The use of spread spectrum technology to enhance telecommunication systems was originally developed during World War II. Both military and diplomatic organizations recognized significant advantages of a technology with the ability to provide reliable communications in a RF environment prone to frequency jamming and the ability to prevent the signal from being intercepted by third parties.

Despite the widespread military use, the technology was not made available for commercial use until 1985 when the FCC issued Rule Part 15.247 - which has also been adopted by the European Authority, ETS 300 328. This ruling permitted the use of spread spectrum technology for commercial applications in the 900, 2400, and 5800 MHz ISM frequency bands. Since then, the world-wide telecommunications industry has also recognized the inherent advantages of spread spectrum microwave radios for use in mobile cellular networks, private networks, and local access applications.

Fundamentals of the Technology

Contrary to conventional narrowband modulation techniques that concentrate a signal in a narrow bandwidth, spread spectrum modulation techniques use a much wider bandwidth. A typical spread spectrum microwave radio transmitter will integrate an E1 channel with a sequence of coding bits, referred to as Pseudo Noise (PN) code, and spreads the signal over a bandwidth usually from 20 to 30 MHz for this channel. The spreading is actually accomplished using one of two different methods, either direct sequence or frequency hopping.

Direct sequence spread spectrum uses the Pseudo Noise code, integrated with the E1 channel, to generate a binary signal that can be duplicated and synchronized at both the transmitter and receiver. The resulting signal (desired signal) evenly distributes the power over a wider frequency spectrum and a lower power level. At the receiver, both the desired and foreign signals are "despread" to effectively regenerate the desired signal and suppress foreign signals. This interference rejection capability, very popular for military communication applications, has also been recognized and widely adopted throughout the telecommunications industry. Specifically, the direct sequence spread spectrum technique is often used to transmit higher speed digital data for E1, T1, or high-speed wireless data networks.

Direct sequence spread spectrum modulation techniques have two principle advantages: the transmitted signal is diluted over a wide bandwidth which minimizes the amount of power present at any given frequency. The net result is a signal that is below the noise floor of conventional narrowband receivers, but is still within the minimum receiver threshold for a spread spectrum receiver.

While the direct sequence receiver is able to detect very low signal powers, the receiver is also designed to reject unwanted carriers, even signals which are considerably higher in power than the desired spread spectrum signal. Each transmitter and receiver is programmed with unique spreading sequences that are used to de-spread the desired signal and spread the undesired signal - effectively canceling the noise.

The second popular type of spread spectrum modulation is frequency hopping. This technique is very different than direct sequence in that the carrier signal is similar to a conventional narrowband carrier. The difference being that the carrier signal will change, or "hop", the actual transmit frequency many times each second. The random hopping sequence will utilize the entire band through the entire hopping sequence, but will only occupy a narrow spectrum at any given time. The result is a highly effective communication link especially for wireless data networks such as Internet Service Providers.

System Planning, Analysis and Implementation

Professional system planning is fundamental to the successful installation, operation and proper performance of any communication system. Since the installation of every radio system will be different, planning and implementation issues can vary widely. Compared with visible light, the spread spectrum RF bands are not appreciably affected by fog, clouds, rain, snow, hail, smoke, smog, etc. Since the wavelength of the RF signal is much larger than the physical size of the components which make up these items (e.g. water droplets and smoke particles), all of these items are virtually transparent to the radio signal.

When RF energy is transmitted from a parabolic antenna, the energy spreads outward, much like a beam of light. This microwave beam can be influenced by the terrain between the antennas, as well as by objects in or along the path. Some level of signal loss will occur due to diffraction when the centerline of a beam from one antenna to another antenna just grazes an obstacle along the path. To minimize the possibility of signal loss due to refraction, diffraction, and reflection, as well as by other effects of obstructions and terrain, microwave paths must be properly engineered to address all of these issues.

For relatively short 2.4 or 5.7 GHz microwave paths, only reflection points and obstructions are usually of real concern. The effects of atmosphere, earth curvature, etc. will not usually come into play, so the engineering of these paths is quite straight forward.

Fresnel Zone Clearance

A discussion on the propagation characteristics of a microwave beam always brings up the subject of Fresnel zones and Fresnel zone clearances. Fresnel zones can be viewed as a series of concentric ellipsoids surrounding the entire microwave path. A cross-section view of the microwave path would be a series of concentric circles. From a propagation point of view, the first Fresnel zone is defined as the "surface" containing every point for which the sum of the distances from that point to the two ends of the path is exactly one-half wavelength longer than the direct end-to-end path. The "nth" Fresnel zone is defined in the same manner, except that the difference is in n half-wavelengths.

Fresnel zone clearance is important both from reflection and obstruction standpoints. From a practical standpoint, a sixty-percent clearance of the first Fresnel zone (0.6F1) radius is a requirement for all microwave paths. Referencing the diagram above, the following table provides you with an indication the first Fresnel zone radius at different points along a 2.4 GHz 10 km (6.3 mile) microwave path.

It is worth noting that the radius of the first Fresnel zone will be the greatest at the mid-path point. As the table indicates, the first Fresnel Zone radius at the mid-point of a 10 km path is approximately 17.66 meters (58.3 feet). A straight line from the center of one antenna to the center of the other, following the sixty-percent clearance requirement (0.6F1), mid-path clearance for this path is slightly more than 10.5 meters (34.7 feet) from the antenna centerline.

Radio Path Analysis

Proper operation of any microwave radio communication system is dependent upon a "line-of-site" path between the microwave antennas at each end of a radio link. In general, if the path between the two sites is unobstructed and within an allowable distance, the microwave system will provide reliable service. However, further investigation is recommended to ensure other path characteristics will not affect propagation of the microwave signal.

Assuming an appropriate line of sight path from radio site to radio site can be established, both the feasibility and viability of a point-to-point microwave radio link will be dependent upon the gains, losses and receiver sensitivity corresponding with the system. Gains are associated with the transmitter power output of the radio, and the gains of both the transmitting and receiving antennas. Losses are associated with the cabling between the radios and their respective antennas, and with the path between the antennas. Other losses can also occur if the path is partially obstructed or if path reflections cancel a portion of the normal receive signal.

One of the first items to consider for any microwave path is the actual distance from antenna to antenna. The further a microwave signal must travel, the greater the signal loss. This form of attenuation is termed Free Space Loss (FSL). Assuming an unobstructed path, only two variables need to be considered in FSL calculations: (1) the frequency of the microwave signal and (2) the actual path distance.

Summary

As with any communication system, careful planning and engineering is essential to ensure the network will meet the system performance objectives. Wireless interconnect solutions are no different, regardless if they are based on narrowband or spread spectrum technologies. While the telecommunications industry has continued its rapid expansion, spread spectrum microwave radios have been identified as a highly reliable and cost-effective wireless solution for local access applications, especially within a mobile service network such as PHS, PCS, or PCN. Let Wireless Telecom, Inc. help you with your wireless needs.

Introduction

For some time now, companies and individuals have interconnected computers with local area networks (LANs). (Note- because of the many acronyms, there is a list at the end of the paper.) This allowed the ability to access and share data, applications and other services not resident on any one computer. The LAN user has at their disposal much more information, data and applications than they could otherwise store by themselves. In the past all local area networks were wired together and in a fixed location as in figure 1 below.

Figure 1: Traditional Wired LAN

Why would anyone want a wireless LAN? There are many reasons. An increasing number of LAN users are becoming mobile. These mobile users require that they are connected to the network regardless of where they are because they want simultaneous access to the network. This makes the use of cables, or wired LANs, impractical if not impossible. Wireless LANs are very easy to install. There is no requirement for wiring every workstation and every room. This ease of installation makes wireless LANs inherently flexible. If a workstation must be moved, it can be done easily and without additional wiring, cable drops or reconfiguration of the network. Another advantage is its portability. If a company moves to a new location, the wireless system is much easier to move than ripping up all of the cables that a wired system would have snaked throughout the building. Most of these advantages also translate into monetary savings. Ad Hoc networks are easily set up in a wireless environment. Ad Hoc networks will be discussed later. Figure 2 is an example of a wireless LAN.

Figure 2: Wireless LAN

Physical Media

There are three media that can be used for transmission over wireless LANs. Infrared, radio frequency and microwave. In 1985 the United States released the industrial, scientific, and medical (ISM) frequency bands. These bands are 902 - 928MHz, 2.4 - 2.4853 GHz, and 5.725 - 5.85 GHz and do not require licensing by the Federal Communications Commission (FCC). This prompted most of the wireless LAN products to operate within ISM bands. The FCC did put restrictions on the ISM bands however. In the U.S. radio frequency (RF) systems must implement spread spectrum technology. RF systems must confine the emitted spectrum to a band. RF is also limited to one watt of power. Microwave systems are considered very low power systems and must operate at 500 milliwatts or less.

Infrared

Infrared systems are simple in design and therefore inexpensive. They use the same signal frequencies used on fiber optic links. IR systems detect only the amplitude of the signal and so interference is greatly reduced. These systems are not bandwidth limited and thus can achieve transmission speeds greater than the other systems. Infrared transmission operates in the light spectrum and does not require a license from the FCC to operate, another attractive feature. There are two conventional ways to set up an IR LAN. The infrared transmissions can be aimed. This gives a good range of a couple of kilometer and can be used outdoors. It also offers the highest bandwidth and throughput. The other way is to transmit omni-directionally and bounce the signals off of everything in every direction. This reduces coverage to 30 - 60 feet, but it is an area coverage. IR technology was initially very popular because it delivered high data rates and relatively cheap price. The drawbacks to IR systems are that the transmission spectrum is shared with the sun and other things such as fluorescent lights. If there is enough interference from other sources it can render the LAN useless. IR systems require an unobstructed line of sight (LOS). IR signals cannot penetrate opaque objects. This means that walls, dividers, curtains, or even fog can obstruct the signal. InfraLAN is an example of wireless LANs using infrared technology.

Microwave

Microwave (MW) systems operate at less than 500 milliwatts of power in compliance with FCC regulations. MW systems are by far the fewest on the market. They use narrow-band transmission with single frequency modulation and are set up mostly in the 5.8GHz band. The big advantage to MW systems is higher throughput achieved because they do not have the overhead involved with spread spectrum systems. RadioLAN is an example of systems with microwave technology.

Radio

Radio frequency systems must use spread spectrum technology in the United States. This spread spectrum technology currently comes in two types: direct sequence spread spectrum (DSSS) and frequency hopping spread spectrum (FHSS). There is a lot of overhead involved with spread spectrum and so most of the DSSS and FHSS systems have historically had lower data rates than IR or MW.

Direct Sequence Spread Spectrum (DSSS)

With direct sequence spread spectrum the transmission signal is spread over an allowed band (for example 25MHz). A random binary string is used to modulate the transmitted signal. This random string is called the spreading code. The data bits are mapped to into a pattern of "chips" and mapped back into a bit at the destination. The number of chips that represent a bit is the spreading ratio. The higher the spreading ratio, the more the signal is resistant to interference. The lower the spreading ratio, the more bandwidth is available to the user. The FCC dictates that the spreading ratio must be more than ten. Most products have a spreading ratio of less than 20 and the new IEEE 802.11 standard requires a spreading ratio of eleven. The transmitter and the receiver must be synchronized with the same spreading code. If orthogonal spreading codes are used then more than one LAN can share the same band. However, because DSSS systems use wide sub channels, the number of co-located LANs is limited by the size of those sub channels. Recovery is faster in DSSS systems because of the ability to spread the signal over a wider band. Current DSSS products include Digital's Roam About and NCR's Wave LAN.


Frequency Hopping Spread Spectrum (FHSS)

This technique splits the band into many small sub channels (1MHz). The signal then hops from sub channel to sub channel transmitting short bursts of data on each channel for a set period of time, called dwell time. The hopping sequence must be synchronized at the sender and the receiver or information is lost. The FCC requires that the band is split into at least 75 sub channels and that the dwell time is no longer than 400ms. Frequency hopping is less susceptible to interference because the frequency is constantly shifting. This makes frequency hopping systems extremely difficult to intercept. This feature gives FH systems a high degree of security. In order to jam a frequency hopping system the whole band must be jammed. These features are very attractive to agencies involved with law enforcement or the military. Many FHSS LANs can be co-located if an orthogonal hopping sequence is used. Because the sub channels are smaller than in DSSS, the number of co-located LANs can be greater with FHSS systems. Most new products in wireless LAN technology are currently being developed with FHSS technology. Some examples are WaveAccess's Jaguar, Proxim RangeLAN2, and BreezeCom's BreezeNet Pro.

Multipath

Interference caused by signals bouncing off of walls and other barriers and arriving at the receiver at different times is called multipath interference. Multipath interference affects IR, RF, and MW systems. FHSS inherently solves the multipath problem by simply hopping to other frequencies. Other systems use anti-multipath algorithms to avoid this interference. A subset of multipath is Rayleigh fading. This occurs when the difference in path length is arriving from different directions and is a multiple of half the wavelength. Rayleigh fading has the effect of completely canceling out the signal. IR is not effected by Rayleigh fading because the wavelengths used in IR are so small. Figure 3 shows the problem of multipath fading.

• Figure 3: Example of Multipath Fading
  

Medium Access Layer

With more and more companies and individuals requiring portable and mobile computing the need for wireless local area networks continues to rise throughout the world. Because of this growth, IEEE formed a working group to develop a Medium Access Control (MAC) and Physical Layer (PHY) standard for wireless connectivity for stationary, portable, and mobile computers within a local area. This working group is IEEE 802.11. Because 802.11 will eventually become the standard for wireless networking, (I have only seen an unapproved draft), I will use 802.11 terminology in the rest of this paper.

802.11 Architecture

Each computer, mobile, portable or fixed, is referred to as a station in 802.11. The difference between a portable and mobile station is that a portable station moves from point to point but is only used at a fixed point. Mobile stations access the LAN during movement. When two or more stations come together to communicate with each other they form a Basic Service Set (BSS). The minimum BSS consists of two stations. 802.11 LANs use the BSS as the standard building block.

A BSS which stands alone and is not connected to a base is called an Independent Basic Service Set (IBSS) or is referred to as an Ad-Hoc Network. An ad-hoc network is a network where stations communicate only peer to peer. There is no base and no one gives permission to talk. Mostly these networks are spontaneous and can be set up rapidly. Ad-Hoc or IBSS networks are characteristically limited both temporally and spatially.

When BSS's are interconnected the network becomes one with infrastructure. 802.11 infrastructure has several elements. Two or more BSS's are interconnected using a Distribution System or DS. This concept of DS increases network coverage. Each BSS becomes a component of an extended, larger network. Entry to the DS is accomplished with the use of Access Points (AP). An access point is a station, thus addressable. So data moves between the BSS and the DS with the help of these access points.

Creating large and complex networks using BSS's and DS's leads us to the next level of hierarchy, the Extended Service Set or ESS. The beauty of the ESS is the entire network looks like an independent basic service set to the Logical Link Control layer (LLC). This means that stations within the ESS can communicate or even move between BSS's transparently to the LLC.

One of the requirements of IEEE 802.11 is that it can be used with existing wired networks. 802.11 solved this challenge with the use of a Portal. A portal is the logical integration between wired LANs and 802.11. It also can serve as the access point to the DS. All data going to an 802.11 LAN from an 802.X LAN must pass through a portal. It thus functions as bridge between wired and wireless.

The implementation of the DS is not specified by 802.11. So a distribution system may be created from existing or new technologies. A point to point bridge connecting LANs in two separate buildings could become a DS. While the implementation for the DS is not specified, 802.11 does specify the services which the DS must support. Services are divided into two sections, Station Services (SS) and Distribution System Services (DSS).

There are five services provided by the DSS. Association, Reassociation, Disassociation, Distribution, and Integration. The first three services deal with station mobility. If a station is moving within its own BSS or is not moving, the stations mobility is termed No-transition. If a station moves between BSS's within the same ESS, its mobility is termed BSS-transition. If the station moves between BSS's of differing ESS's it is ESS transition. A station must affiliate itself with the BSS infrastructure if it wants to use the LAN. This is done by Associating itself with an access point. Associations are dynamic in nature because stations move, turn on or turn off. A station can only be associated with one AP. This ensures that the DS always knows where the station is. Association supports no-transition mobility but is not enough to support BSS-transition. Enter Reassociation. This service allows the station to switch its association from one AP to another. Both association and reassociation are initiated by the station. Disassociation is when the association between the station and the AP is terminated. This can be initiated by either party. A disassociated station cannot send or receive data. Notice that I have not mentioned ESS-transition. That is because it is not supported. A station can move to a new ESS but will have to reinitiate connections. Distribution and Integration are the remaining DSS's. Distribution is simply getting the data from the sender to the intended receiver. The message is sent to the local AP (input AP), then distributed through the DS to the AP (output AP) that the recipient is associated with. If the sender and receiver are in the same BSS, the input and out AP's are the same. So the distribution service is logically invoked whether the data is going through the DS or not. Integration is when the output AP is a portal. Thus 802.x LANs are integrated into the 802.11 DS.

 

Station services are Authentication, Deauthentication, Privacy, and MAC Service Data Unit (MSDU) Delivery. With a wireless system, the medium is not exactly bounded as with a wired system. In order to control access to the network, stations must first establish their identity. This is much like trying to enter a radio net in the military. Before you are acknowledged and allowed to converse, you must first pass a series of tests to ensure that you are who you say you are. That is really all authentication is. Once a station has been authenticated, it may then associate itself. The authentication relationship may be between two stations inside an IBSS or to the AP of the BSS. Authentication outside of the BSS does not take place. There are two types of authentication services offered by 802.11. The first is Open System Authentication. This means that anyone who attempts to authenticate will receive authentication. The second type is Shared Key Authentication. In order to become authenticated the users must be in possession of a shared secret. The shared secret is implemented with the use of the Wired Equivalent Privacy (WEP) privacy algorithm. The shared secret is delivered to all stations ahead of time in some secure method (such as someone walking around and loading the secret onto each station). Deauthentication is when either the station or AP wishes to terminate a stations authentication. When this happens the station is automatically disassociated. Privacy is an encryption algorithm which is used so that other 802.11 users cannot eavesdrop on your LAN traffic. IEEE 802.11 specifies Wired Equivalent Privacy (WEP) as an optional algorithm to satisfy privacy. If WEP is not used then stations are "in the clear" or "in the red", meaning that their traffic is not encrypted. Data transmitted in the clear are called plaintext. Data transmissions which are encrypted are called ciphertext. All stations start "in the red" until they are authenticated. MSDU delivery ensures that the information in the MAC service data unit is delivered between the medium access control service access points. The bottom line is this, authentication is basically a network wide password. Privacy is whether or not encryption is used.

Wired Equivalent Privacy is used to protect authorized stations from eavesdroppers. WEP is reasonably strong. The algorithm can be broken in time. The relationship between breaking the algorithm is directly related to the length of time that a key is in use. So WEP allows for changing of the key to prevent brute force attack of the algorithm. WEP can be implemented in hardware or in software. One reason that WEP is optional is because encryption may not be exported from the United States. This allows 802.11 to be a standard outside the U.S. albeit without the encryption.

Framing

Frame formats are specified for wireless LAN systems by 802.11. Each frame consists of a MAC header, a frame body and a frame check sequence (FCS). The basic frame can be seen in figure 4 below.

Frame Control	Duration ID	Address 1	Address 2	Address 3	Sequence Control	Address 4	Frame Body	FCS
2	2	6	6	6	2	6	0-2312	4

• Field Length is in Bytes Figure 4: 802.11 Frame

  The MAC header consists of seven fields and is 30 bytes long. The fields are frame control, duration, address 1, address 2, address 3, sequence control, and address 4. The frame control field is 2 bytes long and is compised of 11 subfields as shown in figure 5 below.

  Protocol Version	Type	Subtype	To DS	From DS	More Frag	Retry	Pwr Bgt	More Data	WEP	Order
  2	2	4	1	1	1	1	1	1	1	1

  Field Length is in Bits Figure 5: 802.11 MAC Header

  The protocol version field is 2 bits in length and will carry the version of the 802.11 standard. The initial value once 802.11 is approved will be 0, all other bit values are reserved. Type and subtype fields are 2 and 4 bits respectively. They work together hierarchically to determine the function of the frame. The remaining 8 fields are all 1 bit in length. The To DS field is set to 1 if the frame is destined for the distribution system. From DS field is set to 1 when frames exit the distribution system. Note that frames which stay within their basic service set have both of these fields set to 0. The More Frag field is set to 1 if their is a following fragment of the current MSDU. Retry is set to 1 if this frame is a retransmission. Power Management field indicates if a station is in power save mode (set to 1) or active (set to 0). More data field is set to 1 if there are any MSDUs are buffered for that station. The WEP field is set to 1 if the information in the frame body was processed with the WEP algorithm. The Order field is set to 1 if the frames must be strictly ordered.

  The duration/ID field is 2 bytes long. It contains the data on the duration value for each field and for control frames it carries the associated identity of the transmitting station. The address fields identify the basic service set, the destination address, the source address, and the receiver and transmitter addresses. Each address field is 6 bytes long. The sequence control field is 2 bytes and is split into 2 subfields, fragment number and sequence number. Fragment number is 4 bits and tells how many fragments the MSDU is broken into. The sequence number field is 12 bits an indicates the sequence number of the MSDU. The frame body is a variable length field from 0 - 2312. This is the payload. The frame check sequence is a 32 bit cyclic redundancy check which ensures there are no errors in the frame. For the standard generator polynomial see IEEE P802.11.

   

  Medium Access Control Protocol

  Most wired LANs products use Carrier Sense Multiple Access with Collision Detection (CSMA/CD) as the MAC protocol. Carrier Sense means that the station will listen before it transmits. If there is already someone transmitting, then the station waits and tries again later. If no one is transmitting then the station goes ahead and sends what it has. But what if two stations send at the same time? The transmissions will collide and the information will be lost. This is where Collision Detection Comes into play. The station will listen to ensure that its transmission made it to the destination without collisions. If a collision occurred then the stations wait and try again later. The time the station waits is determined by the back off algorithm. This technique works great for wired LANs but wireless topologies can create a problem for CSMA/CD. The problem is the hidden node problem.

  Figure 6: The Hidden Node Problem

  The Hidden Node problem is shown in Figure 6 above. Node C cannot hear node A. So if node A is transmitting, node C will not know and may transmit as well. This will result in collisions. The solution to this problem is Carrier Sense Multiple Access with Collision Avoidance or CSMA/CA. CSMA/CA works as follows: the station listens before it sends. If someone is already transmitting, wait for a random period and try again. If no one is transmitting then it sends a short message. This message is called the Ready To Send message (RTS). This message contains the destination address and the duration of the transmission. Other stations now know that they must wait that long before they can transmit. The destination then sends a short message which is the Clear To Send message (CTS). This message tells the source that it can send without fear of collisions. Each packet is acknowledged. If an acknowledgement is not received, the MAC layer retransmits the data. This entire sequence is called the 4-way handshake as shown by figure 7 below. This is the protocol that 802.11 chose for the standard.

  Figure 7: The 4-way Handshake
  

Summary

Wireless LANs come in many types: infrared, microwave, and radio. Radio is further broken down into direct sequence and frequency hopping spread spectrum. The MAC layer protocol used by wireless LANs as standardized in 802.11 is CSMA/CA. The Negroponte Switch Theory states that all things wired will be wireless and all things wireless will become wired. This will certainly be true in the case of LANs. Traditional wired LANs will become a thing of the past as more and more users become mobile. LANs used to be defined by distance and spatial locality. Today, with the advances of wireless and virtual LAN technology, LANs are defined as a trust relationship regardless of location. Stationary users will become wireless once technology is able to increase throughput and data rate to levels which equal today's wired LANs.

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