HIPERLAN/2 and 802.11a: A Comparative Study PADMA BONDE Reader, Department of Computer Science Shri Vaishnav Institute of Technology and Science Indore, INDIA JAYESH BONDE Executive Engineer, Department of Information Technology Madhya Pradesh Electricity Board Indore, INDIA Abstract: - This paper compares the HIPERLAN/2 and IEEE s 802.11a standards. At present, Wireless Local Area Networks supporting broadband multimedia communication are being developed and standardized. Both WLAN standards will provide data rates up to 54 Mbps in the 5GHz band. An overview of the two standards is presented together with physical layer and MAC layer. Keywords: - High PERformance Local Area Network (HIPERLAN/2); IEEE 802.11a; European Telecommunications Standards Institute (ETSI); Broadband Radio Access Network (BRAN) 1 Introduction Wireless Local Area Networks (WLANs) provide wideband wireless connectivity without the need for cable installation. WLANs can be used in different environments such as home, public, corporate, industry and government. The worldwide demand for broadband wireless communications for multimedia applications supporting higher data rates has prompted development of new standards to achieve compatibility and interoperability between competing technologies. These standards define protocols in the Open Systems Interconnection (OSI) physical and link layers. These standards differ primarily in the Medium Access Control (MAC), but some differences also occur in the physical layers [1,2]. 2 Architecture Layer The HIPERLAN/2 standard is defined such that there are independent Physical (PHY), Data Link Control (DLC), and Convergence Layers (CL). The DLC sublayer is composed of the MAC and Radio Link Control (RLC) sublayers. The CL provides for inter-facing with: Ethernet, Point-to-Point Protocol Internet Protocol, Asynchronous Transfer Mode (ATM), Universal Mobile Telecommunications System (UMTS), and IEEE 1394 infrastructure. IEEE 802.11a similarly defines independent PHY and MAC sublayers. The IEEE 802.11a network is such that the MAC sublayer contained within the Data Link Layer (DLL) and interfaces to the backbone network through the 802.1 Bridging and 802.2 Logical Link sublayers [3, 4]. Each standard specifies a preamble in the MAC sublayer which is used to alert network devices that a data frame is coming which provides synchronization through specific training symbol sequences. The training symbols used for channel estimation are the same, while those provided for synchronization are different. The
architecture for the two standards is depicted in Figure 1. BACKBONE NETWORK H I P E R LA N RLC AIRWAVES 5 6 GHz (a) (b) Figure 1 (a) and (b) Layer Architecture for HIPERLAN/2 and 802.11a, respectively. 3 Physical Layers CL MAC PHY BACKBONE NETWORK 802.2 802.1 MAC PHY AIRWAVES 5 6 GHz LLC The physical layers for both standards provide the interface to the airwaves for wireless networks and use Orthogonal Frequency Division Multiplexing (OFDM) as a coding scheme. OFDM is used to reduce frequency selective fading and to randomize the burst errors caused by a wideband fading channel. Data for transmission is supplied to the D L C 802.11a physical layer in the form of PDU (Protocol Data Unit) which is input to a scrambler that prevents long runs of 1s and 0s in the input data being input to the remainder of the modulation process. Although both 802.11a and HIPERLAN/2 scramble the data with a length 127 pseudo random sequence, but the initialization of the scrambler is different. The scrambled data is input to a convolutional encoder is used by both standards to enable error detection and correction. The encoder consists of a ½ rate mother code and subsequent puncturing. The puncturing schemes facilitate the use of the code rates: 1/2, 3/4, 9/16 (HIPERLAN/2 only) and 2/3 (802.11a only).interleaving is used to minimize burst errors. The encoded and interleaved data are mapped into data symbols according to either a BPSK, QPSK, 16-QAM or 64-QAM modulation scheme [3, 4]. 3.1 HIPERLAN/2 MAC Sublayer The MAC sublayer in HIPERLAN/2 is based on a Time Division Duplex/Time Division Multiple Access (TDD/TDMA) approach, uses time frame with 2 Ms. Time slots are allocated dynamically depending on the need for transmission resources. The control is centralized to an Access Point (AP) which informs the Mobile Terminals (MT) to transmit their data. HIPERLAN/2 MAC sublayer frame structure is shown in Figure 2. It comprises time slots for Broadcast Control (BCH), Frame Control (FCH), Access Feedback Control (ACH), and data transmission in three different phases: Downlink (DL), Uplink (UL), and Directlink (DiL) and Random Access Channel (RCH), used by MT to request capacity from the AP. 2 ms DL, UL, BCH FCH ACH RCH DiL Phase Figure 2 HIPERLAN/2 MAC Frame Structure
DL, UL and DiL phases consist of long and short PDUs corresponding referred as long transport channel (LCH) and Short Transport Channel (SCH). The long PDUs (Figure 3) have a size of 54 bytes and contain control or user data. The payload is 49.5 bytes and the remaining 4.5 bytes are used for the PDU Type, a Sequence Number (SN) and Cyclic Redundancy Check (CRC). PDU Type 2 bits SN 10 bits 54 bytes Payload 49.5 Bytes Figure 3 Format of the Long PDUs CRC 3 Bytes Short PDUs contain only control data and have a size of 9 bytes. The 9-byte payload consists of PDU Type, Information, and the CRC. They may contain resource requests, Automatic Retransmission request (ARQ) messages, etc. Traffic from multiple connections to/from one MT can be multiplexed onto one PDU train. PDU train payload and a preamble form a physical burst (Figure 4) which is to be transmitted via the physical layer. Figure 4 HIPERLAN/2 Physical Burst Format Two protocols defined by RLC sublayer to address interference issues which are Dynamic Frequency Selection (DFS) and Transmission Power Control (TPC). 3.2 IEEE 802.11a MAC Sublayer 802.11a uses a distributed MAC sublayer protocol based on Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA)and uses a packet structure. In IEEE 802.11, if the medium is idle the MT can start transmitting the packet, otherwise the transmission is deferred and a wait process begins. Once the wait time has expired, the terminal can access the medium again. As a collision in a wireless environment is undetectable, a positive acknowledgement is used to indicate that a packet has been successfully received. If this acknowledgement is not received, the terminal will retransmit the packet. Figure 5 shows the format of a complete packet PDU in 802.11a, including the preamble, header and Physical Layer Service Data Unit (PSDU) or payload. The header contains information about the transmission rate, length of the payload, a parity bit, 6 zero tail bits, and 16 service bits. The rate field conveys information about the type of modulation and the coding rate used for the rest of the packet. The length field takes a value between 1 and 4095 and specifies the number of bytes in the PSDU. The Service field is used to initiate the scrambler process. The six tail bits are used to reset the convolutional encoder and to terminate the code trellis in the decoder. The first 7 bits of the service field are set to zero and are used to initialize the descrambler. The remaining nine bits are reserved for future use. The pad bits are used to ensure that the number of bits in the PPDU maps to an integer number of OFDM symbols [3, 5, and 6] 4 Conclusions Table 1 shows a side-by-side comparison of the major features of each standard. At present, there is a great deal of effort on-going within the IEEE 802 Committee to harmonize these two standards as well as to make IEEE 802.11a compatible in other countries such as Japan. IEEE 802.11h has been approved by the IEEE, which harmonizes 802.11a with HIPERLAN/2. IEEE 802.11j has also been approved, which harmonizes 802.11a for use within Japan. It can
be concluded that 802.11a and HIPERLAN/2 PHY layers are compatible, 802.11a and HIPERLAN/2 should be able to exchange data supporting an Ethernet environment, 802.11h supplement has been approved. This opens the door to potential wider deployment of 802.11a networks within Europe. 802.11a is currently working to develop QoS, through the 802.11e WG, to support the features that HIPERLAN/2 already has. References [1]Mark Ciampa, Guide to Designing and Implementing Wireless LANs, pp. 6-8. [2]C.S.R. Prabhu and Pratap Reddi, Bluetooth Technology and its Application with Java and J 2 Me, pp. 292-300 & 307-311, 2004. [3]Martin Johnson, HiperLAN/2- The Broadband Radio Transmission Technology Operating in the 5GHz Frequency Band, HiperLAN/2 Global Forum, 1999. [4] Erina Ferro and Francesco Potorti, Institute of Research Council, IEEE Wireless Communication, Bluetooth and WiFi Wireless protocol: A Survey and comparison, Vol. 12, No. 1 Feb 2004. [5]Dale Barr, A Comparison of IEEE 802.11a and HIPERLAN/2 Wireless Networks, Technology and Program Division,volume II, Number 2,October 2004. [6] Angela Doufexi, Simon Armour, Peter Karlsson, Andrew Nix, David Bull Centre for Communications Research, University of Bristol, UK elia Research AB, Malmoe, A Comparison of HIPERLAN/2 and IEEE 802.11a.
Header Rate 4-bits Reserved 4 bits Lengh 12 bits Parity 1 bits Tail 6 bits Service 16 bits PSDU Tail 6 bits Pad bits BPSK ½ Rate Mode that is indicated RATE PREAMBLE 12 bits SIGNAL One OFDM Symbol DATA Variable Number of OFDM Symbols Figure 5 802.11a Packet PDU (PPDU) Format Table 1 Comparison between HIPERLAN/2 and 802.11a HIPERLAN/2 802.11a Frequency Band 5GHz 5GHz Multiplexing OFDM OFDM Noise Adoption Physical Layer Physical Layer Maximum Signal Rate 54 Mbps 54 Mbps Channel Access Method TDD/TDMA CSMA/CA Connectivity Connection Oriented Connection less Frequency Selection Single carrier with dynamic Single carrier frequency allocation Encryption DES,3DES RC4 stream QoS mechanism Supports Coordination function Fixed Network Support Ethernet,IP,ATM,UMTS,IEEE1394 Ethernet Management HIPERLAN/2 MIB 802.11 MIB Nominal Range 150m 100m Regional Support European Worldwide Cost High High Network Topologies Cellular networks Ad-hoc or Infrastructure