Tài liệu Điện thoại di động mạng lưới Radio P13 doc

Application Layer Presentation Layer Session Layer Transport Layer Network Layer Data Link Layer Physical LayerMedium-Access Control Sublayer Physical Layer Channel-Access Control Higher

Since the introduction of lightweight portable computers (laptops, notebooks),

a great deal of attention has been focused on the development of wirelesscomputer networks (Wireless Local Area Network, WLAN)

Thanks to standardization in the field of local area networks, it is paratively easy to find systems that will still be upgradable even in a fewyears’ 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 thatcan occur are the surfacing of mechanical defects (corrosion) after a few yearsand violations of rules on radiated interference It is difficult to adapt thesenetworks to cope with changing office conditions Mobile network nodes arenot possible

com-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 toconnect terminals to their processing computers The newer wireless LANswork with the most up-to-date radio technology Data is encrypted and ex-tensive error-protection mechanisms are available Integrity of data is alsoguaranteed

Just like wireline LANs, wireless LANs can be divided into different tectures and performance categories Many companies offer products for wire-less point-to-point connections, but only very few build LANs for multipointcommunication Today wireless networks use spread-spectrum, narrowbandmicrowave or infrared signals for transmission (see Table 13.1) Because oflegal regulations, networks using spread-spectrum and narrowband microwavecannot be operated in most countries unless special authorization has beengiven

archi-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 alsobecause of the late standardization in this area In spite of this, suppliers areprojecting a growth in wireless networks over the next few years Standardssuch as IEEE 802.11 or HIPERLAN/1 discussed below will help to increaseuser acceptance of wireless LANs

∗ With the collaboration of Christian Plenge and Andreas Hettich

Table 13.1: Characteristics of different transmission techniques

Figure 13.1: The IEEE 802/ISO 8802 standard

The diversity of LAN systems in terms of cabling, transmission techniques,transmission speeds, access procedures and variations thereof necessitatedstandardization in order to facilitate their acceptance and enable differentLANs 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 beenaccepted

The standard mainly restricts itself to the lower two layers of the ISO/OSIreference model (see Figure 13.1) A separation is made between Logical LinkControl (LLC) and Medium-Access Control (MAC) The LLC layer upwardlyoffers all systems a standard interface for establishing logical connections TheMAC sublayer supports protocols such as token ring, token bus, CSMA/CD(Ethernet )

In Western Europe the standards for wireless radio LANs are specified byETSI The technical group RES 10 (Radio Equipment and Systems) at ETSIhas developed HIPERLAN/1, the European standard for wireless LANs [3]

Table 13.2: Mid-frequencies of HIPERLAN/1

Channel No Mid-frequency [MHz] Channel No Mid-frequency [MHz]

compat-of HIPERLAN/1 and IEEE 802.11 in the following sections

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 providedfor HIPERLAN/1; see Table 13.2

One channel provides a bit rate of 23.5294 Mbit/s for user and controldata 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 thechannel-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 arespecified in the standard A GMSK modulation method with a bandwidthtime product of 0.3 is used on the radio channel

Synchronous applications as well as those with real-time requirements aresupported The transmission time is not a critical factor with asynchronoustraffic, 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 portablecomputers Plans are underway to make them available the size of a PCM-CIA card (Personal Computer Memory Card Interface Association) with the

Since HIPERLAN/1 systems support applications for battery-operated tems, they must offer low power consumption of a few hundred mW HIPER-LAN/1 offers an energy-saving mode

sys-HIPERLAN/1 networks should support the mobility of terminals LAN/1 stations are therefore designed to be able to exchange information withother stations at up to a speed of 10 m/s, which corresponds to 36 km/h, or

HIPER-up to a rotational speed of 360°/s

The ETR 069 technical report [4] produced by the ETSI RES 10 tee defined the services and possibilities planned for HIPERLAN/1 Some ofthe applications that will benefit from new solutions and an overview of theHIPERLAN/1 network topologies are presented in the following [2] HIPER-LAN/1 appears without the “/1” below

Wireless offices A WLAN is a better option than a fixed network in listedbuildings 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 portablecomputers in different locations and connect them easily to a network

Ad hoc networks Ad hoc networks are radio networks without any kind ofpermanent communications infrastructure A group of users can form itsown closed complex This means that at conferences, conventions, andlarge functions, or in the event of accidents or catastrophes, computerscan communicate with each other without having been cabled togetherbeforehand Each user carries his part of the network with him in theform of his computer with a radio-LAN connection

Medicine Within a radio-LAN, doctors would be able to have direct andinteractive access to remote data such as X-rays when visiting theirpatients This would make the work of doctors easier and produce betterand faster diagnosis for patients

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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 aging a large number of machines that are restricted to use in theirlocations A wireless connection to the network would allow these ma-chines (e.g., industrial robots or unmanned vehicles) more freedom ofmovement and the possibility of being used with more flexibility Main-tenance personnel would use laptops to retrieve the data needed fordiagnostic purposes

A HIPERLAN is not organized centrally, and instead has a completely tributed architecture with a dynamic allocation of network and network nodeidentifiers Each station (node) is differentiated from the other stations by

dis-a unique node identifier (NID) A number of stdis-ations dis-are combined into dis-anetwork with a shared HIPERLAN identifier (HID) This network forms aHIPERLAN

In contrast to a wireline network, HIPERLANs using the same radio nel cannot be separated from one another Overlapping can occur Anotherproblem with radio channels is the range restriction Mobile HIPERLANnodes and unfavourable propagation characteristics can cause the fragmenta-tion of a network

chan-The following network topologies exist as a result of the channel istics and the applications presented:

character-Independent HIPERLANs Two HIPERLANs, A and B, are considered to beindependent 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 tion medium and therefore also do not cause any interference in eachother’s network HIPERLAN A could be an ad hoc network set up for

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Figure 13.3: Overlapping HIPERLANs

a conference of company X, and HIPERLAN B an LAN used in thefactory of company Y

Overlapping HIPERLANs If the radio range of some of the stations in work A should overlap with some of the stations in network B thenthese members share the communications medium and its transmissioncapacity in the area where the overlapping occurs (see Figure 13.3)

net-An overlapping of networks produces two effects:

• The senders in the different HIPERLANs use the same frequencyband, thereby increasing the occurrences of interference As a re-sult, optimal use of the frequency band is no longer possible becausenot 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 ferent HIDs All received data packets are evaluated, and onlythose with their own HIDs are accepted As a result, there is adecrease in the maximum possible data transmission capacity andconsequently also the data transmission rate in this area

dif-These effects can be reduced if several frequency channels are introduced.With HIPERLAN, up to five frequency channels in the 5.15–5.30 GHzrange are provided

Multihop networks In addition to their original role as transmitting and ceiving stations for their own terminals, some stations in a multihopnetwork also perform the function of relay stations This allows data

re-to be transmitted over larger distances despite the restricted reachablerange 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 bepossible for HIPERLANs to be connected to the usual fixed networks

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Figure 13.5: Connection to fixed network

(see Figure 13.5) This affects the network layer, and is not part of theHIPERLAN/1 standard

The HIPERLAN reference model defines the components needed to install aprivate radio subnetwork It is based on the ISO/OSI reference model andconsists of the Medium-Access Control (MAC) sublayer, the Channel-AccessControl (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 layerprovides services for the layer above it These services are offered at the

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 ofservice primitives (SP)

The individual sublayers are described in more detail in the following tions Figure 13.7 shows the HIPERLAN service model

The MAC sublayer provides the following functions to ensure smooth andreliable HIPERLAN operation

Because HIPERLAN stations share the radio channel, an overlapping of bouring HIPERLANs can occur that causes the radio ranges of a number ofwireless networks to overlap in the same radio channel (see Figure 13.3)

neigh-A restricted radio range, the mobility of the stations and unfavourablepropagation conditions can cause the fragmentation of a HIPERLAN (seeFigure 13.2), although A and B are separate subnetworks of the same HIPER-LAN.

The standard defines internal address structures A HIPERLAN addressconsists of a HIPERLAN name (HID) and a station identification (NID) TheHID is used by the MAC protocol to differentiate between the MAC commu-nication of the individual HIPERLANs If there is a dynamic allocation ofthe HID because of the possible overlapping of cells, it is less likely that mixedcommunication will occur The MAC protocol reserves the use of special HIDsfor communication between the stations of neighbouring HIPERLANs

It is easier for a user to identify his network using a name rather than thenumerical HID The name is also used by the lookup function for establishingwhich HIPERLANs are operating in the area

Since no clear guidelines and no administrative coordination exist forHIPERLAN identification, it is possible for a MAC communication to takeplace 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 PERLAN addressing

A radio channel can be listened for in a neighbouring area An encryptionalgorithm with the appropriate cryptographic management is therefore pro-vided in the MAC sublayer This protects confidential data from unauthorizedeavesdropping, 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 istransmitted with the encrypted data to the receiver In addition, a commoninitialization vector for encryption and decryption is required and if necessarytransmitted

The level of transmission security increases with the frequency of the change

of key and initialization vectors

For compatibility with the ISO-MAC service definition the MAC service (HMservice) uses 48-bit LAN-MAC addresses for identification of MAC serviceaccess points (MSAP) (see Figure 13.7) The standard recognizes separateaddresses for individual MAC service access points and group addresses forcontacting several MSAPs

An HM entity is linked to a single MSAP, through which it offers MACservices It is also connected to a single HIPERLAN-CAC Service AccessPoint (HCSAP), over which it uses the HIPERLAN-CAC services

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Figure 13.9: Unicast transmission

A single 48-bit LAN-MAC address is used as an MSAP address for 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 ofMSAPs and the users associated with it

The MAC protocol incorporates a multihop relaying facility that enables thetransmission of data beyond the boundaries of a station’s sending area—inother words over several stations An HM entity is either a forwarder or anon-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 whenthe transmission route is not known Each station that recognizes a route tothe destination station forwards the data packet accordingly To avoid datapackets from being routed by several stations at the same time, the protocolensures that only a limited number of stations can forward data Figure 13.8shows a broadcast transmission being sent by station 4

It is more efficient to forward packets that are addressed to a particularreceiver if a unicast transmission is used (see Figure 13.9) A packet is thenrouted to its destination through successive hops in accordance with an (op-timal) route Broadcast relaying must be used if the route is not known, asexplained above

Each HM entity collects and manages routing information in its RoutingInformation Base (RIB) This information is continuously updated The data

in the RIB becomes obsolete and is discarded once it has passed its validityperiod Because of this constant updating of routing information, it is evenpossible to specify quasi-optimal paths for the forwarding of packets in con-tinuously changing HIPERLANs

HMS Provider

HIPERLAN-MAC Service MSAP MSAP

Figure 13.10: HIPERLAN-MAC service model

The power conservation function of the MAC protocol provides a power-savingmechanism ensuring that battery-operated systems use a minimal amount ofpower The power-saving function is an optional function It recognizes twotypes of stations:

• The power-saving terminal (P-saver) defines time periods when it isactivated These are the only times it sends and receives data

• The P-supporter only transmits data to its power-saving neighbour whenthe latter is activated

As a service provider (HMS provider), the MAC sublayer provides its services(HIPERLAN-MAC services, HMS) to the LLC sublayer above it at the MSAP.This makes the LLC layer a HIPERLAN-MAC service user (HMS user ) ofthe services offered by the MAC sublayer (see Figure 13.10)

The following is defined for the transmission of MAC service data units(MSDU):

• An MSDU is transmitted from the source MSAP to an individual tination MSAP or to a group of destination service access points in asingle request to the MSDU transfer service A connectionless service isprovided, i.e., the transfer takes place without the need for a connection

des-to be established explicitly and then later cleared

• Each MSDU transmission is independent of all others

• MSDUs are transmitted without any restriction to their content, format

or coding; MSDUs are never interpreted from the standpoint of theirstructure or their content

• An HMS user can indicate the quality of service desired MAC Quality of Service, HMQoS) using user priority and the MSDUlifetime Other means for influencing the HMS service provider are notplanned

(HIPERLAN-• The HMS provider can carry out the following actions:

Table 13.3: Range of values for quality of service parameters

– Change sequence of MSDUs

The HMS service provider notifies the HMS user through an HMQoS errorreport that it was not possible to meet the desired quality of service for one

of the previous invocations of the MSDU transfer function

HMS users can use the HIPERLAN lookup service to determine whichHIPERLANs are in their area

According to Section 13.5.1.3, there are two types of MSAP addresses:

• Individual MSAP addresses for the identification of an individual MSAP

• Group MSAP addresses that address a group of MAC service accesspoints

Individual MSAP addresses are allowed to be used as source and destinationaddress, but a group MSAP address can only be used to address a destination

When an MSDU is being transferred to User Data Acceptance (see tion 13.5.3.4), a MAC service user can indicate the quality of service pa-rameters (see Table 13.3):

Sec-User priority (UP) The UP indicates the importance of an MDSU The userpriority controls the quality of service of time-critical services

Lifetime of data This specifies the maximum lifetime of an MSDU that isallowed to elapse between transmission and reception

Residual lifetime This indicates how much time is still remaining from thelifetime

Table 13.4: HMS primitives and their parameters

Data

trans-fer

MSDU transfer HM Unitdata req Source Address,

Destina-tion Address, MSDU, UserPriority, MSDU Lifetime

HM Unitdata ind Source Address,

Destina-tion Address, MSDU, UserPriority, MSDU Lifetime,Residual MSDU LifetimeControl HMQoS failure

report

HM Qosfailure ind Destination Address, User

Priority, MSDU LifetimeHIPERLAN

lookup

HM Lookup conf HIPERLAN Information

The HIPERLAN-MAC service primitives derived from the ISO/OSI dard represent the possible interactions between HMS user and HMS serviceprovider The service primitives at the HIPERLAN-MAC service access pointand their parameters are listed in Table 13.4

stan-In addition to the quality of service parameters, the following parametersare also used:

Source Address (SA) The individual sending address of the MSAP

Destination Address (DA) Address of an individual MSAP or a group ofMSAPs to which a packet is directed

MAC Service Data Unit (MSDU) Data that is transported without cation by the HM service provider between HMS users The size of thepackets is between zero and a value specified by the HMS provider.HIPERLAN Information Information about other HIPERLANs in the vicin-ity of an HMS user

sublayer is notified through an HM Unitdata Request message from the LLCsublayer All the information required for transmission to an MSAP or to agroup of MSAPs is supplied at the same time The packet is regarded as beingindependent of the previous or the following packets, and is transmitted inconnectionless mode The receiver is not able to control the rate at which thesender sends the packets Figure 13.11 shows the primitives for a successfultransfer

Figure 13.12: Sequence of primitives in a detected failed data transfer

The LLC sublayer does not receive an acknowledgement indicating whetherthe data packet has been successfully transmitted within the lifetime of theMSDU

of service prescribed by the user until the lifetime of the data is eliminated, theMAC notifies the LLC sublayer through sending an HM Qosfailure Indicationthat the transmission has failed The MAC sublayer then discards the MSDU.The error report gives the HMS user the chance to repeat the transmissionwith adjusted quality of service parameters; see Figure 13.12

Figure 13.13 presents an overview of the elements of the MAC sublayer, whichwill be discussed in detail below

User Data Transfer Function Conservation Function

Power Routing

Information Exchange Function Lookup

HIPERLAN Function HIPERLAN-MAC Service

HMPDU Transfer Function HIPERLAN-CAC Service

MAC Sublayer

HIPERLAN-Figure 13.13: Structure of HIPERLAN-MAC sublayer

The MAC protocol, which is a prerequisite for the services offered bythe HIPERLAN Channel-Access Control sublayer and introduced in Sec-tion 13.6.2, is used in the MAC sublayer for communication between HMentities For the communication, HIPERLAN-MAC protocol data units (HM-PDU) transmitted over an HC connection are exchanged between the HMentities For detection of the PDUs already received, each packet is assigned

a sequence number from the range 0–65 535

Initiated by the HMS user—in other words spontaneously (for ment)—HMPDUs are created within the MAC sublayer All HMPDUs arecollected in a queue, where they remain until they are transmitted or, because

manage-of the expiration manage-of their lifetime, are discarded

The HIPERLAN-MAC protocol provides for service data units (HMSDU)with maximum size 2385-byte length

The lookup function is used to establish the communications environment of

a HIPERLAN The following subfunctions are supported for this purpose:Find A HIPERLAN known by its name can be found if the HIPERLANidentifier (HID) associated with this name is established

Create For the HIPERLAN name indicated, an HID that is not being used inthe current communications environment is selected for creating a newHIPERLAN

Destroy A HIPERLAN is implicitly destroyed if there is no user using itsHID

Join A user implicitly joins a HIPERLAN by calling up its HID and the cryption and decryption key used and sending his subsequent data pack-ets using this HID

en-Leave A user implicitly leaves a HIPERLAN when he no longer uses its HID

in his data packets and evaluates packets with this HID

The lookup function is invoked after receipt of an HM Lookup Request

stations must arrive before it runs out (see Table 13.16)

All receivers of the LR-HMPDU create an LC-HMPDU addressed to thesender of the LR-HMPDU The HIPERLAN name and the HID of the HIPER-LAN to which the station belongs are entered in this packet

The information of all LC-HMPDUs which arrive at the HM entity beforethe timer runs out is filed in a table An HM Lookup Confirm service primitive

is used for forwarding this data to the HMS user making the inquiry

13.5.3.2 Routing Information Exchange Function

The Routing Information Exchange Function enables HM entities to establish

a routing information base (RIB) The RIB contains lists in which ing relationships between two HM entities and information about individual

neighbour-HM entities are administered A differentiation is made between the followingdata records:

• A neighbour tuple (N-tuple) with the MSAP address of a neighbourand the neighbouring relationship to this station All N-tuples togetherform a neighbour table (N-table) N-tuples that contain neighbouringforwarders are combined into a multipoint relay table (MRT) Wheneverthere is a change to this list, the allocated sequence number (MRSC-SN) is increased

• Source multipoint relay tuples (SMR tuples) contain the MSAP addressand the MRSC-SN of a neighbour that is using this station as a for-warder All SMR tuples are combined into a source multipoint relaytable (SMR table)

• A topology tuple (T-tuple) contains the MSAP address of an HM entity

in the network, the corresponding MRSC-SN and the MSAP address ofthe last hop on the route to this station The corresponding list is thetopology table (T-table)

• The routing tuple (R-tuple) indicates to an HM entity the MSAP address

of the next station on the route to the HM entity and the total number

of hops required All R-tuples together form a routing table (R-table).The forwarder administers all the lists mentioned above for all known sta-tions in the HIPERLAN; a non-forwarder only needs the SMR table

The Routing Information Exchange Function allows an exchange of mation that enables communication beyond the boundaries of the radio area.The procedures presented in the following sections enable stations to ascertaintheir immediate neighbourhood and the configuration of the HIPERLAN

by the MAC sublayer for creating the neighbour table, the multipoint relaytable and the source multipoint relay table; see Table 13.16 In this connectionall N-tuples are entered in an H-MPDU, which is then sent to all neighbours.The sequence number of the user’s own multipoint relay table along with thestation type (forwarder/non-forwarder) is also transmitted

HM entities that receive an H-HMPDU adapt their MRT and N and SMRtables according to the data contained in the information field If an N-tupledoes not yet exist for the sender, one is produced If the sender uses this

started for all data records that have been produced or changed, and when itruns out, the expired N-tuples or SMR tuples are discarded (aging)

Topology Control The topology control procedure allows the exchange ofinformation for the topology of the HIPERLAN A TC-HMPDU is created by

an HM entity at fixed times (every Ttcr) and whenever there has been a change

to the SMR table It contains a list of the SMR tuples and correspondingsequence number The packet is sent to all HM entities, and should also beforwarded by them A timer with Ttcris started for all the list elements andthe appropriate tuple is destroyed after the timer has run out

The receiver of a TC-HMPDU creates a T-tuple for each of the SMR tuplescontained in it or restores T-tuples that already exist Because of the existingT-tuples, the routing table is recalculated after any changes to the T-table

If the maximum number of hops has not yet been reached and an R-tuple,

in other words a path, exists for the creator of the HMPDU, the HMPDU is then forwarded with an increased hop timer

N-table that contains the forwarders important to this station is recalculated

sequence number is increased and the topology control procedure is invoked

To save power, an HM entity (P-saver ) is allowed to participate in the

provides the means for notifying other stations of the active and inactive ods Certain stations serve as P-supporters whereby they delay the messagesfor particular HM entities (unicast traffic) The supported stations must beentered on the P-saver list administered by the P-supporter

oper-ations, it sends a Wake Pattern Declaration packet (WPD-HMPDU) On thepacket it enters the time of the last wake state, the time interval between twowake states and the duration of the state This packet is transmitted to allneighbouring stations

After a P-supporter has received the packet, it adds the station with thenecessary parameters to the list of supported P-savers; in other words, itadjusts the values Each entry only has the lifetime Twp, and is then discarded

reception also of broadcast traffic, the P-supporters define time intervals bywhich they delay all received broadcast messages, thereby also allowing thedeferred reception of broadcast traffic P-savers can participate through anappropriate selection of their wake states

For coordination of the broadcast traffic the P-supporter transmits aZDMPD-HMPDU at fixed intervals and before every deferred transmission

It contains the time since the last transmission window, the time interval tween two areas, and the duration of the state This packet is sent to allneighbouring stations.

be-After receipt of the DMPD packet, P-savers who are using the transmittingtimes of the sender in order to receive multicast traffic adjust the durationand the time for their next active state according to the values supplied

A P-saver can also define additional wake states in order to participate inthe broadcast traffic This involves a new entry in the corresponding list Thevalidity of the entry expires after the time Tdmp

The User Data Transfer Function undertakes the transmission of data tween two HMS users on the basis of the specified quality of service parametersand in accordance with the existing routing information

be-The MSDU is normally transferred from the HMS user to the HM entity.Packed in a DT-HMPDU, the packet is passed from HM entity to HM entityuntil it has reached its destination or the preset parameters (HMQoS) can nolonger be maintained The destination HM entity forwards the MSDU to itsHMS users

an HMS user, the User Data Acceptance function must decide whether it cancarry out the transmission with the quality of service requested It eitheraccepts the MSDU or returns an error message The criteria for the decisioncould be average channel delays or other parameters which are not established

contain alias addresses for MSAPs which are required for routing the HMPDUs (see also alias resolution) Parameters and timers for the PDUscan be found in Section 13.8

DT-The routing function is called to forward the next HM entity on the route

to its destination

expired, the corresponding packet is discarded The HMS user then receivesthe appropriate error report

to this HMS user The MSDU is obtained from the DT-HMPDU and delivered

Table 13.5: Selection of DT-HMPDU on basis of alias addresses

with details about the quality of service achieved to the HMS user in an

HM Unitdata Indication primitive over the MAC service access point

may occur between alias addresses and MSAP addresses This function isnecessary for routing DT-HMPDUs, generates DT-HMPDUs of the corre-sponding type (DT0 to DT3) and sets the corresponding address fields.Alias resolution is initiated by the user data acceptance function and theforwarding function before the routing procedure determines the path of apacket

The alias table is searched for alias addresses for the source (SA) anddestination (DA) addresses The alias source address (ASA) and the aliasdestination address (ADA) are used to create new DT-HMPDUs with appro-priately set address fields Table 13.5 shows which DT-HMPDU is selecteddepending on the alias address

The alias learn function is invoked after each receipt of a DT-HMPDU

of the type DT1, DT2 or DT3 (these are the only ones that contain aliasaddresses) for compiling and updating the alias tables This function entersalias combinations that are not yet known into its tables All elements of thealias table are subject to aging They are discarded after expiration of thetime Tah

address of the next HCSAP in order to reach the next HM entity as well

as the number of hops remaining on the path to the destination HM tity This function is invoked by the user data acceptance function after an

en-HM Unitdata Request and by the forwarding function If no routing tion is available for the destination, the address of a neighbouring forwarder

informa-is selected If thinforma-is also informa-is not known, the reserved address for all adjacentstations is used

re-ceived, the packet must be forwarded by a forwarder The forwarding functiononly accepts packets from a unicast transmission when the number of hops

Table 13.6: Mapping quality of service parameters to channel-access priority

The HMPDU transfer function sends MAC protocol data units between HMentities At the start of transmission, the data packet with the highest channel-access priority is determined by the HMPDU transfer function to enable therespective service to be provided with the quality of service requested.The normalized residual MSDU lifetime, NRML is calculated from the resid-ual lifetime of the HMPDUs and the number of hops still required to reachthe receiver

Channel-access priority is then generated from the NRML and the user ority UP as per Table 13.6 and is updated during each transmitting cycle

from the HMPDU queue according to the following rules:

• Select HMPDU(s) with the highest channel-access priority

• From these select the one(s) with the lowest NRML

• And then select any one of them

Data where the residual lifetime has expired is not transmitted

through the selection of a key from the existing set for the coding of databefore transmission by the HMPDU transmission function while an HCSDU(HIPERLAN-CAM service data unit) is being generated The number of thekey used and the initialization vector are entered in the PDU

Taking the key number and the initialization vector, the receiver of a codedDT-HMPDU can then decrypt the packet.

of transmission errors

service provider for transmission whenever an HC Sync Indication primitiveexists or anytime after an HC Free Indication primitive (see Section 13.6.2.3).This involves selecting the packet with the greatest penetration, per the pro-cedure described above Through an HC Unitdata Request primitive, the HM-PDU is transferred with the necessary parameters to the CAC sublayer at theHIPERLAN-CAC Service Access Point (HCSAP) Without any changes, theHMPDU becomes the HCSDU The end of the transmission is indicated by

HC Status Indication

Upon receipt of a HC Data Indication primitive, the receiving HM entitymust accept the HCSDU received as an HMPDU and process it according toits type or its destination

Channel-access takes place in the Channel-Access Control (CAC) sublayerthrough a Channel-Access Mechanism (CAM) It is here that it is decidedduring different phases which station has access to the radio channel to enable

it to send asynchronous or time-critical traffic over the physical layer to thereceiver The CAC sublayer contains the functions described below

Although the CAC protocol for multiple radio channel operation is defined,channel selection is not specified in the standard

The MAC protocol expects the CAC sublayer with the CAM to provide ahierarchical independent channel-access mechanism for the support of time-critical traffic

An expanded Non-Preemptive Priority Multiple-Access procedure (NPMA)

is used for HIPERLAN This ensures that traffic with high priority is notinterrupted by traffic with a lower priority

The NPMA procedure works in channel-access cycles on the radio channel

An access cycle follows the preceding cycle or runs any time a channel isconsidered to be available Each transmission attempt is carried out according

Figure 13.15: HIPERLAN-CAC service model

to access priority, and is synchronized with the access cycles currently running

collision will occur if too many stations start to send at the same time

As a HIPERLAN-CAC service provider (HCS provider) the CAC sublayerprovides its services (HIPERLAN-CAC services HCS) to the MAC sublayerabove it at the CAC service access point In this case the MAC sublayer

is a HIPERLAN-CAC user (HCS user ) of the services offered by the CACsublayer A model illustrating this relationship is shown in Figure 13.15.The following is defined for the transmission of HIPERLAN-CAC servicedata units (HCSDUs):

• An HCSDU is transmitted from the source HCSAP to an individualdestination HCSAP or to a group of destination service access points in

a single request of the HCSDU transfer service The MAC service usercan identify himself and submit one or more destination HCSAPs The

Table 13.7: Possible values for channel-access priority

service is a connectionless one, i.e., the transfer does not require that anexplicit connection be set up and then terminated again later

• Each HCSDU transmission is independent of all the other ones

• Activation of the HCSDU transfer service is controlled by the HCprovider according to the NPMA channel-access procedure

• Each activation of the HCSDU transfer service is followed by an tion of whether or not the transmission was successful

indica-• An HCS user can provide the desired channel-access priority for thetransmission Aside from these parameters the user has no influence onthe means used by the HCS service provider for the transmission

• HCSDUs are transmitted without any restrictions in terms of content,format or coding; HCSDUs are never interpreted from the standpoint oftheir structure or content

of HCSAPs and their service users

The only quality-of-service parameter known to the CAC service is access priority This priority, which is defined in the NPMA, indicates therelative importance of the CAC service data unit during access to a sharedchannel (see Table 13.7) Numerically smaller values indicate a higher channel-access priority

Table 13.8 presents an overview of the service primitives that are possible atthe CAC service access point The parameters required for transmission arelisted in the last column for each primitive

Table 13.8: HCS primitives and their parameters

Data

transfer

HCSDU

transfer

HC Unitdata Request (Source Address, Destination

Address HCSDU, HIPERLANIdentifier, Channel Access Pri-ority

HC Unitdata Indication (Source Address, Destination

Address HCSDU, HIPERLANIdentifier)

Table 13.9: Transmission status

Transfer Transfer Successful The HCSDU transmission was successful.status Transfer Unsuccessful The HCSDU transmission was unsuccess-

ful This status is also used when the HCSprovider is no 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 serviceprovider that is transported between HCS users

HIPERLAN identifier HID of destination HIPERLAN

Transfer status Table 13.9 shows the possible status for the acknowledgement

of transmission by the HC Status Indication primitive

Activation of the HCSDU transfer function by an HCS user is basically ordinated by the transfer control function of the sublayer This function isused to notify the MAC sublayer when the CAC sublayer is ready to accept

co-an HCSDU to commence a chco-annel-access cycle

The HC service provider differentiates between two types of HCSDU fer In a synchronized HCSDU transfer the transmissions directly follow each

trans-Layer N +1

Site B Layer

t

t

HC_Unitdata_Indication HC_Unitdata_Request

Site A

HC_Sync_Indication

HC_Status_indication

N +1 N

Layer

SYNC

SYNC

Figure 13.16: Sequence of primitives with

successful transmission over a synchronized

transmission of the current packet has ended This is necessary to ensure thatthe NPMA channel-access procedure is functioning correctly If a channel hasnot been used for a longer period of time, transmission can begin immediatelyafter the HCSDU request There is no synchronization with other transfers.Every successful or unsuccessful transmission is acknowledged by the CACsublayer

al-lowed, the CAC sublayer notifies the MAC sublayer accordingly through an

signals to the HCS user that the CAC sublayer can accept data for the lowing access cycle on the radio channel

fol-If the MAC sublayer has HCSDUs available for transmission, it must, assoon as it receives the HC Unitdata Indication message, immediately create

an HC Unitdata Request primitive with the packet being sent to initiate theHCSDU transfer The HCSDU transfer function is not allowed to be invoked

at any other time Figure 13.16 illustrates the sequence of primitives for asuccessful transmission for synchronized access of the channel

Owing to the channel-access mechanism, it is possible that an HCSDUcould not be transmitted because another station seized the channel or because

a collision occurred This results in a negative acknowledgement of the transferrequest (see Figure 13.17)

that the CAC sublayer has to synchronize itself If the radio channel is Idlefor a longer period of time, the HCS provider notifies the MAC sublayeraccordingly through HC Free Indication The CAC sublayer thereby indicatesthat the HCS user can invoke the HCSDU transfer function at any time untilfurther notice If there is a change to the channel status, the HCS user isnotified through an HC Status Indication

Layer N +1 Layer N

Layer N +1

Site B

HC_Status_Indication HC_Free_Indication

HC_Status_Indication

HC_Free_Indication

HC_Unitdata_Request

HC_Unitdata_Indication

Figure 13.19: Sequence of primitives for

successful transmission with an idle

chan-nel

Layer N Layer N +1 Layer N +1

HC_Unitdata_Request HC_Free_Indication

HC_Status_indication

Site A Site B

Figure 13.20: Sequence of primitivesfor unsuccessful transmission with anidle channel

Figure 13.18 illustrates a case when there are no HCSDUs waiting for mission in the MAC sublayer while a channel is idle A request for the HCSDUtransfer function is not allowed once an HC Status Indication primitive hasbeen received

trans-If the MAC sublayer receives or generates an HCSDU during the time inwhich the channel is regarded as idle, it can send it for transmission to theCAC sublayer by invoking the HCSDU transfer function On the other hand,the transfer function can fail The sequence of service primitives for successfuland unsuccessful HCSDU transfer is presented in Figures 13.19 and 13.20

In each case the HCS user receives an acknowledgement of the HC data Request call through an HC Status Indication primitive

The CAC protocol is used for communication between two HC entities Itsupports the services offered to the HCS user and is described in Section 13.6.2.The HC protocol provides for the transport of HCSDUs with a maximumlength of 2422 bytes HC (HIPERLAN-CAC) protocol data units (HCPDUs)are exchanged between the HC entities and transmitted over a connection inthe physical layer (see Table 13.10)

Table 13.10: HIPERLAN-CAC protocol data units

Figure 13.21: Channel activities with EY-NPMA procedure

The structure of the HCPDU can be in two parts: one that is transmitted

at a low bit rate (low-bit-rate part, LBR part) and one that is transmitted at

a high bit rate (high-bit-rate part, HBR part) The HBR part always consists

of 1–47 blocks, each with 52 bytes Because of their structure, two types ofHCPDUs are possible:

• LBR-HCPDU, which only contains an LBR part

• LBR-HBR-HCPDU, which has an LBR and an HBR part

Transmission of an HCPDU over a channel is described in Section 13.7.The procedures required for the protocol are explained below

ETSI RES10 specified Elimination Yield–Non-Preemptive Priority MultipleAccess (EY-NPMA) as the channel access procedure for the HIPERLAN stan-dard With this procedure the collision rate is largely unaffected by the num-ber of stations competing for a channel It is based on the NPMA procedurediscussed in Section 13.6.1.2 As an extension of NPMA, a combination of anelimination phase and a yield phase is used for the contention phase

During the elimination phase, as many stations as possible should lose theirright to channel-access—but not all of them The remaining stations competeone more time during the yield phase

Figure 13.21 shows the phases and channel activities for an EY-NPMA

phases of EY-NPMA channel-access; the constants for the parameters defined

in the HIPERLAN standard can be found in Section 13.8

Defer Access

ESV PA

Detection Priority

Bursting Elimination

PA Elimination

Contention Phase Priority Phase

Detection PriorityA

cycle that follows the channel-access synchronization (CS) of the previouscycle, or they wait until the channel is free and then commence with the

the packet to be transmitted has channel-access priority n, the station detects

the radio channel is free at this time, the station immediately sends a burst(priority assertion, PA) However, if a burst has been sent by another station

access cycle immediately and waits for the next cycle (see Figure 13.22) Thepriority phase ends with the transmitted burst PA

The priority phase is followed by a contention phase during which a lection of stations takes place if a number of stations want to transmit datapackets with the same priority The decision is made about which stationsare allowed to transmit their packets over the radio channel The contentionphase consists of an elimination and a yield phase

elimi-nation slots (ES) long Each station sends an elimielimi-nation burst (EB) of length

0− mES slots The probability that the burst will continue into the next slot

is pE The probability that a station will send its burst for the duration of n

ES is derived from the geometric distribution:

pE(n) =

(pE)n(1− pE), 0≤ n < mES

(13.2)

At the end of its last burst, the station observes the channel for the duration

of an elimination survival verification interval (ESV) If the channel is idleduring this period, the station (A) then continues with the yield phase If,however, EBs of other stations are still being transmitted on the channel, itterminates (B) the cycle as shown in Figure 13.23

Defer Access Listening

Yield

A

B

Bursting Elimination

Bursting Elimination

Yield Phase Elimination Phase

Contention Phase Transmission Phase

Frame Transmission

Listening Yield

Figure 13.24: Yield phase with EY-NPMA procedure

Each station listens in on the radio channel for the duration of 0− mY S slots,with pY indicating the probability that a station will also hear a slot sequence

in the next slot The probability of a station observing the channel for theduration of n yield slots is geometrically distributed:

pY(n) =

(pY)n(1− pY), 0≤ n < mY S

(13.3)

If no other transmission was detected during the n yield slots of a station,the channel is interpreted as being free This means that the station has comethrough the contention phase and immediately begins to transmit its data.Stations that detect transmission by another station during the yield phaseare eliminated They are not allowed to attempt another access to the channeluntil the next cycle

Figure 13.24 shows two stations A and B trying to occupy a channel with

A succeeding in obtaining the channel and B having to delay its transmission

HC entities during the transmission phase:

un-der the following two conditions:

• Channel-access when there is a free channel In this case the channel

• Synchronized channel-access If a channel is not free, synchronization

is carried out at the end of the current transmission A full EY-NPMA

synchronization)

Different parts of the LBR part of an HCPDU are provided with a 4-bitchecksum during the generation of a packet An error detected in the checksumtest in an LBR-HCPDU when the packet is received will cause the entirepacket to be discarded In an LBR-HBR-HCPDU the information is alsocontained in the HBR part, so the packet is not invalid

Like the LBR part, the HBR part is also protected through error detection bythe means of checksums A 32-bit checksum is generated over the entire HBRpart when an HCPDU is created When received, the packet is discarded if

an error has occurred in the checksum

The HIPERLAN standard stipulates the five frequencies listed in Table 13.2

channels is specified by national authorities If use of channels 3 and 4 ispermitted then they are released over a CP-HCPDU

The CP-HCPDU is produced by authorized stations to notify neighbouringstations of the release of channels 3 and 4 The receiving station evaluates thepacket and, depending on the entries, either releases the channels or blocksthem again The validity of this release is checked regularly on a timed basis.Channels 3 and 4 are excluded from use once the timer runs out

The User Data Transfer Function is responsible for transporting data betweentwo HCS users in accordance with the CAC service definition HCSDUs aretransferred by the HCS user (of the MAC sublayer) with channel-access pri-ority for transmission to an HCS user or group of users The transfer functionshould only be invoked if the preceding request has been acknowledged Amulticast transmission is never acknowledged, and therefore its transfer isconsidered successful

The procedures of the transfer function are described in the following tions

the MAC sublayer that transmission is possible