illustrated in Figure 13.25 If the channel is free, the CAC sublayer is signalled that data can be transmitted Table 13.11 presents an overview of the di erent possible channel states Data cannot be transmitted directly, but instead must... occur in the di erent layers if a transfer takes place 13.7.3 Transmission Rates and Modulation Procedures Acknowledgements and data packets are modulated di erently and with different signal rates by the physical layer Data packets are modulated with 688 13 Site A Layer N +1 Layer N Wireless Local Area Networks Site B Layer N +1 HPh_Channel-State_Indication HPh_Unitdata_Request HPh_Unitdata_Indication . in the form of his computer with a radio- LAN connection. Medicine Within a radio- LAN, doctors would be able to have direct and interactive access to remote. restriction of the radio medium (e.g., ra- dio range), both standards must contain functions for the management and maintenance of the radio network that