`
`10.3 WLAN Standards
`
`243
`
`AHLAN 670
`
`
`-//HLAN655
`
`
`
`AFILAN 655
`
`Figure 10.1 Access application for WLAN.
`
`_lreless
`-RLAN655
`
`_epeater-
`
`-CMC|A
`
`ARLAN 670
`
`ARLAN 620
`
`Up to 6 Mlles
`
`
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`mechanism. Once it is determined that the network is free, the end sta-
`
`tion ramps up to full power and sends a preamble (a standard signaling
`message) to the access point. The preamble is a repeated bit pattern fol-
`lowed by a special bit sequence. It allows the access point to lock on to
`the signal before the data are sent. After the link is established, the end
`station sends address and protocol information. The header is followed
`by the data, which are transmitted at the on-air data rate. After the error
`check word is sent, the end station listens for acknowledgment from the
`destination. If no acknowledgment is received, the data are resent. The
`sequence is repeated until all the data have been sent and acknowledged.
`The IEEE 802.11 committee has specified that data rates for wire-
`less systems must be either 1 or 2 Mbps. Either the user chooses the rate
`or the system selects the best one according to the conditions. The on-air
`data rate includes message headers, retransmissions, and latency (the
`time between when a network station begins to seek access to a trans—
`mission channel and when that access is granted). Header overhead and
`retransmissions primarily affect performance of large data transfers.
`Whereas, latency has the greatest effect on short, bursty data transfers,
`because the latency involved in setting up a transmission introduces
`more delay than the transmission of message overhead or retransmis—
`sions. Therefore, throughput on a WLAN is lower for short messages
`than for longer messages. The actual throughput of an IEEE 802.11 sys-
`tem on an on-air data rate of 2 Mbps is about 1—1.5 Mbps for long mes-
`sages and 0.5—1.0 Mbps for short messages. Throughput is also affected
`by the range of the system. In a typical office environment. the range of
`an IEEE 802.11 WLAN is 200—300 feet, which is sufficient to cover most
`
`partitioned areas and an outside rim of walled offices.
`Sensitivity of the system is crucial because signal power can be
`affected drastically by obstacles. Sensitivity figures are the smallest
`amounts of received power that the radio can use. The IEEE 802.11 stan-
`dard requires a sensitivity of less than —80 dBm. One issue that is not
`addressed by the standard is roaming capability. Roaming is made possi—
`ble with overlapping WLAN cells in a configuration similar to that used
`for analog cellular phones. Roaming is considered to be part of the appli-
`cation- or driver-level technology, so vendors will be likely to resort dif-
`ferent schemes for achieving it.
`
`10.3.2 Wireless Information Networks Forum
`
`The Wireless Information Networks Forum (WINForum) addresses
`WLAN and wireless private branch exchange (WPBX) services and
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`10-3 WLAN standards
`245
`
`focuses on spectrum etiquette to provide fair access to an unlicensed
`band Widely used for different applications and devices. The etiquette
`does not preclude any common air interface standards or access technolo—
`gies. It demands listen-before-talk (LBT); thus, a device may not trans-
`mit if the spectrum it will occupy is already in use within its range The
`power is limited to keep the range short and allow operation in densely
`populated office areas. The power and connection time are related to the
`Occupied bandwidth to equalize the interference and provide a fair access
`to frequency—time resources. In the view of WINForum, the asynchro—
`nous transmission used in WLAN applications is bursty, begins trans-
`mission within milliseconds. uses short bursts that contain large
`amounts of data, and releases the link quickly. On the other hand, the
`isochronous transmission, typified by voice services such as a WPBX,
`uses long holding time, periodic transmission, and flexible link access
`times that may be extended up to a secoud. The asynchronous subbands
`may range from 50 kHz to 10 MHz, whereas the isochronous subbands
`may be divided into 1.25-MH2 segments. The two types are technically
`
`contrasting and cannot share the same spectrum.
`
`10.3.3 High-Performance Radio Local Area Network
`
`ETSI’s subtechnical committee RES 10 has been assigned the task
`of developing a standard for the High-Performance Radio Local Area Net-
`work (HIPERLANJ. The committee secured two bands at 5.12—5.30 GHz
`and 17.1—17.3 GHz for the HIPERLAN to operate at a minimum useful
`bit rate of 20 Mbps for point-to-point application with a range of 50 m. It
`is expected that. at this rate and range, a data rate of 500—1000 Mbps,
`comparable with fiber distributed data interface (FDDI) for a standard
`building floor of approximately 1000 m2, can be achieved. RES 10 is
`responsible to define a radio transmission technique, including type of
`modulation. coding, and channel access, as well as the specific protocols.
`
`10.3.4 ARPA
`
`The US. Advanced Research Project Agency (ARPA) has sponsored
`WLAN projects at the University of California at Berkeley (UCB) and
`University of California at Los Angeles (UCLA). The UCB Infopad
`project is based on a coordinated network architecture with fixed coordi-
`nating nodes and D888 (CDMA), whereas the UCLA project is for peer-
`tO-peer networks and uses FHSS. Both APRAusponsored projects are con-
`centrated on the BOO-MHz ISM band.
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`246
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`
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`
`10.4 ACCESS METHODS
`
`10.4.1 Fixed—Assignment Access Methods
`
`In the fixed—assignment access method, a fixed allocation of channel
`
`resources (frequency or time or both) is made on a predetermined basis
`to a single user. The three basic access methods—FDMA, TDMA, and
`CDMA—are the examples of the fixed—assignment access method. In this
`
`section, we discuss only the CDMA method. With CDMA, multiple users
`operate simultaneously over the entire bandwidth of the time—frequency
`signal domain, and the signals are kept separate by their distinct user—
`signal codes. As we discussed in chapter 2, the number of users that can
`be supported simultaneously by a DS-CDMA system is
`
` Gp
`1
`M_[Eb/N0]xE—Bxccx— x1
`
`.
`(10.1}
`
`= processing gain = Btu/Rs,
`where Gp
`= bandwidth,
`3,,
`= symbol transmission rate,
`R3
`Eb/N = bit energy-to-noise ratio,
`[3
`= interference factor,
`or
`= power control factor,
`
`1)
`it
`
`2 voice activity factor (= 1 for data service),
`= gain due to sector antenna.
`
`
`Example
`10.1
`
`We consider a CDMA system that uses QPSK modulation and convolutional
`coding. The system has a bandwidth of 1.25 MHz and transmits data at 9.6
`kbps. Find the number of users that can be supported by the system and
`the bandwidth efficiency. Assume a three-sector antenna with effective gain
`= 2.6, or = 0.9, and an interference factor B = 0.5. A bit—error rate of 10'3 is
`required.
`
`125 x 106
`GP = '—3 = 130.2
`9.6 x 10
`
`Pb = 10
`
`—3
`
`’E5
`1
`= -erfc 2T0
`
`7dB(5)
`
`Eb
`170..
`
`
`_ 4
`I
`1302
`.-.M_ _5._ x 1+0.s x 2.6 x 0.9-
`
`a
`
`40
`
`0.6
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`———-———‘._____fl*—_
`
`10.4 Access Methods
`
`247
`
`um” : w; = 0.307 bit/Hz-see
`1.25 x 10'
`
`
`10.2
`Example
`
`
`We consider a QPSK/DSSS WLAN that is designed to transmit in the 902-
`to 928-MHz ISM band. The symbol transmission rate is 0.5 Megasymbolsf
`sec. An orthogonal code with 16 symbols is used. A bit-error rate of 105 is
`required. How many users can be Supported by the WLAN? A three-sector
`antenna with gain 2 2.6 is used. Assume an interference factor-fl = 0.5 to
`account for the interference from users in other cells, and a = 0.9. What is
`the bandwidth efficiency of the system?
`
`Band width.Bw = 928 — 902 = 26 MHZ
`
`Data rate. R = R5 log2 16 = 05 log2 24 = 2 Mbps
`
`G ——i —-
`P
`R,
`s
`
`2
`
`5
`
`.lEb
`l
`s
`Pb— 10 —§LI’1’C N—U
`
`Eb
`-N—U~10dB(10)
`
`
`52
`|
`, =8.128
`,
`:_
`10X1+0.5X26X09
`
`
`mm: 8 X 2= 0.62bin’Hz—scc
`26
`___—__—________________
`
`10.4.2 Random Access Methods
`
`When each user has a steady flow of information to transmit (e.g., a
`data file transfer or a facsimile transmission), fixed‘aSSignment access
`methods are useful because they use communication resources efii-
`ciently. However, when the information to be transmitted is bursty 1?
`nature, the fixed—assignment access methods result in wasting commum'
`cation resources. Furthermore, in a cellular system where Sugscnber:
`are charged based on a channel connection time, the fixed'3581gn1:8n_
`access methods may be expensive to transmit short messages- Ran. oma
`access protocols provide flexible and efficient methods for managlfigds
`channel access to transmit short messageS- The random-acceSS met :he
`give freedom for each user to gain access to the network Whenever
`
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`
`user has information to send. Because of this freedom, these schemes
`
`result in contention among users accessing the network. Contention may
`
`cause collisions and may require retransmission of the information. The
`commonly used random~access protocols are pure ALOHA, slotted—
`ALOHA, and CSMA/CD. In the next section we describe briefly details of
`
`each of these protocols and provide necessary throughput expressions.
`
`10.4.2.1 Pure ALOHA In the pure ALOHA scheme, each user
`transmits information whenever the user has information to send. A user
`
`sends information in packets. After sending a packet, the user waits a
`length of time equal to the round—trip delay for an acknowledgment
`(ACK) of the packet from the receiver. If no ACK is received, the packet is
`
`assumed to be lost in a collision, and it is retransmitted with a randomly
`
`selected delay to avoid repeated collisions.1 The normalized throughput S
`(average packet arrival rate divided by the maximum throughput) of the
`pure ALOHA protocol is given as
`
`S = Ge—m
`
`(10.2)
`
`Where G = normalized offered traffic load.
`
`From equation (10.2), note that the maximum throughput occurs at
`traffic load G = 50 percent and is S = 1/(22). This is about 0.184. Thus, the
`
`best channel utilization with the pure ALOHA protocol is only 18.4 percent.
`
`10.4.2.2 Slotted-ALOHA In the slotted—ALOHA system, the trans—
`mission time is divided into time slots. Each time slot is made exactly
`
`equal to packet transmission time. Users are synchronized to the time
`slots, so that whenever a user has a packet to send, the packet is held
`and transmitted in the next time slot. With the synchronized time—slots
`scheme, the interval of a possible collision for any packet is reduced to
`one packet time from two packet times, as in the pure ALOHA scheme-
`The normalized throughput S for the slotted-ALOHA protocol is given as
`
`s = 06°”
`
`(10.3)
`
`where G = normalized offered traffic load.
`
`1. It should be noted that the protocol on CDMA access channels as implemented in TIA
`IS-95A is based upon the pure ALOHA approach. The mobile station randomizes its
`attempt for sending a message on the access channel and may retry if an acknowledgment
`i5 ”Gt TeCEiVEd from the 13851‘! station. For further details, one should reference section
`6.6.3.1. 1.1 of'I‘IA IS-95A
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`M 1
`
`0.4 Access Methods
`
`249
`
`The maximum throughput for the slotted—ALOHA occurs at G = 1.0
`[equation (10.3)], and it is equal to He or about 0.368. This implies that
`at the maximum throughput, 36.8 percent of the time slots carry the suc-
`cessfully transmitted packets.
`
`10.4.2.3 Carrier-Sensed Multiple Access CSMA protocols have
`been widely used in both wired and Wireless LANs. These protocols pro-
`vide enhancements over the pure and slotted ALOHA—protocols. The
`enhancements are achieved through use of additional capability at each
`user station to sense the transmissions of other user stations. The carrier—
`
`sensed information is used to minimize the length of collision intervals.
`For carrier sensing to be effective, propagation delays must be less than
`packet transmission times. Two general classes of CSMA protocols are
`nonpersistent and p—persistent. Each of these classes can be used with
`the slotted or unslotted operation.
`
`' Non—persistent CSMA. A user station does not sense the channel
`
`continuously while it is busy. Instead, after sensing the busy condi-
`tion, it waits for a randomly selected interval of time before sensing
`again. The algorithm works as follows: If the channel is found to be
`idle, the packet is transmitted; if the channel is sensed busy, the
`user station backs off to reschedule the packet to a later time. After
`backing off, the channel is sensed again, and the algorithm is
`
`repeated again.
`° p-persistent CSMA. The slot length is typically selected to be the
`maximum propagation delay. When a station has information to
`transmit, it senses the channel. If the channel is found to be idle, it
`transmits with probability p. With probability q = 1 — p, the user
`station postpones its action to the next slot, where it senses the
`channel again. If that slot is idle, the station transmits with proba-
`bility p or postpones again with probability 9. The procedure is
`repeated until either the frame has been transmitted or the channel
`is found to be busy. When the channel is detected busy, the station
`then senses the channel continuously. When it becomes free, it
`starts the procedure again. If the station initially senses the chan-
`nel to be busy, it simply waits one slot and applies the preceding
`procedure.
`° l-persistent CSMA. l-Persistent CSMA is the simplest form of the
`p-persistent CSMA. It signifies the transmission strategy, which is
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`
`to transmit with probability 1 as soon as the channel becomes idle.
`
`After sending the packet, the user station waits for an ACK. If it is
`
`not received within a specified amount of time, the user station
`
`waits for a random amount of time and then resumes listening to
`
`the channel. When channel is again found to be idle, the packet is
`
`retransmitted immediately.
`
`For more details, refer to references [2, 21].
`
`The throughput expressions for the CSMA protocols follow:
`
`0 Unslotted Nonpersistent CSMA
`
`—a0
`8 — i— (10.4)
`_ Guanine”G
`
`0 Slotted Nonpersistent CSMA
`
`s =
`
`-oG
`61090
`l-—e41 +a
`
`(10.5)
`
`. Unslotted 1-Persistent CSMA
`
`—G(l+lal
`s 2W (10.6)
`G“ +2a)_{1“e—OG)+(1 +aG)e‘G(1+“)
`
`- Slotted 1-Persistent CSMA
`
`G84?“ +a)[l + a '2flG]
`S _
`h (1+ a)(1—e_aG)+ ae‘G“ ““
`
`(10.7)
`
`where S = normalized throughput,
`G = normalized offered traffic load,
`a = T/Tp,
`1: = propagation delay,
`Tp = packet transmission time.
`
`—-—--—-—-——-———.——_—_—_—.——————'__-
`Example
`10.3
`
`We consider a WLAN installation in which the maximum propagation delay
`is 0.4 us. The WLAN operates at a data rate of 10 Mbps, and each packet
`has 400 bits. Calculate the throughput with (1) an unslotted nonpersistent,
`(2) a slotted persistent, and (3) a slotted 1-persistent CSMA protocol.
`
`400
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`10.5 Error Control Schemes
`
`251
`
`a=l=0+4=om
`Tp
`40
`
`(a
`4,
`G=40><IO x10x10 =
`400
`
`l
`
`' Slotted Nonpersistent:
`—(J.t)l
`s =W : 0.496
`I — e "
`+ 0.01
`
`' Unslotted Nonpersistent:
`
`l x 24101
`S = ’————;m = 0.493
`(I + 0.02) + e"
`
`° Slotted 1-Persistent:
`
`940]“ + 0.01 _ eiflfli)
`S -
`(1 + 0.01m — e'O'O‘) + onle‘m'
`
`
`: 0.531
`
`10.5 ERROR CONTROL SCHEMES
`
`Channel coding and automatic repeat request (ARQ) schemes are used to
`increase the performance of mobile communication systems. In the phys-
`ical layer of DS-CDMA system, error detection and correction techniques
`such as forward error correction (FEC) schemes are used. For some of the
`data services, higher-layer protocols use ARQ schemes to enable retrans-
`mission of any data frames in which an error is detected. The ARQ
`schemes are classified as follows [21, 22].
`
`Stop and Wait. The sender transmits the first packet numbered 0
`after storing a copy of that packet. The sender then waits for an ACK
`numbered 0, ACKO of that packet. If the ACKO does not arrive before a
`time—out, the sender makes another copy of the first packet, also nums
`bered 0, and transmits it. If the ACKO arrives before a time-out, the
`sender discards the copy of the first packet and is ready to transmit the
`next packet, which it numbers 1. The sender repeats the previous steps,
`using number 1 instead of 0. The advantages of the Stop and Wait proto—
`col are its simplicity and its small buffer requirements. The sender needs
`to keep only a copy of the packet that it last transmitted, and the
`receiver does not need to buffer packets at the data link layer. The main
`disadvantage of the Stop and Wait protocol is that it does not use the
`communication link efficiently.
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`lllfii
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`4——————IEH
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`
`ereless Data
`
`
`
`The total time taken to transmit a packet and to prepare for trans-
`
`mitting the next one is
`
`T: rp+2r
`
`T
`prop + 2 proc
`
`+T
`
`a
`
`(10.8)
`
`The protocol efficiency without error is
`
`T
`= _P
`
`0
`
`(10.9}
`
`= total time for transmitting a packet,
`where T
`= transmission time for a packet.
`’I‘p
`TmJD = propagation time of a packet or an ACK.
`Tm: = processing time for a packet or an ACK.
`T“
`= transmission time for an ACK.
`
`Ifp is the probability that a packet or its ACK is corrupted by trans-
`mission errors and a successful transmission of a packet and its ACK
`takes T seconds and occurs with probability 1 — p, the protocol efficiency
`for full duplex (FD) and half duplex (HD) operation are given as
`
`(1 — p)Tp
`= __—_
`(l —p)T+pr
`
`“FD
`
`(I — p)T)
`
`(10.10)
`
`Selective Repeat Protocol (SRP). The data link layer in the
`receiver delivers exactly one copy of every packet in the correct order. The
`data link layer in the receiver may get the packets in the wrong 01":19r
`from the physical layer. This occurs, for example, when transmission
`errors corrupt the first packet and not the second one. The second paCkEt
`arrives correctly at the receiver before the first. The data link layer in the
`receiver uses a bufi'er to store the packets that arrive out of order. Once
`the data link layer in the receiver has a consecutive group of packets in
`its buffer, it can deliver them to the network layer. The sender also uses a
`buffer to store copies of the unacknowledged packets. The number of the
`packets, which can be held in the sender/receiver buffer is a design
`parameter. Let W be the number of packets that the sender and receiver
`bufi'ers can each hold and SRP be the number of packets modulo-2W. The
`protocol efficiency without any error and with an error probability of p is
`given as
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`10.5 Error Control Schemes
`
` _ WT
`
`THO) = rmn{ T”. I}
`
`For very large W, the protocol efficiency is
`
`253
`
`(10.12)
`
`mp) = l — p
`
`(10.13)
`
`where T = time-out = WTp
`
`mm =
`
`2+p(W—l)
`
`2—————+p(3W _ 1)
`
`(10.14)
`
`SRP is very efficient, but it requires buffering packets at both the sender
`and the receiver.
`
`Go-Back-N (GBN). The Go-Back-N protocol allows the sender to
`have multiple unacknowledged packets without the receiver having to
`store packets. This is done by not allowing the receiver to accept packets
`that are out of order. When a time-out timer expires for a packet, the
`transmitter resends that packet and all subsequent packets. The Go-
`Back-N protocol improves on the efficiency of the Stop and Wait protocol
`but is less efficient than SRP. The protocol efficiency is given as
`
`1
`"PD: ——_
`
`l+[ p )W
`
`l-P
`
`10.15
`
`)
`
`(
`
`Window-Control Operation Based on Reception Memory
`ARQ- In digital cellular systems, bursty errors occur by multipath fad-
`ing, shadowing, and handoffs. The bit-error rate fluctuates from 10‘1 to
`10*. Therefore, the conventional ARQ schemes do not operate well in
`digital cellular systems. Window-control operation based on reception
`memory (WORM) ARQ has been suggested for control of dynamic error
`characteristics. It is a hybrid scheme that combines SRP with GBN. GBN
`protocol is chosen in the severe error condition, whereas SRP is selected
`
`in the normal error condition.
`
`Variable Window and Frame Size GBN and SRP. Since CDMA
`systems have bursty error characteristics, the error control schemes
`should have a dynamic adaptation to bursty channel environment. The
`SRP and GBN with variable window and frame size have been proposed
`in [22] to improve error control in the CDMA systems. Table 10.3 PTO-
`vides the window and frame size for different bit-error rates. If the error-
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`
`rate increases, the window and frame size are decreased. In the case of
`
`error-rate being small, the window and frame size are increased. The
`
`optimum threshold values of bit-error rate (BER) and window and frame
`
`sizes were obtained through computer simulation.
`
`Table 10.3 Bit-Error Rate Versus Window and Frame Size
`
`Bit-Error Rate (BER)
`
`Window Size (W)
`
`Frame Size. bits
`
`BER s 10“1
`
`10“1 < BER < 10‘3
`
`10‘3 < BER < 10‘2
`
`32
`
`8
`
`4
`
`172
`
`80
`
`40
`
`
`
`210‘2 < BER 16
`
`
`
`In CDMA systems, the forward link consists of pilot, sync, paging.
`and trafiic channels. System informatiou sent on the pilot, sync, and pag-
`ing channels allows each mobile station to evaluate the BER easily by
`
`measuring the ratio of the number of retransmitted frames to the num-
`ber of transmitted frames over a 2—second period. Thus, the mobile sta-
`tion can change the window and frame size according to the BER.
`
`
`
`Example 10.4
`
`We consider a WLAN in which the maximum prepagation delay is 4 ps. The
`WLAN operates at a data rate of 10 Mbps. The data and ACK paCkEt
`lengths are 400 and 20 bits, respectively. The processing time for a data or
`ACK packet is 1 ps. Ifthe probability p that a data packet or its ACK can be
`corrupted during transmission is 0.01, find the data link protocol efficiency
`with (1) Stop and Wait protocol, full duplex, (2) SRP with window size W =
`8, and (3) Go-Back-N protocol with window size W = 8.
`
`T __ 400
`p—W=40HS
`
`T = 40+2x4+2x1+2= 52 [18
`
`Stop and Wait:
`
`*(1—o.01)x4o=0
`7} _ ____—52
`
`.762
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`10.6 The Data Servlces Standard for CMDA CellularIPersonal Communlcatlons Systems
`
`255
`
`SRP:
`
`2 + 001(8 #1)
`= —— =
`2 + 001(24 u I)
`
`n
`
`.“
`0354
`
`Go‘Bach:
`
`11 = —-—-1-——-—-——~ = 0.925
`1 + 8(
`U.U|
`)
`l — 0.0]
`
`10.6 THE DATA SERVICES STANDARD FOR CMDA CELLULAR!
`
`PERSONAL COMMUNICATIONS SYSTEMS
`
`CDMA systems send data using the reference model shown in figure 10.3
`as standardized in TIA 18—99 [14]. The following is the description of the
`
`reference points:
`
`' Reference point Rm is a physical interface that connects a Terminal
`Equipment 2 (TE?) to an Mobile Terminal Type 2 (MT2). An MT2
`provides a non-ISDN user interface.
`"1
`° Reference point U is a physical interface that connects a Mobile
`Terminal Type 0 (MTO) or MT2 to a base station. Um is the air inter»
`face. An MTO is a self-contained data—capable mobile terminal that
`
`does not support an external interface.
`° Reference point A, is a physical interface connecting a base station
`to the PSTN.
`
`
`
`T52:
`
`MT2:
`
`BS:
`
`Terminal Equipment 2
`
`Mobile Termlnatlon 2
`
`Base SIatlon
`
`DCE:
`
`Data Clrcult-termlnal Equipment
`
`PSTN:
`
`Publlc Switchlng Telephone Network
`
`Figure 10.3 Reference model for asynchronous data transmission. (Reproduced underwritten
`”Mission 01 lhe copyright holder [TIA].)
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`Chapter 10
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`ereless Data
`
`° Reference point W is a physical interface that connects data circuit-
`terminating equipment (DCE) to the PSTN.
`0 Reference point R1, is a reference point between a TE2 and a Data
`Communications Equipment (DCE). It may be a physical interface.
`or it may be internal to the user equipment. CCITT V-Series
`modems are the examples of TE2; Group—3 FAX is an example of a
`DCE. TE2 is equivalent to data terminal equipment (DTE') as
`defined in EIA/TIA-ZSZE [15]; similarly, MT2 is equivalent to DCE.
`
`10.6.1 Asynchronous Data and Group-3 Facsimile
`
`The general approach taken in TIA lS-95A [13] for data services
`reuses the previously specified physical layer of the IS—95A protocol stack
`as the physical layer. Figure 10.4 shows the air interface ( Um} protocol
`stack.
`
`The current TIA standards define three primary services: asynchro-
`nous data, Group-S facsimile, and short message service ISMS). Stan-
`
`Async Data
`
`FAX
`
`
`
`
`Application Interface
`
`TCP
`ICMP
`
`
`
`TCP:
`
`Transmission Control Protocol
`
`ICMP:
`
`Internet Control Message Protocol
`
`IP:
`
`Internet Protocol
`
`SNDCF: Sub-Network Dependent Convergence Function
`
`lPCP:
`
`Internet Protocol Control Protocol
`
`LCP:
`
`Link Control Protocol
`
`PPP:
`
`RLP:
`
`Point-to-Point Protocol
`
`Radio Link Protocol
`
`Figure 10.4 The Um protocol stack.
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`257
`
`dard activities are in progress to define packet data, synchronous data,
`and other primary services.
`
`IS—QSA asynchronous data has been structured as a circuit-
`switched service in which a dedicated path is established between the
`data devices for the duration of the call. It is used for connectivity
`through the PSTN when point—to-point communications to a PC or FAX
`user is required. For example, for a file transfer involving PC-to-PC coma
`munications, the asynchronous data service is the preferred cellular ser—
`vice mode.
`
`The radio link protocol (RLP) employs automatic repeat request,
`forward error correction, and flow control. Flow control and retransmis-
`
`sion of data blocks with errors are used to provide an improved perfor-
`mance in the mobile segment of the data connection at the expense of
`variations in throughput and delay. Typical raw channel data-error rates
`for cellular transmission are approximately 10*. However, an acceptable
`data transmission usually requires a bit-error rate of about 1045. In order
`to achieve this, it requires the design of efficient ARQ and error-correc~
`tion codes to deal with error characteristics in the mobile environment.
`
`The CDMA protocol stack for data and facsimile (fig. 10.4) has the
`following layers:
`
`' The Application Interface Layer includes an application inter-
`face between the data source/destination in the mobile terminal
`
`(MTO) or terminal equipment (TEZ) and the transport protocol
`layer. In the base station, the application interface resides between
`the data source/destination on the network (A, interface) side and
`the transport protocol layer. The application interface provides
`modem control, AT command processing,2 negotiation of air inter-
`face data compression, and data compression over the air interface
`(optional).
`' The Transport Layer for CDMA asynchronous data and FAX ser-
`vices is based on Internet transport layer protocol known as trans—
`mission control protocol (TCP) [6]. The implementation complies
`with the requirements for TCP with modifications as described in
`1395 [13]. If the modified procedure is disabled, there is no maxi-
`
`2. The AT commands were originally defined by the Hayes Microcomputer company‘for
`their wireline modems. The command set has now been adopted by most wrrelme and Wire-
`less modems. The name AT is derived from the use of AT to preface all commands to the
`modem.
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`W 2
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`53
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`Chapter
`
`10 Wireless Data
`
`mum number of retransmission attempts during synchronization,
`and an established TCP connection remains open until explicitly
`closed by the mobile station or base station. The application inter-
`face sets the value of R 2 in the protocol. The base station follows
`either the procedure of the Internet control message protocol
`(ICMP) [7] or the preceding given procedure.
`- The Network Layer for CDMA async data and FAX services is
`
`based on the Internet network layer protocol known as the Internet
`
`protocol (IP) [5]. The network layer includes the ICMP [6]. The
`implementation complies with the requirements of the IP I5] and
`the requirements for Internet hosts [8] with modifications as
`described in 18-95 [13]. The interface between the network and
`
`transport layer complies with the requirements of’the ICMP l7].
`
`0 The Subnetwork Dependent Convergence Function
`(SNDCF) performs header compression on the headers of the
`transport and network layers. This function is negotiated using
`point-to-point protocol (PPP) and internet protocol control protocol
`(IPCP) [10}. Mobile stations support Van Jacobson TCP/IP header
`compression. A minimum of one compression slot is negotiated.
`Base stations support TCP/IP header compression compatible with
`that required for mobile stations. Negotiation of the parameters of
`header compression is carried out using IPCP. The SNDCF sublayer
`accepts a network layer datagram from the network layer, performs
`header compression as required, and passes the datagram to the
`PPP layer, indicating the appropriate PPP identifier. The SNDCF
`sublayer receives network layer datagrams with compressed or
`uncompressed headers from the PPP layer, decompresses the data-
`gram header as necessary, and passes the datagram to the network
`layer.
`
`' The Data Link Layer uses PPP [11]. The PPP link control proto-
`col (LCP) is used for initial link establishment and for the negotia-
`tion of optional link capabilities. The data link layer uses the PPP
`and IPCP to negotiate IP addresses and TCP/IP header compres-
`sion. The data link layer accepts network layer datagrams from the
`SNDCF and encapsulates them in the PPP information field. The
`packet is framed using the octet synchronous framing protocol,
`except that there is no interframe fill. No flag octets are sent
`between a flag octet that ends one PPP frame and the flag octet that
`begins the subsequent PPP frame. The framed PPP packets are
`passed to the RPL layer for transmission. The data link layer
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`10.6 The Data Servlces Standard for CMOA Cellular/Personal Communicallons Systems
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`
`accepts received octets From the RLP layer and reassembles the
`original PPP packets. The PPP process discards any PPP packet for
`which the received frame check sequence (FCS) is not equal to the
`computed value.
`0 The Internet Protocol Control Protocol Sublayer supports
`negotiation of the IP-address (type = 3) and IP-compression protocol
`(type = 2) parameters. [PCP negotiates a temporary IP address for
`the mobile station whenever a transport layer connection is actively
`opened. Mobile stations maintain the temporary IP address only
`while a transport layer connection is open or being opened; they dis-
`card the temporary IP address when the transport layer connection
`is closed.
`
`' The Link Control Protocol layer messages with a protocol identi-
`fier of OXCOZI to the PPP layer which processes the packet accord-
`ing to PPP LC P. For other supported protocol identifiers, the PPP
`layer removes the PPP encapsulation and passes the datagram and
`protocol identifier to the SNDCF. For unsupported protocol identifi-
`ers, the LCP protocol-Reject is passed to the RLP layer for trans-
`mission. The mobile station supports the PPP LCP Configure-
`Request, Configure—ACK, Configure-NAK, Configure-Reject, Termi-
`nate-Request, Terminate—ACK, Code-Reject, and Protocol-Reject.
`Other LCP packet types may also be supported. The PPP LCP nego-
`tiates the following configuration options:
`
`X ASYDC control character map. The mobile station does not
`require any mapping of control characters. The base station
`may negotiate mapping of control characters.
`X Protecol field compression. This option applies when the
`protecol number is less than OxFF.
`_
`X Address and contI'Ol field compression. This option
`applies When the protocol number is not 0xCO2I.
`The mobile station may support Other configuration options ((1%,
`maximum regeiVe unit- authentlcation protocol, hnk quality1 pro 0
`ml,” magic number). When a? option that 35 at Wm 15 I
`received, the C
`e_Reject 15 sent as an indication to the pee ‘. e
`0 The Radio L‘ onfi'gmrwcol Layer provides an octet stream.1 57”“
`over the forw 111k Pro everse traffic chann