Francine Lalooses

Wireless ATM

Francine Lalooses

David Lancia

Arkadiusz Slanda

Donald Traboini

Table of Contents

I.What is ATM?

II.Transition to Wireless ATM

III.Advantages

IV.Disadvantages

V.Implementation: The Magic WAND

VI.Goals and Results of the Simulation

VII.Works Cited

What is ATM?

Before the discussion of wireless ATM can begin, the concept of ATM in general needs to be discussed. ATM, or by its more formal name Asynchronous Transfer Mode, is a basic packet-based networking system designed for the simultaneous transmissions of voice, video, and data. In the mid 1980s, the major telecommunication companies decided that they needed a new network to handle the surge of video and data, along with voice, traffic being sent over their existing networks. From this, the concept of ATM was born. From an increasing need to handle data traffic, which is inherently packet-based, as well as voice traffic, ATM was designed to work as a packet-switched network. In a packet-switched network, all traffic is broken into small pieces, which are easier to transmit than one large chunk of data. The problem with using this type of network design for ATM is that the old telephone network is circuit-switched, or in other words creates a physical direct connection between the source and destination during the transmission. ATM, therefore, is designed so that it can handle circuit-switched traffic on its packet-switched backbone. To accomplish this, ATM creates virtual circuit connections over the packet-based network between the source and the destination. These virtual circuit connections provision a set number of network resources dedicated to the connection between a specific source and destination, making it appear to the old telephone network that a circuit connection is established. This allows an ATM network to guarantee the same or greater quality of service for voice traffic as the old telephone network does, while at the same time providing a much greater level of service for data and video traffic than was previously available.2

The idea of breaking voice into packets created quite a problem for the standards process. The US telecom carriers wanted to set the packet size to 64 bytes, which is the size of one voice data packet. The European telecom carriers wanted to set the packet size to 32 bytes so that their transmission lines would not require echo cancellation equipment. Instead of making a technical compromise, a more radical solution was implemented. The two proposals were averaged together to get the packet size. The standard was set to 48 bytes with a 5-byte header, creating a 53-byte ATM cell, or packet. Since 48 is not a power of 2, it is completely out of character for normal data standards. The final data packet size of 53 bytes is technically imperfect, but is still useful for all of the types of traffic that would need to be transmitted over an ATM network. 3

In order for ATM networks to be useful to the telecoms, they needed to be able to transmit large amounts of data. ATM was defined to allow data rates ranging from 25 to 622 Mbps (megabits/sec). To achieve these data rates, ATM is designed to assume that no packets or a very minimal number of packets will be lost during transmission, so that retransmission can be avoided. This is accomplished by requiring that ATM networks use fiber optic cable as the transmission medium. Fiber optic cable is the only medium that has a low enough transmission error rate for the small 53-byte ATM cells to be transmitted at a frequency fast enough to achieve 622Mbps. 3

With the technical capabilities outlined above, it is possible to understand why there is such an effort to create wireless ATM networks. These networks would allow for the rapid transmission of all types of traffic without wires while using existing ATM equipment. However, even though ATM was developed by the telecom companies with all of the technical characteristics mentioned above, ATM networks have begun to fall out of favor in the last few years. Instead of ATM, networks are being constructed using mostly standard IP (Internet Protocol) based equipment because it has become both cheaper and faster than ATM. Even though ATM may be technically superior for voice, IP data traffic has become much more important to the telecom companies in terms of revenues and amounts of traffic, so ATM appears to be a dying breed.

Transition to Wireless ATM

Wireless ATM is mainly considered as an “access to an ATM network” issue. Different types of wireless networking need to be addressed depending on what kind of ATM network is accessed. The proposed technology provides an extension of the network to mobile users, simplifies wiring and reconfiguration. When making the transition to wireless access, there are a number of things that need to be considered. Consideration needs to given to the physical, data, and the multi-access layer issues. 1

The physical layer deals with the transmission of data over the physical medium by means of a radio or an optical transmitter/receiver pair. Circuit switched or packet switched operations, operating frequency, licensed vs. unlicensed bands, channel coding, and the need for multiple antennas are only some of the issues in consideration when implementing the physical layer for wireless ATM.

• Radio is the preferred solution since it is not restricted to line of sight, does not require pointing, and multi-access is simpler to attain. Infrared is an option but it mostly operates as a collection of point-to-point communication links. Overcoming multi-path reception from stationary and moving objects is the main challenge of this endeavor because the above-mentioned produces space- and time- varying dispersive channels.
• A pure circuit-switched system is not viable for wireless ATM due to the presence of variable bit rate services and a hybrid of circuit and packet – switched systems makes the best alternative. The trade-off, however, is that it makes the implementation more complicated.
• When dealing with frequency, the ideal frequency and whether it is licensed or unlicensed needs to be decided upon. Licensed bands require FCC approval, which is a long and difficult process.
• To spread the transmission over a larger number of wireless channels, wireless ATM implements OFDM (Orthogonal Frequency Division Multiplexing). OFDM chooses wireless channels that are orthogonal to each other in the electromagnetic spectrum, which means that they arecompletely independent from each other. This minimizes interference and cross talk, while at the same time maximizing bandwidth.
• As far as channel coding is concerned, there is no consensus on the feasibility of channel coding for wireless channels. Due to the busty nature, it either has errors or it is flawless. System performance improves by incorporating error correction in unconventional ways. By replacing the CRC (Cyclic Redundancy Check) in the data link layer with an error detecting and correct code, the advantages of error correction can be obtained at most with a slight increase in the CRC field size.
• Multiple antennas implemented with the proper selection algorithm provide for the best transmission performance, but the number needed, that will provide optimal performance, must be decided. 1

Encapsulation, header compression, and ARQ vs. FEC are some of the many issues involved in the data link layer implementation.

• Encapsulation is a technique used for transporting data units of a protocol within those of another. This technique provides the advantage of transparency, but the disadvantages are added overhead and delays due to encapsulation and decapsulation.
• In header compression, the information content of a header is represented with a fewer number of bits than the 40 bits used in conventional ATM in order to compensate for the intolerable 10% overhead that normally occurs.
• Various ARQ (Automatic Repeat Request) and FEC (Forward Error Correction) techniques have been compared for various channels. In studying these, the consensus reached is that the best technique is a combination of ARQ and FEC. Although there are several hybrids currently present, new research results under the newly assumed conditions.1

In transitioning from ATM to wireless, two major issues arise. One is the shared use of unreliable transmission links, and the other is the mobility of the terminals. The multi-access (MAC) protocols attempt to efficiently and equitably allocate use of a communications channel to independent, competing users. Many schemes have been implemented over the last few decades and each has its advantages and limitations. When transitioning to wireless, it necessitates the need to consider one of these numerous protocols. Mobility management is another issue that needs consideration in this transition because the following three basic issues arise: location management, connection management, and handoff management. 1

Advantages

There are many advantages of having a wireless ATM network. The first and foremost major advantage is that all of the benefits of ATM are made mobile. There is the capability of having a high-speed network, such as ATM, in the palm of your hand. Therefore, the use of mobile equipment to communicate and exchange multimedia traffic over small, handheld portable equipment is one of the options that wireless ATM proposes to offer. Another advantage is flexible bandwidth allocation that would provide end-to-end communications in a Wide Area Network. Companies would no longer have to buy extra equipment like routers to interconnect their LANs (Local Area Networks). 4

Wireless ATM efficiently multiplexes traffic from bursty data and multimedia sources. It chooses signals to avoid congestion and maintain a constant flow of information sent to the stations. The basic structure of wireless ATM is that there are a large number of small transmission cells, called pico cells, that are each served by a base station. All base stations in the network are connected through the wired ATM network. The use of ATM switching for inter-cell traffic avoids developing a new backbone network with sufficient throughput to support communication among the large number of cells. The basic role of a base station is to act as an interconnection between the LAN and the wireless subnets and to transfer packets to the wired ATM network from the mobile units. Each wireless base station has two virtual circuits open to each base station and to each router. As the packet arrives to the base station of the wireless unit, it chooses the circuit that leads to the correct destination. Using two connections guarantees that routing information will not be confused with data because data packets never travel on the virtual circuits used for routing, and routing packets never travel on the circuits used for data. The circuits can also be assigned priority to guarantee that stations receive and process routing updates quickly. Since ATM switching equipment for inter-cell switching is available for wireless ATM, the availability of using existent ATM switching is another advantage. 4

The wireless scheme used in wireless ATM allows for the reuse of spectrum. In other words, to avoid needing to stop and restart transmitting when moving between pico cells, the base stations can operate on the same frequency. Small cell sizes give flexibility, thus avoiding the problem of running out of bandwidth. Along with this reuse of spectrum, the network is capable of soft handoff without any data loss. Handoff is when a mobile unit leaves the area of one cell and enters the area of another. Therefore, soft handoff without any data loss is important and implemented transparently. With wireless ATM arises the capability of being able to roam freely. Mobile units are allowed to roam freely from cell to cell without any user intervention. 4

Disadvantages

The major disadvantage of wireless ATM is that it brings delay to multi-path interference. Reducing the size of pico cells (transmission cells) brings delay to the multi-path network as well as a lack-of-sight path resulting in high attenuation. There are a small number of mobiles within the range of any base station. As cell size is reduced, hand-over rate increases. By using the same frequency, there is no hand-over required at the physical layer. 4

The hop-by-hop routing method is not adequate to cope with wireless systems because every node of the wireless network must store the location of every mobile system to which a route exists, which represents a large amount of information. It is impossible to keep routing information up to date and consistent throughout the network, since a very large number of mobiles will exist. A possible solution would be to have each mobile controller node with network-layer software enhanced by an additional sublayer that performs routing to mobile systems. 4

Another major drawback is the fact that establishing virtual connection takes longer. In a wide or local area network, virtual connection establishment is fast, but it is likely to take longer time in wireless networks. It is not practical to re-establish all virtual circuits whenever a mobile moves between pico-cells. A possible solution would be to isolate small-scale mobility of the mobile from the rest of the wired network. 4

Wireless ATM also embodies high noise interference and poor physical level characteristics. Since wireless ATM is designed for low-level error control, it cannot deal with large-scale networks. Wireless ATM may use either 16 or 24 byte cells, so segmentation and reassembly is required, which allows one to use a segment counter that uses the two least significant bits of the error control sequence number to try to cope with errors in these networks. Another disadvantage is finding a suitable channel sharing media access control technique at the data link layer. Shared media access leads to poor quantitative performance in wireless networks. 4

Implementation: The Magic WAND

The Magic WAND (Wireless ATM Network Demonstrator) was a joint European project established to specify and implement a wireless access system for ATM. The project started in 1996 and was projected to end in 1998. Large corporations such as Nokia, Intracom, IBM, Lucent Technologies, and Eurecom, sponsored it. 5

The major goals of the project were to specify and implement an access system for ATM networks that maintain the service characteristics and benefits of the ATM to the mobile users. It also promoted the standardization of wireless ATM access. The last objective set by this project was to demonstrate that wireless access to ATM was technically feasible, thus capable of providing real-time multi-media services to mobile users. This would have been demonstrated through user trials in hospitals where doctors would access databases wirelessly, retrieve patient information from the network, consult expert doctors, and share documents. The user trials would also be conducted in universities where college students would “surf the web” at high speeds. 5

The WAND project was to consist of three major phases. In the first phase, system and component design would be undertaken. In the second phase, the implementation of the design would be started. In the implementation, the mobile terminals would be as close as possible to the access points. In the final phase, the wireless ATM network would be tested with real life users to verify the correct operation. 5

Although the project was never completed, some accomplishments did occur. The first achievement was the finalization of specifications by the project group for wireless ATM. It was decided that the communication between the mobile units and the access points was to be set in the 5 GHz frequency range. The WAND radio was to operate in the 5.15-5.3 GHz frequency bands. The maximum theoretical bandwidth was set at 20 Mbps, the theoretical Bit Error Rate (BER) at 10-6, and the range of communication between the mobile units and the access points was set up to 50 meters. 5

The project was productive in the area of standardization. The project designers wanted the Magic Wand to set the standard for future research and study in the area of wireless ATM. The radio channel model was developed and verified by measurements in the 5 and 17 GHz frequency bands. However, since no further work was completed by the Magic Wand project its research was terminated in 1997 and wireless ATM was never implemented. 5

Goals and Results of the Simulation

Since wireless ATM was never fully implemented in real life applications, the options for the simulation of the behavior in the network are limited. Therefore, our simulation will focus on the physical layer and examine the effects of noise and mobility on a wireless channel. In the first Matlab program, we analyze the detrimental effects of random noise on the availability of a channel. Making the assumption that noise is of random nature, we generate a random noise matrix with values ranging from zero to one. In the matrix, we assume that values below 0.4 indicate a channel that is not available for transmission. Knowing that wireless ATM uses OFDM (Orthogonal Frequency Division Multiplexing) with 16 channels, we set the noise matrix to be 16x1. Running the program through a user specified number of trials, the user is able to calculate the probability of finding a good channel. The plots generated represent the number and probability of having useful channels present at any given time. Once all probabilities are calculated for the specified number of trials, the average probability is displayed.