HIPERLAN 2 [9–11] aims to provide high speed access (up to 54 Mbps at the physical layer) to a variety of networks including 3G networks, ATM and IP based networks and for private use as a wireless LAN system. Supported applications include data, voice and video, with specific QoS parameters taken into account. In contrast to the WLAN systems described in Chapter 9, HIPERLAN 2 is a connection-oriented system which uses fixed size packets.
HIPERLAN 2 is compatible with ATM. Its connection-oriented nature makes support for QoS applications easy to implement. In the following subsections, we describe the main aspects of HIPERLAN 2.
10.3.1 Network Architecture
The HIPERLAN 2 standard adopts an infrastructure topology. As shown in Figure 10.5, the network coverage area comprises a number of cells, with traffic in each cell being controlled by an Access Point (AP). Mobile terminals within a cell communicate with the cell’s AP through the HIPERLAN 2 air interface. Direct communication between two mobile terminals is also possible, however. this procedure is still in the development phase. Each mobile terminal can communicate only with one AP (that of the current cell). In order for such a communication to take place, an association procedure must first take place between the AP
Figure 10.5 HIPERLAN 2 network architecture
and the mobile terminal. After the association takes place, mobile terminals can freely move within the coverage area of the HIPERLAN 2 network while maintaining their logical connections. Moving to another cell is made possible through a handover procedure. The APs automatically configure the network by taking into account changes in topology due to mobility. Association and handover are revisited later in this section.
Being compatible with ATM, HIPERLAN 2 is a connection-oriented network using fixed size packets. Signaling functions are used to establish connections between the mobile nodes and the AP in a cell and data is transmitted over these connections as soon as they are established, using a time division multiplexing technique. The standard supports two types of connections: bi-directional point-to-point connections between a mobile node and an AP, and unidirectional point-to-multipoint connections carrying traffic to the mobile nodes.
Finally, there is a dedicated broadcast channel used by the AP to transmit data to all mobiles within its coverage.
The connection-oriented nature of HIPERLAN 2 makes support for QoS applications easy to implement. Each connection can be created so as to be characterized by certain quality requirements, like bounded delay, jitter and error rate. This support enables the HIPERLAN 2 network to support multimedia applications in a way similar to the ATM network.
HIPERLAN 2 also provides support for issues like encryption and security, power saving, dynamic channel allocation, radio cell handover, power control, etc. However, most of these issues are either not standardized yet or left to the vendors to implement.
10.3.2 The HIPERLAN 2 Protocol Stack
The protocol stack for the HIPERLAN 2 standard is shown in Figure 10.6. It comprises a control plane part and a user plane part following the semantics of ISDN functional partition- ing. The user plane includes functionality for transmission of traffic over established connec- tions, and the control plane provides procedures to control established connections. The protocol has three basic layers: the Physical Layer (PHY), the Data Link Control (DLC) layer, and the Convergence Layer (CL). At the moment, only the DLC includes control plane functionality. The various layers are discussed below.
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Figure 10.6 The HIPERLAN 2 protocol stack
10.3.2.1 HIPERLAN 2 Physical Layer
HIPERLAN 2 is characterized by high transmission rates at the physical layer, up to 54 Mbps.
The use of OFDM in the physical layer effectively combats the increased fading occurrence experienced in indoor radio environments, such as offices, etc., where the transmitted radio signals are subject to reflection from a number of objects, thus leading to multipath propaga- tion and consequently ISI. The channel spacing is 20 MHz with 52 subcarriers used for each channel. Of these, 48 subcarriers carry actual data and the remaining four are used as pilots in order to perform coherent demodulation.
HIPERLAN 2 is able to adapt to changing radio link quality through a Link Adaptation (LA) mechanism. Based on received signal quality which depends both on the AP-mobile terminal relative position and interference from nearby cells, LA dynamically selects the method of modulation and the Forward Error Correction (FEC) code to use in an effort to provide a robust physical layer. The alternative modulation methods are BPSK, QPSK, 16 QAM and 64 QAM. FEC is performed by a convolutional code with rate 1/2 and constraint length 7. The physical layer alternatives offered by LA are shown in Figure 10.7.
10.3.2.2 HIPERLAN 2 Data Link Control (DLC) Layer
The DLC layer is used to establish the logical links between APs and the MTs. The DLC layer comprises a number of sublayers providing medium access and connection handling services to upper layers. The DLC layer consists of three sublayers: the Medium Access Control (MAC) sublayer, the Error Control (EC) sublayer and the Radio Link Control (RLC) sublayer.
10.3.2.2.1 MAC Protocol and Channel Types The MAC protocol used by HIPERLAN 2 is based on time-division duplex (TDD) and dynamic time-division multiple access (TDMA).
MAC control is centralized and performed by each cell’s AP. The wireless medium is shared in the time domain through the use of a circulating MAC frame containing slots dedicated either to uplink or downlink traffic. The length of the MAC frame is fixed at 2 ms and comprises a number of parts which are not fixed. Rather, their lengths are variable in nature and are determined by the AP. Uplink and downlink slots within a frame are allocated dynamically depending on the need for transmission resources. All data from both mobile terminals and APs is transmitted in dedicated time slots. For mobile terminal
Figure 10.7 HIPERLAN 2 physical layer alternatives
transmission, slots are allocated after bandwidth requests made to the AP. The exact form of the MAC frame is shown in Figure 10.8, where one can see that apart from the parts dedicated to uplink and downlink traffic there are also broadcast, direct link and random access phases.
The broadcast frame carries the broadcast control channel and the frame control channel (both are described below). The direct link phase enables exchange of user traffic between mobile terminals without intervention of the AP. As mentioned above, this is optional.
Finally, the random access phase carries the random access channel (described below).
This phase is used by mobile terminals either for purposes of association with an AP, for control signaling when the terminal has not been allocated uplink slots within the MAC frame and during handover to a new AP for the purpose of switching ongoing connections to the new AP.
The MAC frame consists of several transport channels:
† The Broadcast Channel (BCH) is a downlink channel used to convey to the mobiles control information regarding transmission power levels, wake-up indicators for nodes in power save mode, length of the FCH and the RCH channels (described below) and the means to identify the HIPERLAN 2 network and the AP to which the mobile belongs.
† The Frame Control Channel (FCH) is a downlink channel used to notify the mobile nodes about resource allocation within the current MAC frame both for uplink and downlink traffic and for the RCH.
† The Random Access Channel (RCH) is used in the uplink both in order to request trans- mission in the downlink and uplink portions of future MAC frames and to transmit signaling messages. The RCH comprises contention slots which are used by the mobiles to compete for reservations. Collisions may occur and the results from RCH access are reported back to the mobiles in the Access Feedback Channel (ACH). When the request for transmission resources from the MTs arise, the AP can allocate more resources for the RCH in order to serve the increased demand.
† The Access Feedback Channel (ACH) is used on the downlink to notify about previous access attempts made in the RCH.
The above transport channels are used as a means to support a number of logical HIPERLAN 2 channels. The mapping is shown in Figures 10.9 and 10.10. The logical channels are as follows:
† The Slow Broadcast Channel (SBCH). All nodes within a cell can access the SBCH. It is a
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Figure 10.8 Structure of the 2 ms MAC frame
Figure 10.9 Mapping from logical to transport channels (downlink)
downlink channel that conveys broadcast control information concerning all the nodes within a cell. This transmission is initiated upon decision of the AP and may contain information regarding (a) the seed to be used for encryption, (b) handover acknowledg- ments, (c) MAC address assignments to non-associated mobile terminals, and (d) broad- cast of RLC and CL information.
† The Dedicated Control Channel (DCCH) is of bidirectional nature and is implicitly estab- lished when a terminal associates with the AP within a cell. After association with an AP has taken place, a terminal has its dedicated DCCH which is used to convey control signaling. The DCCH is realized as a DLC connection upon which RLC messages regard- ing association and control of DLC connections are exchanged.
† The User Data Channel (UDCH) transports user data cells between a mobile node and an AP and vice versa. A UDCH for a specific mobile node is established through signaling transmitted over the node’s DCCH. The UDCH establishment takes place after negotiation of certain quality parameters that characterize a connection. The DLC guarantees in- sequence delivery of the transmitted data cells to the convergence layer. The use of ARQ techniques is possible in UDCH operation, although there might be connections where ARQ is not used, such as multicasts and broadcasts. For uplink traffic, mobile requests for UDCH bandwidth are conveyed to the AP which then notifies the mobile whether or not it has been granted bandwidth through the FCH. For downlink traffic, the AP can reserve UDCH bandwidth without requests from mobiles.
† The Link Control Channel (LCCH) is a bidirectional channel used to exchange informa- tion regarding error control (EC) over a specific UDCH. The AP determines the necessary transmission slots for the LCCH in the uplink and the grant is announced in an upcoming FCH.
† The Association Control Channel (ASCH) is used by the mobile nodes either to request association or disassociation from a cell’s AP. Such messages are exchanged only (a) when a mobile terminal de-associates with an AP and (b) when a handover takes place.
10.3.2.2.2 Error Control Protocol The Error Control (EC) protocol of the HIPERLAN 2 protocol stack uses a selective repeat ARQ scheme in order to provide error-free, in-sequence data delivery to the convergence layer. Positive and negative acknowledgments are transmitted over the LCCH channel. In-sequence delivery is guaranteed by assigning proper sequence numbers to all frames of the connection. The number of retransmission attempts per frame is configurable. Furthermore, in an effort to support QoS for applications that are vulnerable to delay, the EC layer includes an out-of-date frame
Figure 10.10 Mapping from logical to transport channels (uplink)
discard mechanism. If a data cell becomes obsolete, then the sender EC layer can decide to discard it together with frames in the same connection with lower sequence number. In such a case, the responsibility for dealing with the data loss belongs to upper layers.
10.3.2.2.3 Radio Link Control Protocol The Radio Link Control (RLC) protocol provides services to the Association Control Function (ACF), Radio Resource Control function (RRC), and the DLC user Connection Control function (DCC). These signaling entities implement the DLC control plane functionality for exchange of control information between the AP and the mobile terminals.
The ACF is used by mobile nodes for purposes of:
† Association.In this case, a mobile node chooses among multiple APs the one with the best link quality. These measurements are made by listening to the BCH from the various APs, since the BCH provides a beacon signal to be used for this purpose. If association takes place, the AP grants the mobile terminal a unique MAC identity number. Then, the ASCH is used to exchange information with the AP regarding the capabilities of the DLC link to be established. For example, a mobile terminal may request from the AP information regarding the capabilities and characteristics of the links it can offer, such as the physical layer used, whether encryption is possible or not, supported authentication and encryption procedures and algorithms, supported convergence layers, etc. The AP replies with a set of supported PHY modes, a single convergence layer and a selected authentication and encryption procedure; an alternative is support for no authentication/encryption.
Supported encryption algorithms are DES and 3-DES. The two alternatives for authenti- cation are public key-based and pre-shared key authentication. If encryption is to be employed then the mobile terminals start a Diffie–Hellman key exchange procedure in order to determine the secret session key. This is used for encryption of all unicast traffic between the AP and the mobile terminal. Moreover, broadcast and multicast traffic can also be encrypted. This procedure takes place by using common keys at the mobile terminals and the AP (all mobile terminals under the same AP use the same common key). Common keys are distributed encrypted through the use of the unicast encryption key. Periodic changes of the various encryption keys increases system security. After the mobile node and the AP have associated, the AP can assign a DCCH to the mobile node which is used by the latter to establish one or more DLC connection, possibly each of different QoS.
† Deassociation.This can have either an explicit or an implicit form. In both cases the AP frees the resources which were allocated to the deassociated mobile terminal. In the first case, the AP is notified by the mobile terminal that the latter wants to deassociate (e.g.
when the terminal is about to shut down). In the second case, the AP deassociates with a specific terminal, when the latter remains unreachable from the AP for a specific time period.
No user data traffic transmission can take place unless at least one DLC connection has been established between the mobile terminal and the AP. Thus, the DCC function is used to establish DLC user connections by transmitting signaling messages over the DCCH. The AP assigns a unique connection identifier to each DLC connection. The signaling scheme is quite straightforward, comprising a request for a specific QoS connection followed by an acknowledgment when the request can be fulfilled. There also exist connections that manage
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both DLC connection release and modification of the parameters that characterize an existing DLC connection.
The RRC function manages the following procedures:
† Handover.For a mobile terminal handover is initiated when the quality of the link between the terminal and the current AP is inferior to that of a link to another AP. There are two handover methods in HIPERLAN 2: reassociation and handover via signaling across the fixed network. The first method takes place when the mobile terminal deassociates with an AP and reinitiates association with another AP. The second method involves exchange of information regarding association and connection control between the old and new APs.
This information transfer between the APs takes place across the fixed network. The method for making link quality measurements for handovers is not defined in HIPERLAN 2. Rather, each vendor is free to either base it on signal strength or another quality criterion.
† Dynamic frequency selection.This RRC function automatically allocates frequencies to the various APs of a HIPERLAN 2 network. Allocation is made in a way that avoids use of interfering frequencies through measurements made by APs and mobile terminals. The latter contribute to the procedure upon request of their AP to perform measurements regarding the radio signals received from nearby APs. Since the radio environment is due to be dynamic, APs are likely to change operating frequency while already involved in an ongoing connection. Thus, RRC also includes signaling functionality to inform mobile terminals of an upcoming change in the operating frequency of their AP.
† ‘Mobile terminal alive’.This procedure enables second case of deassociation mentioned above. When mobile terminals are idle, their AP tracks them by periodically transmitting
‘alive’ messages to these terminals. Alive messages are followed by responses from idle terminals and thus APs are able to supervise them. If an idle terminal does not respond to the ‘APs’ alive messages, it is deassociated from the AP. Alternatively APs do not transmit alive messages but rather monitor idle terminals for a specific time period. When this period has elapsed the terminal is deassociated.
† Power saving. This is a process controlled by the mobile terminal. A mobile terminal selects a sleeping time of durationNframes, with 2#N#216. After theseNframes have elapsed, the following scenarios are possible: (a) the AP wakes up the mobile terminal due to data pending for this terminal at the AP; (b) the mobile terminal wakes up due to data pending for transmission at this terminal; (c) the mobile terminal goes to sleep for another Nframes; (d) the mobile terminal, after failing to receive the wake-up messages from the AP, wakes up afterNframes and performs the ‘mobile terminal alive’ procedure.
10.3.2.3 HIPERLAN 2 Convergence Layer (CL)
The CL of the protocol stack carries out two functions. The first is to segment the higher layer PDUs into fixed size packets used by the DLC. The second is to adapt the services demanded by the higher layers to those offered by the DLC. This function requires reassembly of the fixed-size DLC packets to the original variable-size packets used by the higher layers. There are currently two different types of CLs defined:
† Cell-based CL. The cell-based CL serves interconnection to ATM networks and transpar-
ently integrates HIPERLAN 2 with ATM. In the cell-based CL, Segmentation and Reas- sembly (SAR) functionality is not included because ATM cells fit into the HIPERLAN 2 DLC PDU. Nevertheless, a compression of the ATM cell header is necessary, transmitting only its most important parts.
† Packet-based CL.The packet based CL is used to interconnect WATM mobiles to legacy wired LANs like Ethernet. The packet-based CL comprises a common part and several Service Specific parts (SSCS), as shown in Figure 10.11. SSCSs allow for easy interfacing with fixed networks. The common part has the responsibility of segmenting packets received from SSCSs before handing them down to lower layers. Similarly, it is a respon- sibility of the common part to reassemble segmented packets received from the DLC before these are handed to the appropriate SSCS. Furthermore, the common part is responsible for adding padding bytes in an effort to make common part Protocol Data Units (PDUs) and an integral number of DLC Service Data Units (SDUs).
The overall performance of a HIPERLAN 2 system depends on a number of factors, including available channel frequencies, propagation conditions and interference experi- enced. Tests have shown that, in most cases, data rates above 20 Mbps (at the DLC layer) are likely to be achieved.