Problems To Chapter

Architecture of wireless LANs

Learning objectives

After completing this chapter, you are able to

• demonstrate an understanding of wireless LANs;

• explain the difference between DSSS and FHSS;

• demonstrate an understanding of IEEE 802.11 WLAN architecture;

• demonstrate an understanding of Bluetooth architecture;

• explain the states of a Bluetooth device;

• explain how a connection is established between a host and Bluetooth device;

• explain the functions of L2CAP.

Practice problems

8.1: What are the WLAN's operating speeds?

8.2: What is the radio frequency (RF) band in which the LANs operate?

8.5: What is the function of DSSS?

8.6: What is the function of FHSS?

8.7: What is the addressable unit in IEEE 802.11?

8.9: What is the architecture of the DS?

8.10: What is the role of a portal?

8.11: What are the architectural services of IEEE 802.11?

8.12: What are the possible states of a Bluetooth device?

8.13: How is a connection between a host and Bluetooth device established?

8.14: How does the host control a Bluetooth module?

8.15: What are the functions of L2CAP?

Practice problem solutions

8.1: A WLAN is capable of operating at speeds in the range of 1, 2, or 11Mbps depending on the actual system. These speeds are supported by the standard for WLAN networks defined by the international body, the IEEE.

8.2: WLAN communications take place in a part of the radio spectrum that is designated as license-free. In this band, 2.4 to 2.5 GHz, users can operate without a license as long as they use equipment that has been type-approved for use in the license-free bands.

8.3: RF is very capable of being used for applications in which communications are not line-of-sight and are over longer distances. The RF signals travel through walls and communicate where there is no direct path between the terminals.

8.4: Infrared is primarily used for very short distance communications, less than 3 ft where there is an LOS connection. It is not possible for the Infrared light to penetrate any solid material; it is even attenuated greatly by window glass, so it is really not a useful technology in comparison to Radio Frequency for use in a WLAN system. The application of Infrared is as a docking function and in applications in which the power available is extremely limited such as a pager or PDA. The standard for such products is called Infrared Data Association (IrDA), which has been used by Hewlett Packard, IBM, and others. This is found in many notebook and laptop PCs and allows a connectionless docking facility up to 1 Mbps to a desktop machine and up to two feet, line of sight.

8.5: A DSSS system takes a signal at a given frequency and spreads it across a band of frequencies where the center frequency is the original signal. The spreading algorithm, which is the key to the relationship of the spread range of frequencies, changes with time in a pseudorandom sequence that appears to make the spread signal a random noise source.

8.6: Frequency Hopping Spread Spectrum (FHSS) is based on the use of a signal at a given frequency that is constant for a small amount of time and then moves to a new frequency. The sequence of different channels determined for the hopping pattern, that is, where the next frequency will be to engage with this signal source, is pseudorandom.

8.7: In IEEE 802.11 the addressable unit is a station (STA), which is a message destination, but not (in general) a fixed location. IEEE 802.11 handles both mobile and portable stations. MSs access the LAN while in motion, whereas a Portable Station (PS) can be moved between locations but it is used only at a fixed location. MSs are often battery powered, and power management is an important consideration since we cannot assume that a station's receiver will always be powered on.

8.8: The IEEE 802.11 architecture provides a WLAN supporting station mobility transparently to upper layers. The Basic Service Set (BSS) is the basic building block consisting of member stations remaining in communication. If a station moves out of its BSS, it can no longer directly communicate with other members of the BSS.

The IBSS is the most basic type of IEEE 802.11 LAN, and may consist of at least two stations that can communicate directly. This LAN is formed only as long as it is needed, and is often referred to as an ad hoc network. The association between an STA and a BSS is dynamic since STAs turn on and off, come within range and go out of range.

8.9: A BSS may form the Distribution System (DS), which is an architectural component used to interconnect BSSs. IEEE 802.11 logically separates the Wireless Medium (WM) from the Distribution System Medium (DSM). Each logical medium is used for different purposes by a different component of the architecture. The IEEE 802.11 LAN architecture is specified independently of the physical characteristics of any specific implementation. The DS enables mobile device support by providing the logical services necessary to handle address-to-destination mapping and seamless integration of multiple BSSs. An Access Point (AP) is an STA that provides access to the DS by providing DS services in addition to acting as an STA. The data move between a BSS and the DS via an AP. All APs are also STAs and they are addressable entities. The addresses used by an AP for communication on the WM and on the DSM are not necessarily the same.

The DS and BSSs allow IEEE 802.11 to create a wireless network of arbitrary size and complexity called the Extended Service Set (ESS) network. The ESS network appears the same to an LLC layer as an IBSS network. Stations within an ESS may communicate and MSs may move from one BSS to another (within the same ESS) transparently to LLC.

8.10: A portal is the logical point at which MAC Service Data Units (MSDUs) from an integrated non-IEEE 802.11 LAN enter the IEEE 802.11 DS. All data from non-IEEE 802.11 LANs enter the IEEE 802.11 architecture via a portal, which provides logical integration between the IEEE 802.11 architecture and existing wired LANs. A device may offer both the functions of an AP and a portal, for example, when a DS is implemented from IEEE 802 LAN components.

8.11: Architectural services of IEEE 802.11 are as follows: authentication, association, deauthentication, disassociation, distribution, integration, privacy, reassociation, and MSDU delivery. These services are provided either by stations as the Station Service (SS) or by the DS as the Distribution System Service (DSS).

The SS includes authentication, deauthentication, privacy, and MSDU delivery. The SS is present in every IEEE 802.11 station, including APs and is specified for use by MAC sublayer entities.

The DSSs include association, disassociation, distribution, integration, and reassociation. The DSSs are provided by the DS and accessed by an AP, which is an STA that also provides DSSs. DSSs are specified for use by MAC sublayer entities.

8.12: A Bluetooth device is in one of the following states:

• Standby state, in which the device is inactive, no data is being transferred, and the radio is not switched on. In this state the device is unable to detect any access codes.

• Inquiry state, when a device attempts to discover all the Bluetooth-enabled devices in its local area.

• Inquiry scan is used by devices to make themselves available to inquiring devices.

• Page state is entered by the master, which transmits paging messages to the intended slave device.

• Page scan is used by a device to allow paging devices to establish connection with it.

• Connection-Active is a state in which the slave moves to the master's frequency hop and timing sequence by switching to the master's clock.

• Connection-Hold mode allows the device to maintain its active member address while ceasing to support ACL traffic for a certain time to free bandwidth for other operations such as scanning, paging, inquiry, or low-power sleep.

• Connection-Sniff mode allows the device to listen for traffic by using a predefined slot time.

• Connection-Park mode allows the device to enter low-power sleep mode by giving up its active member address and listening for traffic only occasionally.

8.13: A connection between a host and Bluetooth device is established in the following steps:

1. Host requests an inquiry.

2. Inquiry is sent using the inquiry hopping sequence.

3. Inquiry scanning devices respond to the inquiry scan with FHS packets that contain all the information needed to connect with them.

4. The contents of the FHS packets are passed back to the host.

5. The host requests connection to one of the devices that responded to the inquiry.

6. Paging is used to initiate a connection with the selected device.

7. If the selected device is page scanning, it responds to the page.

8. If the page-scanning device accepts the connection, it will start hopping using the master's frequency hopping sequence and timing.

8.14: The host controls a Bluetooth module by using the following HCI commands:

1. Link control, for instance, setting up, tearing down, and configuring links.

2. Power-saving modes and switching of the roles between master and slave.

3. Direct access to information on the local Bluetooth module and access to information on remote devices by triggering LMP exchanges.

4. Control of baseband features, for instance, timeouts.

5. Retrieving status information on a module.

6. Invoking Bluetooth test modules for factory testing and for Bluetooth qualification.

8.15: Logical Link Control and Adaptation Protocol (L2CAP) send data packets to HCI

or to LM. The functions of L2CAP include

• multiplexing between higher layer protocols that allow them to share lower layer links;

• segmentation and reassembly to allow transfer of larger packets than those that lower layers support;

• group management by providing one-way transmission to a group of other Bluetooth devices; and

• Quality-of-service management for higher layer protocols.

L2CAP provides the following facilities needed by higher layer protocols:

• Establishing links across underlying ACL channels using L2CAP signals.

• Multiplexing between different higher layer entities by assigning each one its own connection ID.

• Providing segmentation and reassembly facilities to allow large packets to be sent across Bluetooth connections.

Mobile Telecommunications Protocols For Data Networks. Anna Hac Copyright © 2003 John Wiley & Sons, Ltd.

ISBN: 0-470-85056-6

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