1 Supplementary material: Digital Enhanced Cordless Telecommunications (DECT) 1.1 Introduction History Analog cordless phones came into use in the early 1980s. They enabled communications between handsets and BSs that were at most a few hundred meters away. The BS served as an interface to the public telephone network or to a private branch exchange. Around 1990, di erent digital cordless phone standards were developed. The digital systems o er a larger variety of services and options than the analog versions. In this chapter we give a brief overview of the DECT standard, which was developed by ETSI (European Telecommunications Standard Institute). One purpose of introducing DECT to the reader is to show the di erences between a cellular standard (GSM, see Chapter 21) and a cordless standard. This comparison illustrates how systems for di erent usage scenarios di er in their design parameters. The reader may infer from this how the target application is re ected in the choice of some basic parameters of, for example, the air interface. A very brief overview of the worldwide available digital cordless phone systems is provided as well Usage scenarios and basic structure One of the goals of the development of DECT was to specify a standard that supports a variety of application scenarios. These include cordless private phones, cordless o ce phone systems, systems for the public sector (quasi-cellular systems), cordless Local Area Networks (LANs) and wireless access techniques for public networks without mobility support (Wireless Local Loop ± WLL). 1 A DECT system consists of one or more Mobile Stations (MSs) 2 and one or more Base Stations (BSs). The MSs are connected to the BS via the speci ed air interface. One BS can serve up to 12 MSs at the same time. Normally, the BSs are connected directly to the public or private network. However, it is also possible that several BSs may be connected to an optional 1 Due to various deregulation issues this latter application never became practically important (see also Section 1.1). 2 In the DECT literature, the expressions ``portable'' and ``portable terminal'' (PT) are often used instead of the ``Mobile Station (MS)'' we use in this book. This is motivated by the fact that even though DECT supports some mobility (handover, etc.), the possible speed of the terminals and the cell ranges are rather limited when compared with the Global System for Mobile communications (GSM). Wireless Communications Andreas F. Molisch # 2005 John Wiley & Sons, Ltd
2 2 Supplementary material: Digital Enhanced Cordless Telecommunications (DECT) unit that controls all of them; in this case, this unit establishes the connection to the public network or the private branch exchange. In the case that a system consists of several BSs that are located in di erent places, a picocellular structure of the system may be established. When an MS is leaving the coverage area of one BS a handover may be performed to another BS associated with the same system. However, in contrast to cellular systems, DECT does not require assigning one particular carrier to each cell, so that no frequency planning is required. Private cordless phones The simplest setup for a DECT system in a private environment consists of one MS and one BS. Usually, this BS serves also as the charging station for the MS and is connected to the Public Switched Telephone Network (PSTN). Most of the available DECT BSs are capable of supporting several MSs and can connect to several PSTN lines or ISDN (Integrated Services Digital Network). If multiple MSs are used in the same DECT system, the calls from one MS to the other MS are free of charge, because they do not involve the PSTN. Of course, the calls to the regular landline networks are charged at the normal (landline) rates by the provider. Cordless o ce communication systems Cordless o ce communication systems usually support the same services as regular private branch exchanges. A picocellular setup is often used to provide enough capacity and to maintain coverage over bigger o ce complexes or commercial buildings. The setup normally consists of a regular wired switching unit and an additional entity supporting the cordless functionality. The additional unit has (wireline) connections to the DECT BSs. This unit supports the mobility management functions of the cordless system ± e.g., it enables the handover between di erent BSs. Regular (landline) devices, such as o ce phones, can also be connected to the private branch exchange. Cordless phone systems in the public sector ± quasi-cellular systems The DECT standard supports setting up a cluster of BSs that provide access to the PSTN. This cluster consists of picocells. As the range of the DECT BSs is rather limited, the density of BSs per coverage area must be bigger than for GSM, for example. DECT requires a density of roughly 100 BSs per square kilometer. Thus, it is not feasible to provide nationwide coverage with DECT. However, the coverage of so-called hotspots, like airports, train stations, or even entire city centers, with a public DECT system can be attractive. Similar to GSM, a user is required to have a contract with a particular provider. This provider then charges the user (subscriber) for the accessed services. In contrast to GSM, DECT relies partly on the functionalities (like call forwarding, etc.) of modern landline networks (ISDN) to support the user's mobility. ETSI developed special standards ± Cordless Terminal Mobility (CTM) ± to enable MSs that can operate in a private system, an o ce setup, or in the public sector. These devices are able to automatically access one of the services, depending on the availability and the user preferences. 3 Furthermore, there is a standard specifying the cooperation between DECT and GSM, the GSM Interworking Pro le (GIP). The DECT BSs may therefore be connected to the GSM 3 For example, the portable tries to use the private-owned BS as much as possible. However, once the coverage of this BS is left the portable accesses the public DECT network of a particular provider.
3 Wireless Communications 3 Mobile Switching Centers (MSCs). So-called dual-mode MSs have been developed that are capable of accessing both the GSM and the DECT air interface. All the provisions mentioned above proved to be, however, rather unsuccessful ± DECT did not gain much acceptance for quasi-cellular systems. In the European and American market, cellular systems were built in such a way that they do not to require additional hotspot service. In Japan, the Personal Handyphone System (PHS), which has similar characteristics to DECT, is used instead of DECT. PHS supported services for up to 45 million subscribers in the public sector. 4 Cordless local area networks DECT supports the integration of voice and data communications over the same air interface. There are several cordless LAN products based on DECT. Data rates of up to 552 kbit=s are feasible with DECT. The medium access functions of DECT support burst and asymmetric tra c. DECT lost most of its importance in this sector to the IEEE 802:11 standards. Nevertheless, there are several important applications based on DECT ± e.g., cordless terminals in gastronomy. Wireless local loops WLLs or Radio Local Loops (RLLs) are radio systems replacing the wired connections on the so-called last mile ± i.e., the connection between the end user and the local switching center (see also Chapter 1). WLLs are interesting in countries where no su cient wired infrastructure is available and services should be established fast, like fast-developing countries or postwar areas. In the U.S. and Europe, WLLs were interesting for providers that wanted to enter the deregulated market 5 at the end of the 1990s. DECT had a market share of more than 30% in this sector. However, since the turn of the century, DECT faces increased competition in this eld by standards specially developed for such scenarios ± e.g., IEEE 802: Digital enhanced cordless telecommunications application pro les To support the access to di erent networks ± e.g. PSTN, ISDN and GSM ± so-called application pro les have been speci ed. The DECT application pro les contain additional speci cations, which de ne how the DECT air interface supports particular applications and how the di erent devices interact with each other. With these speci cations the interoperability of units from di erent manufacturers is ensured. Furthermore, the pro les allow adaptation of DECT signaling to the requirements of the networks accessed The digital enhanced cordless telecommunications protocol stack The protocol architecture of DECT is based on the Open Systems Interconnection (OSI) layer model. The DECT standard speci es the lower three layers of the OSI model: the PHysical Layer (PHL), the Data Link Layer (DLL), and the network layer. Figure 1.1 illustrates the DECT protocol stack. In DECT the data link layer is divided into the Data Link Control (DLC) and Medium Access Control (MAC) layer. The functionality above the MAC layer is divided into a control plane and a user plane (C-plane and U-plane). The C-plane handles the controlling 4 As in March 2001, source: NTT Docomo. 5 Until the 1990s, most European countries had a single state-owned provider for the regular telephone services. Similarly, the U.S. had a ``regulated monopoly'' until the mid-1980s.
4 4 Supplementary material: Digital Enhanced Cordless Telecommunications (DECT) Figure 1.1 Digital enhanced cordless telecommunications protocol stack. Adapted with permission from Walke , copyright John Wiley & Sons. and signaling functions, whereas the U-plane is concerned with the actual user data transmission. 6 The idea behind the di erent planes in the DLC is that the transmission of user and controlling data has to be ensured by di erent means ± e.g., the successful transmission of control data is ensured via an Automatic Repeat request (ARQ) procedure, which is unnecessary for voice transmissions. The network layer only processes packets in the C-plane, whereas a transparent link is provided in the U-plane. The so-called management entity deals with cross-layer functions and therefore cannot be integrated in the OSI model. 1.2 The physical layer The DECT standard mainly de nes the air interface for cordless phone services. Table 1.1 gives an overview of the important parameters (see Tuttlebee , ETSI , Molisch et al. [1998a]) Multiple access and duplexing Frequency Division Multiple Access DECT employs a combination of FDMA/TDMA with Time Division Duplex () for the multiple access on the air interface. The used frequency span from 1880 to 1900 MHz is divided into ten FDMA bands: f c ˆ 1,897:344 MHz i 1,728 khz with i ˆ 0; 1; :::; 9 1:1 so that the spacing between carriers is 1:728 MHz, and guardbands at the upper and lower end of the frequency band reduce the out-of-band interference. The carrier should have no more than 50 khz o set from this de nition during transmission. 6 A similar partitioning of the layers into planes can be found in the ISDN speci cations.
5 Wireless Communications 5 Table 1.1 DECT parameter. Parameter Frequency range Medium access DECT 1,880±1,900 MHz FDMA/TDMA/ Number of carriers 10 Channel access procedure Distance between carriers DCS MHz Modulation scheme GFSK (BT ˆ 0.5) Gross data rate Slots per timeframe Duration of each timeframe Peak transmission power Voice encoding 1,152 kbit/s 24 full slots 10 ms 250 mw 32 kbit/s ADPCM Time Division Multiple Access The time axis is segmented into TDMA frames of duration 10 ms, during which a total of 11,520 bits are transmitted (hence the bit rate of 1:152 Mbit/s). Each frame consists of 24 timeslots. The rst 12 slots are used for the downlink, whereas the second 12 slots serve for the uplink. Every voice communication radio link has a duplex channel (uplink and downlink) assigned. Thus, up to 12 connections can be supported per carrier. Every BS and every MS can access all of the 10 carriers. All in all, 120 duplex links are possible (see Fig. 1.2). Changes of the carrier frequency timeslot RFchannel downlink uplink frame (10 ms) Figure 1.2 Division of the time and frequency in timeslots and subbands. Adapted with permission from Walke , copyright John Wiley & Sons.
6 6 Supplementary material: Digital Enhanced Cordless Telecommunications (DECT) Figure 1.3 Structure of a frame for basic connection. are possible from one timeslot to the next. Therefore, connections to the same BS can use di erent carriers for di erent timeslots. However, in order to access multiple carriers simultaneously, the BS needs multiple transceivers. When only one transceiver is implemented at the BS only one carrier can be used in each timeslot. Thus, the number of duplex connections is limited to 12 per transceiver BS. As stated above, a frame usually contains 24 full slots. A full slot is used to transmit regular data packets, Basic Packet P32, or short packets for signaling purposes, Short Packet P00. Furthermore, a full slot may be divided into two half-slots, which may be used to transmit short data packets, P08j. Two full slots my be combined to form a double slot during which a long data packet, P80, may be transmitted. Figure 1.3 illustrates the division of a frame in slots. The basic connection, regularly used for the voice transmission, uses the rst 12 slots for the downlink and the second 12 slots for the uplink. If the downlink is located on slot number k the uplink uses slot number k 12. Advanced connections have a more exible assignment. Structure of the di erent packets in the physical layer User and control data are transmitted in packets. As mentioned above there are four di erent kinds of packets and Figure 1.4 gives an overview of their form. All packets consists of a 32-bit S- eld for synchronization and a D- eld of variable length (depending on the kind of packet), which contains the user and control data. Its composition is explained in Section After the D- eld a Z- eld may be transmitted, which repeats the last 4 bits of the D- eld. The receiver compares the two versions of these 4 bits to recognize interferences by 1. P32: This packet type is used by regular voice transmission connections, as well as other services. The S- eld consists of 32 bits; the rst 16 of which are alternating 1 and 1 and serve as a preamble that can be used for timing and frequency acquisition. The second 16 bits support the packet synchronization; for their speci c values, please refer to the standard [ETSI 1992]. The D- eld, which follows the S- eld, carries a total of 388 bits and is composed in the MAC layer (see Section 1.3.1). Of these 388 bits, up to 320 bits of the D- eld are actually user data. Given the frame duration of 10 ms this results in a data rate of 32 kbit=s. A Z- eld may follow the D- eld. The duration of a full slot corresponds to 480 bit durations. A P32 packet is 56 bit durations shorter than the actual timeslot. 7 This transmission gap serves as a guard time between packets to ensure that packets originating from di erent MSs are not overlapping at the receiver. 7 When no Z- eld is transmitted, it is 60 bit durations shorter.
7 Wireless Communications 7 Figure 1.4 The di erent packet types of the physical layer. Adapted with permission from Walke , copyright John Wiley & Sons.
8 8 Supplementary material: Digital Enhanced Cordless Telecommunications (DECT) 2. P00: The P00 packet is used for transmitting brief information ± e.g., about control functions. Its D- eld has only 64 bits and it is never sent with a Z- eld. Therefore, it features a much bigger guard time (384 bits) than the other packets. 3. P08j: P08j packets are used in half-slots. 8 The D- eld consists only of 148 bits. The Z- eld is optional. Thus, the guard time is again 56 or 60 bits long. 4. P80: P80 packets occupy double-slots for transmission. The D- eld is 868 bits long, of which up to 800 bits are pure user data. This results in bit rates of up to 80 kbit=s Modulation and transmit power Modulation format The modulation used in DECT is Gaussian Minimum Shift Keying (GMSK) 9 with a timebandwidth product B G T ˆ 0:5: Thus, a logical 1 re ects a positive frequency shift and a logical 1 a negative shift (compare Chapter 11). The frequency modulation index is speci ed to be 288 khz, but the implementation may di er within some tolerance range. As the range of the devices is rather limited, the delay spread due to multipath is usually much smaller than the symbol duration. Therefore equalizers are not commonly implemented and no training sequence is foreseen in the standard. Transmission power and receiver sensitivity The peak transmission power of a DECT MS is 250 mw. As the MS is transmitting only in 1 of 24 slots per frame its average transmission power is about 10 mw. The speci cations require a receiver sensitivity of at least 86 dbm to achieve a Bit Error Rate (BER) of As the voice encoding used (ADPCM) is quite robust, no forward error correction is required and a BER of less than 10 3 results in su cient voice quality Voice encoding DECT uses Adaptive Di erential Pulse Code Modulation (ADPCM) with a data rate of 32 kbit=s for voice encoding (see Chapter 15). This voice encoding provides very good voice quality, comparable with that achieved in ISDN, and is rather robust against single-bit errors Security During setup of a connection the BS may evaluate whether the MS has access rights. The BS sends a random sequence as a so-called challenge to the MS. From the challenge and a secret key, the MS computes the response, which it transmits back to the BS. The BS compares the response with the result it calculated itself by applying the secret key to the original random sequence. A matching of the results veri es that the MS holds the secret key. Intercepting the challenge and the response is of course possible. However, it is extremely complex to infer the actually used secret key from the challenge/response pair. The secret key itself is set up only once, often involving the manual entry of a PIN in the device. The secret key may also be used to encrypt the transmitted data. Encryption is part of the standard but not mandatory. 8 If half-slots are used, up to 24 duplex connections per carrier are possible. 9 More precisely, Gaussian Frequency Shift Keying (GFSK) with modulation index 0:5, which results in a nominal frequency deviation of 288 khz. However, deviations of the modulation frequency are allowed; frequencies between 202 and 403 khz are admissible.
9 Wireless Communications 9 Similar to the GSM Subscriber Identity Module (SIM), a DECT Authenti cation Module (DAM) can be used to distinguish between subscriber and used device Comparison between digital enhanced cordless telecommunications and global system of mobile communications Table 1.2 compares several technical parameters of DECT with those of GSM. 1.3 Medium access control layer and connection setup Medium access control layer The MAC layer accomplishes three things:. It is responsible for the assignment and access to the physical medium.. It matches and multiplexes the logical channels, which may either contain user or control data from higher layers, onto the di erent elds of the transmitted channel.. It establishes reliable transmission by backward error correction (ARQ) of the control data and ± if requested ± the user data. Assignment and access to the physical medium The MAC layer is responsible for the transmission and evaluation of the beacon and broadcast channel at the BS and the MS, respectively. Furthermore, the MAC layer hosts the functions for dynamic channel selection (see below) ± e.g., the transmission and evaluation of the scanning sequence of the BS. Furthermore, the MAC layer may support the association of multiple physical channels with one connection to achieve higher data rates. Multiplexing of the logical channels and backward error correction Multiframe Sixteen of the regular timeframes form a multiframe. The multiframe structure allows distribution of the data from the di erent logical channels over several transmission packets. D- eld All transmission packets contain a D- eld, whose length depends on the type of packet that is transmitted. The composition and evaluation of this D- eld is accomplished in the MAC layer. In the following, we describe the composition of the D- eld in the P32 packets, which is illustrated in Fig The D- eld has a length of 388 bits and is subdivided in the A- eld for control data, which contains 64 bits, and the B- eld, which consists of 324 bits, mainly for user data:. A- eld: The A- eld consists of 48 bits of signaling information, and is protected by a 16-bit CRC eld. It is thus used to provide a permanent signaling channel. This channel is used to transmit control data, like MAC-layer information, higher layer information, the broadcast channel for paging, BS ID, etc, or the dynamic link control.. B- eld: The B- eld might be used to transmit user data in unprotected mode or in protected mode. Unprotected mode is used for voice communications. In this case the 324 bits of the B- eld contain 320 bits of actual user data and a 4-bit X- eld, which is a Cyclic
10 10 Supplementary material: Digital Enhanced Cordless Telecommunications (DECT) Table 1.2 Comparison of the technical parameters of Digital Enhanced Cordless Telecommunications (DECT)/Global System for Mobile communications (GSM). Parameter DECT GSM Frequency range 1,880±1,900 MHz 880±915 MHz (uplink) 925±960 MHz (downlink) 1,710±1,785 (uplink) 1,805±1,880 (downlink) 1,850±1,910 (uplinkðu.s.) 1,930±1,990 (downlinkðu.s.) Medium access FDMA/TDMA/ FDMA/TDMA/FDD Selection of physical channel DCS Fixed channel allocation Intracell handover Frequency hopping Carrier distance MHz 0.2 MHz Modulation approach GMSK B G T ˆ 0:5 GMSK B G T ˆ 0:3 E ective frequency usage 144 khz/channel 50 khz/channel per duplex speech connection Gross bit rate on the air interface 1,152 kbit/s 271 kbit/s Symbol duration 0.87 ms 3.7 ms Channels per carrier 24 full slots 8 full slots (32 kbit/s user data) (13 kbit/s user data) Frame duration 10 ms 4.6 ms Maximal RF transmission power 250 mw 2 W at the MS Voice encoding 32 kbit/s 13 kbit/s ADPCM RPE-LTP Diversity Antenna diversity (opt.) Channel coding with interleaving Channel equalization Antenna diversity (opt.) Frequency hopping (opt.) Maximal cell range Ca. 300 m (outdoor) 35 km Dedicated physical channels for Not necessary Necessary signaling Power control Optional Mandatory Redundancy Code (CRC) eld for the other 320 bits. Note that this X- eld is the eld which is copied into the Z- eld. In protected mode the 320 bits of the B- eld are divided into blocks of 80 bits each. Of these 80 bits, 64 are actual user data and 16 bits are a CRC- eld (RB- eld). The X- eld is the same as in unprotected mode. The e ective data rate in protected mode is 25:6 kbit=s instead of 32 kbit=s. In special cases the B- eld may also be used for the transmission of signaling data.
11 Wireless Communications 11 Figure 1.5 Composition of a P32 packet. Adapted with permission from Walke , copyright John Wiley & Sons Dynamic channel selection DECT uses a dynamic decentralized procedure to select the physical channel for each connection. This procedure is called Dynamic Channel Selection (DCS). This procedure is decentralized in the sense that every MS selects individually the carrier for its link. It is dynamic in the sense that the MS or the BS may change the carrier from one frame to the next frame. DCS is a very e cient way of accessing the channels. Each DECT device monitors the channels it does not use to transmit or receive. The MS then identi es the link to a BS that gives the highest signal power and the least interference Continuous broadcast service Each DECT BS transmits continuously on at least one channel. This is called the beacon channel. This beacon allows an MS to log on to a BS. The beacon may be transmitted within a channel of a regular duplex connection or, in the case that there is no regular connection, the BS transmits a so-called dummy bearer, which only contains the beaconing signals. The beaconing channel transmits broadcast information. This information is multiplexed over several frames (multiframe) and includes the BS identity, the supported services, the status of the BS, and paging signals for the associated mobiles. The beacon allows an MS to synchronize to the frame and slot timing of the BS. Afterwards, it evaluates the signaling information transmitted via the broadcast channel to check whether the MS has access rights to the BS, whether its requested services are supported, and whether a duplex channel is available, in case a connection has to be established Establishing a connection In order to establish a connection from or to the MS, it has to be within range of a BS to which it has access rights and synchronized to the frame timing of this BS. It is always the MS that
12 12 Supplementary material: Digital Enhanced Cordless Telecommunications (DECT) initiates the channel request, both when the MS initiates a call, and when the BS pages the MS because of an incoming call. Connection requested by the MS The MS selects the best available channel and starts to transmit on it. 10 The BS scans the unused channels sequentially to detect when a mobile is accessing one of these. The scan sequence is transmitted via the broadcast channel. Knowing this sequence the MS selects the frame during which the BS observes the chosen channel, to send the rst burst. This ensures that the BS immediately detects the channel access of the MS. Connection requested by the BS In case there is an incoming connection request from the network the BS pages the designated MS via the broadcast channel. Upon receiving this paging signal the MS initiates a connection in the same manner as described above Handover DECT o ers the functionality to hand over a connection to another BS, a so-called intercell handover, or to switch the channel within a cell, a so-called intracell handover. The MS is constantly observing the quality of the free channels. In case the channel in use is subject to interferences ± e.g., from neighboring cells ± the MS initiates an intracell handover to one of the free channels with low interference. Since the uplink may be corrupted by interferences of which the MS is not aware, also the BS can request an intracell handover via the signaling protocol. When the MS is in the process of leaving the coverage area of the BS the signal strength and the quality of the link decrease. By observing the unused channels the MS detects the beacons of the neighboring BS. When the signal strength of the neighboring BS is larger than the one of the current BS the MS starts to access a new free channel in this cell and thus initiates a handover. DECT performs seamless handover by rst establishing a second channel, only dissolving the original channel after the second channel is operative. 1.4 Overview of other digital cordless phone standards Table 1.3 provides an overview of the di erent digital cordless phone systems and their air interfaces European standards In addition to DECT, also the Cordless Telephone 2nd Generation (CT2) standard was developed in Europe. It is also known as the Common Air Interface (CAI), and was the rst standard for digital cordless phones. It gained some popularity in the early 1990s for telepoint services, but has now mostly vanished. 10 Let us note that the slots during which another connection to the BS is already in use are so-called blind slots. During these slots the channels on other than the used carriers cannot be used, because the BS cannot support them. These channels are excluded from the set of free channels out of which the mobile chooses the best one.
13 Wireless Communications 13 Table 1.3 Some parameters of the air interfaces of the di erent cordless systems. System Frequency Carrier Modulation Multiple Gross bit rate Frame Speech range distance scheme access on the channel duration channels per (MHz) (khz) (kbit/s) (ms) carrier DECT 1,880±1,900 1,728 GFSK FDMA 1, (Europe) TDMA CT2 864± GFSK FDMA (Europe) PHS 1,895±1, =4-DQPSK FDMA (4) (Japan) TDMA PACS 1,850±1, =4-DQPSK FDMA (licensed in TDMA the U.S.A.) FDD PACS-UA 1,910±1, =4-DQPSK FDMA (unlicensed in TDMA the U.S.A.) PACS-UB 1,910±1, =4-DQPSK FDMA (unlicensed in TDMA the U.S.A.) PWT 1,910±1,930 1,250 =4-DQPSK FDMA 1, (unlicensed in TDMA the U.S.A.) PWT-E 1,850±1,990 1,000 =4-DQPSK FDMA 1, (licensed in TDMA the U.S.A.) PCI 1,910±1, GFSK FDMA (unlicensed in the U.S.A.) Japanese standards The Japanese standard, Personal Handyphone System (PHS), is similar to the European DECT. The used frequencies' range is 1895±1918 MHz. In contrast to Europe, the frequency band is divided into two subbands, one of which is reserved for public (quasi-cellular) services and the other one is used for both public and private applications. Three di erent providers started to o er services based on PHS in All of them started to o er a 32-kbit/s data transmission service in Initial capacity shortages were overcome after 1999 by the widespread implemention of smart antennas (see Section 20.1) North American standards The frequency band for Personal Communication Systems (PCS) is divided into a licensed and an unlicensed subband in the U.S.A.:
14 14 Supplementary material: Digital Enhanced Cordless Telecommunications (DECT). Access to the licensed bands is bought by the provider at an auction. Providers may use their licensed frequency range to implement any mobile communication standards they choose.. Devices according to a standard or to proprietary solutions can be used in the unlicensed band from 1910 to 1930 khz. So-called etiquette rules, which all devices must follow, ensure the coexistence of the di erent systems without excessive mutual interference. Especially, the transmission power has to be limited. Therefore, the devices operating in this band are mainly used indoors in the private and the business sector. In the licensed PCS band, the Personal Access Communications Systems (PACS) standard is widely used. It was designed mainly for use in wireless local loops, but can also be employed for microcellular systems. A combination of TDMA and FDD is employed. A derivative (PACS-UB), which uses, is used in the unlicensed band. The modulation format is =4- DQPSK and a timeframe has a duration of 2:5 ms. In addition, several systems based on spread sprectrum techniques are used in the unlicensed spectrum, especially in the 2.45-GHz ISM band. 1.5 Acronyms ADPCM CAI CAP CT2 CTM DAM DCS DECT DQPSK ETS ETSI GAP GFSK GIP GMSK GSM IAP IIP IN IS ISDN LAN LPC MAC MSC OSI PACS PACS-UA PACS-UB PCI Adaptive Di erential Pulse Code Modulation Common Air Interface CTM Access Pro le Cordless Telephone 2nd Generation Cordless Terminal Mobility DECT Authentication Module Dynamic Channel Selection Digital Enhanced Cordless Telecommunications Di erential Quadrature Phase Shift Keying European Telecommunication Standard European Telecommunication Standards Institute Generic Access Pro le Gaussian Frequency Shift Keying DECT/GSM Interworking Pro le Gaussian Minimum Shift Keying Global System for Mobile communications ISDN Application Pro le ISDN Interworking Pro le Intelligent Network Interim Standard Integrated Services Digital Network Local Area Network Linear Prediction Coding Medium Access Control Mobile Switching Center Open System Interconnection Personal Access Communications System Personal Access Communications System ± Unlicensed A Personal Access Communications System ± Unlicensed B Personal Communications Interface
15 Wireless Communications 15 PCS PDC PHL PHS PWT PWT-E RAP RLL WLL Personal Communication Systems Personal Digital Cellular PHysical Layer Personal Handyphone System Personal Wireless Telecommunications Personal Wireless Telecommunications ± Enhanced RLL Access Pro le Radio in the Local Loop Wireless Local Loop References Walke 2001 B. Walke, Mobile Radio Networks: Networking, Protocols and Tra c Performance (2nd edn), John Wiley & Sons (2001).