Time-controlled Communication and the 802.11 Recommendation

A Baltimore-based study has found that computers are asocial by design. The only time family members are likely to gather around a computer is when something is wrong with it.

The IEEE 802.11 recommendation is the basis for a few wireless communications standards. One of these is Bluetooth, and since Bluetooth is an important part of the FlyWay concept, we have included some background information about Bluetooth on this website. And, since Bluetooth cannot be properly understood if one is not familiar with the IEEE 802.11 recommendation, we must include a little something about this recommendation as well on this website.

The IEEE 802.11 recommendation provides considerable leeway as to how it should be applied. It only sets the foundation, so to speak; it only specifies the lower protocol levels, and sets some limits to what upper protocols can and cannot do. It is rather important to know which functions are part of the IEEE 802.11 recommendation and what belongs to the Bluetooth standard.

Time-controlled distributed communication The IEEE 802.11 Recommendation IEEE 802.11a, b and g Interference See next page for some Bluetooth abbreviations & acronyms, and the Bluetooth profiles.

1. Time-controlled Distributed Communication

utomatically controlled transport systems, such as SwedeTrack System´s FlyWay®, depend for their function on reliable communication between geographically distributed computers, some of which are mobile. The basis for such a system clearly has to be radiowave-based. It should also preferably be time-controlled, and we will here motivate the reason why. Complex systems will not always behave as anticipated. Not only because they might contain bugs, but also because it is extremely time consuming to try to anticipate, during system design, every conceivable situation that could occur. Reliability at information transfer can always be achieved through data verification at the receiving end, and subsequent re-transmission of the same data block, if needed. But this is not good enough in time-critical systems such as FlyWay. While re-transmission of garbled or obstructed information is usually an option here, and is included in the Bluetooth protocol, it cannot be solely relied upon. The reason is, of course, that the waiting for a negative acknowledgement and handling of the re-transmission takes too long time, considering that the vehicles are moving, sometimes at high speed, and the re-transmission might occur at the precise moment that a vehicle moves out of range of the stationary transmitter/receiver.

We thus have 2 options: Time-controlled systems, where every node globally or in a defined group has been allocated a recurring timeslot. This is how Bluetooth function. Event-controlled systems, where transmission occurs when a unit is prompted into action by an event, or generally has something to say. To avoid call collision, one usually has to introduce priority between the participating nodes giving some nodes preference. This order of priority would of course depend on the urgency of the transmission, according to some set rules, and would have to alter dynamically as the situation alters. Of these 2 options, FlyWay and similar systems would clearly be best served by using a time-controlled systems, mainly for the following reasons:

Missing information can be quickly identified in a time-controlled system, because even if this information is not expected at the receiving end, the sending unit would still be expected to send something, such as a signature, in its alloted timeslot. An absence of transmission in a timeslot would thus indicate that transmission has been blocked for some reason, and appropriate action can then be taken by the receiving unit. Using time-controlled systems reduces system complexity and simplifies the design process, since every node can be implemented independently of other nodes´ presence or absence in the system. Communication which entails continual control of some sort can be more "smooth", insofar as information from the controlling unit arrives at regular and short intervals. This means that controls that depend on regular feed-back from the controlled unit are best served by time-controlled systems. Alloting a timeslot to every unit means that no unit can block another units´ transmission because of (assumed) higher priority.

2. The IEEE 802.11 Recommendation

luetooth is based on the The IEEE 802.11 standard. This standard defines the protocol for two types of networks; Ad-hoc and client/server networks. An Ad-hoc network is a simple network where communications are established between multiple stations in a defined coverage area, without the use of an access point or server. The standard specifies the etiquette that each station must observe so that they all have fair access to the wireless media. It provides methods for arbitrating requests to use the media to ensure that throughput is maximized for all of the users. The client/server network uses an access point that controls the allocation of transmit time for all stations, and allows mobile stations to roam from cell to cell. The access point is used to handle traffic from the mobile radio to the wired or wireless backbone of the client/server network. This arrangement allows for point coordination of all of the stations in the basic service area and ensures proper handling of the data traffic. The access point routes data between the stations and other wireless stations or to and from the network server. The 802.11 Physical Layer The IEEE 802 standards committee formed the 802.11 Wireless Local Area Networks Standards Working Group in 1990. The 802.11 working group took on the task of developing a global standard for radio equipment and networks operating in the 2.4GHz unlicensed frequency band for data rates of 1 and 2 Mbps. The standard does not specify technology or implementation but simply specifications for the physical layer and Media Access Control (MAC) layer. The Physical Layer in any network defines the modulation and signaling characteristics for the transmission of data. At the physical layer, two RF transmission methods and one infrared are defined. Operation of the WLAN in unlicensed RF bands requires the spread of spectrum modulation to meet the requirements for operation in most countries. The 2 RF transmission modes specified in the 802.11 standard are: Frequency Hopping Spread Spectrum (FHSS) and Direct Sequence Spread Spectrum (DSSS).

Both architectures are defined for operation in the 2.4GHz frequency band, typically occupying the 83 MHz of bandwidth from 2.400 GHz to 2.483 GHz. Differential BPSK (DBPSK) and DQPSK is the modulation for the direct sequence. Frequency hopping uses 2-4 level Gaussian FSK as the modulation signaling method. The radiated RF power at the antenna is set by the rules governed by FCC part 15 for operation in the United States. Antenna gain is also limited to 6 dBi maximum. The radiated power is limited to 1W for the United States, 10mW per 1Mhz in Europe and 10mW for Japan. There are different frequencies approved for use in Japan, United States and Europe. Figure 2:2 The difference between 802.11a and 802.11b The 802.11 is divided up into an a and a b part. These parts cover slightly different ways of operating, and there will probably be some additions to the specifications. The 802.11a standard, as specified by the Silicon Valley company "Atheros", will operate over the 5 GHz band, a relatively uncrowded frequency, and will offer connections as fast as 72 Mbps. As stated on the previous page, Wi-Fi" is based on 802.11b. Today's Wi-Fi products, which transmit in the unlicensed spectrum at 2.4-2.5 GHz, are capable of speeds of up to 11 Mbps.

Figure 2:3	DSSS & FHSS Bluetooth units will use the FHSS method. But the other method should also be mentioned briefly. The DSSS physical layer uses an 11-bit Barker Sequence to spread the data before it is transmitted. Each bit transmitted is modulated by the 11-bit sequence. This process spreads the RF energy across a wider bandwidth than would be required to transmit the raw data. The processing gain of the system is defined as 10x the log of the ratio of spreading rate (also know as the chip rate) to the data. The receiver despreads the RF input to recover the original data. The advantage of this technique is that it reduces the effect of narrowband sources of interference. This sequence provides 10.4dB of processing gain which meets the minimum requirements for the rules set forth by the FCC.	The FHSS physical layer has 22 different hop patterns to choose from. The frequency hop physical layer (see figure 2:2 above) is required to hop across the 2.4GHz ISM band covering 79 channels. Each channel occupies 1Mhz of bandwidth and must hop at the minimum rate specified by the regulatory bodies of the intended country. A minimum hop rate of 2.5 hops per second is specified for the United States. Bluetooth goes far beyond that, with its 1600 hops/second rate. Each of the physical layers use their own unique header to synchronize the receiver and to determine signal modulation format and data packet length. The physical layer headers are always transmitted at 1Mbps. Predefined fields in the headers provide the option to increase the data rate to 2 Mbps for the actual data packet.

The MAC Layer and Collision Avoidance in 802.11 The Media Access Control layer specification for 802.11 has similarities to the 802.3 Ethernet wired line standard. The protocol for 802.11 uses a protocol scheme know as carrier-sense, multiple access, collision avoidance (CSMA/CA). This protocol avoids collisions instead of detecting a collision like the algorithm used in 802.3. It is difficult to detect collisions in an RF transmission network and it is for this reason that collision avoidance is used. The MAC layer operates together with the physical layer by sampling the transmitted energy over the medium transmitting data. The physical layer uses a clear channel assessment (CCA) algorithm to determine if the channel is clear. This is accomplished by measuring the RF energy at the antenna and determining the strength of the received signal. This measured signal is commonly known as RSSI. If the received signal strength is below a specified threshold the channel is declared clear and the MAC layer is given the clear channel status for data transmission. If the RF energy is above the threshold, data transmissions are deferred in accordance with the protocol rules. The standard provides another option for CCA that can be alone or with the RSSI measurement. Carrier sense can be used to determine if the channel is available. This technique is more selective sense since it verifies that the signal is the same carrier type as 802.11 transmitters. The best method to use depends upon the levels of interference in the operating environment. The CSMA/CA protocol allows for options the can minimize collisions by using request to send (RTS), clear-to-send (CTS), data and acknowledge (ACK) transmission frames, in a sequential fashion. Communications is established when one of the wireless nodes sends a short message RTS frame. The RTS frame includes the destination and the length of message. The message duration is known as the network allocation vector (NAV). The NAV alerts all others in the medium, to back off for the duration of the transmission.

The receiving station issues a CTS frame which echoes the senders address and the NAV. If the CTS frame is not received, it is assumed that a collision occurred and the RTS process starts over. After the data frame is received, an ACK frame is sent back verifying a successful data transmission. A common limitation with wireless LAN systems is the "hidden node" problem. This can disrupt 40% or more of the communications in a highly loaded LAN environment. It occurs when there is a station in a service set that cannot detect the transmission of another station, and thus cannot detect that the media is busy. In figure 2:5, stations A and C can communicate, and likewise B and C. However, an obstruction prevents station B from receiving station A directly and B thus cannot determine when the channel is busy. Therefore both stations A and B could try to transmit at the same time to station C. The use of RTS, CTS, Data and ACK sequences helps to prevent the disruptions caused by this problem. Figure 2:5

Security in 802.11 Security provisions are addressed in the 802.11 recommendation as an optional feature for those concerned about eaves dropping. The data security is accomplished by a complex encryption technique know as the Wired Equivalent Privacy Algorithm (WEP). WEP is based on protecting the transmitted data over the RF medium using a 64-bit seed key and the RC4 encryption algorithm. WEP, when enabled, only protects the data packet information and does not protect the physical layer header. Consequently, other stations on the network can listen to the control data needed to manage the network. However, the other stations cannot decrypt the data portions of the packet. That was deemed to be sufficient, as far as protecting data is concerned. However, this WEP security is rather easily hacked. There is even a "sniffer"-program called "Airsnort" that can dekrypt messages in real time, i.e. as they are being sent. The WEP algorithm is repetitive, so an intelligent decryption program just has to compare a few strings of data with each other. "Airsnort" would read up to 1 MByte of transmitted data and then, by comparison and deduction, decrypt the code in about one second! This means that security-sensitive data transmission should not rely on WEP, but rather on some VPN-encryption. These are parts of communications standards that make use of 802.11.

3. IEEE 802.11a, b and g

As of this writing, the IEEE 802.11 standard has evolved into 3 complementary recommendations, called A, B and G. 802.11A IEEE 802.11a devices use a different radio technology from 802.11b and operate in the 5 GHz bands. IEEE 802.11a therefore is a supplement to the basic IEEE 802.11 standard. Although the IEEE 802.11a standard operates in a different unlicensed radio band, it shares the same proven Medium Access Controller (MAC) protocol as Wi-Fi. In more technical terms, IEEE 802.11a standardizes a different physical layer (PHY). Since products conforming to the IEEE 802.11a standard will operate in different radio bands, they will not be interoperable with Wi-Fi radios, which follow the b-recommendation (see below). 802.11B 802.11b contains some further definitions of the physical layer, and provides for interoperability of Wi-Fi™ WLAN products. Wi-Fi products operate in the worldwide 2.4 GHz Industry, Science, and Medicine (ISM) band.	Figure 3:1	802.11G As of this writing, the IEEE 802.11g recommendation has been accepted, but not implemented. An example, using Intersil´s idea, is shown in figure 3:1. OFDM (Orthogonal Frequency Division Multiplexing) is a compulsory part of IEEE 802.11g and provides for transmission speeds up to 54 Mbit/sec. It would be compatible with WiFi. It also supports CCK (Complementary Code Keying) in order to be compatible with existing radio units that adhere to IEEE 802.11b. The CCK transmission mode, also used by WiFi, uses one single carrier, while OFDM is a new technique, just entering the WLAN-market. It can be used both at 2.4 and 5 GHz carrier frequencies. OFDM is quite interesting. Different blocks of the same data transmission is divided between sub-carriers, thus enhancing receptivity also in environment having strong signal distorsion. It also has greater transmission capacity than CCK.

4. Interference

As noted elsewhere, different transmitters within each other´s range can occasionally transmit on the same frequencies at the same time. Figure 4:1 shows an example of a single Bluetooth piconet´s transmission pattern, using 79 channels, 1 MHz apart. Figure 4:2 shows what can happen when two piconets share the same physical space. Occasional interference can occur, but tests have shown that up to ten piconets can share the same space without serious degradation of performance. The definition of "serious degradation" would of course depend on how the service is used, and what demands are put on its performance. UWB achieves its great bandwith by doing just that; extending its bandwith. Each transmission is extremely short in duration but, as shown in figure 4:3, if a UWB system exist in the same space as a Bluetooth piconet, and has the same transmission range, it cannot help but interfere with the Bluetooth transmissions. The only way to avoid this would be to restrict UWB to a frequency range outside of the Bluetooth range.

Figure 4:1

Figure 4:2

Figure 4:3