OLATUNDE OLYMPIO
ACSG 525
STUDY GUIDE FOR CHAPTER 7
Chapter 7
Low Speed Wireless Local Area Networks
This chapter focuses on the study of two Low Speed WLAN technologies and
standards (IEEE 802.11). Use the following questions and remarks as a study guide to
read and understand the contents of this chapter:
Low-Speed WLAN:
1) What is meant by Low-Speed WLAN? What are the applications for Low-Speed
WLAN?
WLAN technology supports a very broad spectrum of applications. Most
notebook computers are manufactured with embedded WLAN interface. Low
speed WLAN supports speed up to 11Mbps.
2) What is a wireless NIC and its functions?
The most important component of WLAN is the wireless network interface card,
which allows a device to connect wirelessly to the network. When wireless NIC
transmit, they: change the computer internal data from parallel to serial prior to
transmission, divide data into packets and attach the sender and the receiver
addresses, determine when to send the data, and transmit the data.
3) How is the wireless NIC implemented on the various devices (desktop PC,
notebook, and PDA)?
A wireless NIC is most often a separate card that is inserted into one of the
desktop computer internal expansion slot or the peripheral component interface
(PCI) expansion slot. Another option is an external wireless adaptor that can be
connected to a computer’s universal serial bus (USB) port. For notebook
computers, wireless NICs are available in two different formats: the standard PC
Card Type II slot and the Mini PCI (a small card with a standard PCI expansion
card functionality). PDAs have two options for wireless NIC: a Type II PC Card
for device with the expansion slot and a compact flash (CF) card wireless NIC or
secure digital (SD) card wireless NIC.
4) What is an Access Point?
An access point provides wireless LAN devices with a point of access into a
wired network. APs consist of three major parts: a radio transmitter and receiver
to generate signals that are used to send and received wireless data, an antenna to
radiate these signals, and an Rj-45 wired network interface port that uses a cable
to connect the AP to a standard wired network.
5) What are the two basic functions of an AP?
The two basic functions of an AP are to act: as the wireless communication base
station for the wireless network (all the devices communicate through the AP) and
a bridge between the wireless and wired network.
6) What is the range of an AP acting as the base station?
The range of an AP acting as a base station is approximately 375 feet or 115
meters.
7) What is a dynamic rate selection?
The data rate tends to drop as the signal strength or quality (or both) begins to
fade. The exact point at which the data rate begins to drop depends on the specific
environment, the type and number of obstructions, and any sources of
interference. The AP will automatically select the highest possible data rate for
transmission depending on the strength and quality of the signal. This process is
the dynamic rate selection.
8) What is the largest number an AP can support?
The largest number of users an AP can support is generally over 100. Since the
radio signal is shared among all the users, many vendors recommend one AP per
50 users if they are only performing basic email, light web surfing, and occasional
transferring of medium sized files. 20 users per AP is recommended for large files
sharing.
9) What is Power over Ethernet (PoE)?
Power over Ethernet is an enhancement to IEEE 802.3 which define how
manufacturers can implement the use of the unused wires in a standard unshielded
twisted pair (UTP) Ethernet cable to deliver power to the AP without the use of an
AC adapter.
10) What is the main advantage of PoE?
The implementation of PoE eliminates the need for expensive electrical wiring to
be installed in the ceiling and makes the mounting of APs more flexible.
11) How does ad hoc mode work?
In ad hoc mode or peer-to-peer mode also known as the Independent Basic
Service Set (IBSS), wireless clients communicates directly among themselves
without using an AP. This mode is use for quick and easy setup of a wireless
network anywhere that a network infrastructure does not already exist or it’s not
permanently required.
12) How does infrastructure mode work?
The infrastructure mode or the basic service set (BSS) consists of wireless clients
and an AP. More APs can be added to extend the range and the number of users to
create an extended service set (ESS). ESS allows wireless clients to move in the
coverage zone without losing connection. When the wireless client enters into
range of more than one AP, it will chose an AP which to associate with based on
signal strength.
13) What is the IEEE 802.11 standard?
The IEEE 802.11 standard defines a local area network that provides wireless data
access for clients that are either mobile or in a fixed location at a rate of either 1
Mbps or 2 Mbps using infrared or RF transmission. This standard also defines the
implementation of WLAN using FHSS or DSSS.
14) What is the function of the PHY in diffused infrared transmission?
The PHY layer performs the function of both reformatting the data received from
the upper layers and transmitting the light impulses. The PHY layer is divided in
two sublayers: PLCP (physical layer convergence procedure) and PMD (physical
medium dependent). The PLCP receives data from the MAC layer and put it into
a frame and send it to the PMD which in turn will convert the binary signal into
pulses.
15) Describe and explain the different parts of a diffused infrared PLCP frame
A diffused infrared PLCP frame has three main parts:
i) A preamble with two fields
(1) Synchronization: the emitter may send between 57 and 73 pulses, each in
its own time slot. The receiving device then synchronizes with the
incoming signal.
(2) Start frame delimiter: is always the same bit pattern and is the part of the
frame that defines its beginning. The start frame delimiter for infrared is
always 1001, where 1 represents a high intensity pulse and 0 is a lower
intensity pulse.
ii) A header with four fields
(1) Data rate: determines the speed of the transmission.
(2) Direct current level adjustment: contains a pattern of infrared pulses that
allows the receiving device to determine the signal level.
Transmission
speed (Mbps)
Data rate
values
Direct current level adjustment values
1.0
000
0000000010000000000000001000000
2.0
001
00100010001000100010001000100010
(3) Length: indicates the time needed to transmit the entire frame. It is also
used by the receiving device to determine the end of the frame.
(4) Header error: contains a value that the receiving device can use to
determine if the data was received without error.
iii) A data field: contains the data and can be from 1 to 20,000 time slots.
16) What kind of modulation is used for infrared transmission at 1 Mbps and 2
Mbps?
For transmission at 1 Mbps, the 16-pulse position modulation (16-PPM) is used,
which translates four data bits into 16 light pulses. And for transmission at 2
Mbps, a 4-pulse position modulation is used, which translates 2 bits into 4 light
pulses which allows it to transmit twice as fast as the 16-PPM.
17) How did the IEEE solve the problem of the slow maximum bandwidth of only 2
Mbps?
The project IEEE 802.11 was revisited shortly after it was released and made
changes that will allow for greater speed. In 1999 the 802.11b and 802.11a were
published, increasing the speeds to 11 Mbps and 54 Mbps respectively. In 2003,
the IEEE published the 802.11g standard raising the speed of 802.11b compatible
network to a maximum of 54 Mbps.
18) What are the enhancements made in 802.11b?
802.11b is designated as Higher Speed Extension in the2.4 GHz ISM band. The
standard added two higher speeds, 5.5 Mbps and 11 Mbps to the original 1 or 2
Mbps and specified RF and direct sequence spread spectrum (DSSS) as the only
transmission technology. The 802.11b also known as Wi-Fi became quickly the
standard got RF WLANs. The PHY of 802.11b is also divided in two sublayers:
PLCP and PMD. Changes to 802.11 only happen at the PHY layer. PLCP
standard are based on DSSS and data received from the MAC are formatted into
frame that the PMD can transmit. The PLCP frame has the same field as in 802.11
with a service field.

Synchronization: allows the receiver to synchronize with the incoming
signal

Start frame delimiter: always the same bit pattern 1111001110100000; its
defines the beginning of a frame

Signal data rate: indicates the speed of the transmission

Service: most of the bit in this field are reserved for future use and must be
set to zero.

Length: indicates how long the data portion of the frame is in
microseconds. The value ranges from 16 – 65535.

Header error check: contains a value that the receiver device can use to
determine if the data was received correctly.

Data: can be up to 4097 bytes
The preamble and the header of the frame are always transmitted at 1 Mbps to
allow for communication between slower and faster devices. When the frame is
passed to the PMD sublayer of the PHY, it is the job of the PMD to translate the
binary into radio signal that can be used for transmission. The data link layer is
also divided into two sublayers: LLC (logical link control) and MAC (media
access control). The LLC provides the same functions as in 802.11. To allow
devices from the WLAN to share the same RF spectrum, 802.11b uses the
distributed coordination function (DCF) to avoid collision. The DCF specifies that
a modified procedure CSMA/CA be used. Whereas CSMA/CD is designed to
handle collisions when they occur, CSMA/CA attempts to avoid collision. In
other words, the CSMA/CA is a device mechanism in which all devices must
listen to the medium to determine if the channel is free before transmitting.
CSMA/DC is based on the same principle but for Ethernet technology, in which
devices connected using a shared medium, such as a coaxial cable, should first
listen on the cable to see if any other device is transmitting and only start
transmitting when the medium is free. To prevent multiple devices to start
sending their transmission when the medium is clear, CSMA/CA makes all
devices wait a random amount of time (the backoff interval) after the medium is
clear. CSMA/CA also reduces collisions by using explicit packet
acknowledgement (ACK). An acknowledgement packet is sent by the receiving
device back to the sending device to confirm receipt of the intact data packet. In
addition to CSMA/CA, 802.11b provides two additional mechanisms to reduce
collision such as the request to send/clear to send (RTS/CTS) protocol and
fragmentation. RTS frame is transmitted by the wireless client to the AP. The AP
alerts all other wireless devices that a particular device needs to reserve the
medium for a specific period of time. The AP then reply to the device with a CTS
frame that also tells all the wireless clients that the medium is now reserved and
they should suspend any transmissions. The RTS/CTS protocol imposes
additional overhead and it is not used unless there is poor network performance
due to excessive collisions. The second mechanism (fragmentation) involves
dividing the data into several smaller ones. Sending many smaller frames instead
of one large frame reduces the amount of time that the wireless medium is being
used so less probability of collisions occurrence. Another method for channel
access is polling. Each device is sequentially asked if it wants to transmit. If the
answer is yes, then it is giving permission to transmit while all other devices must
wait. If the answer is no, then the next device in sequence is polled. 802.11b
provides additional polling function known as point coordination function (PCF).
The AP serves as the polling device and ask each device in sequence to see if a
client needs to transmit.
19) What are association and reassociation in 802.11b?
The MAC layer in 802.11b provides the functionality for a client to join a WLAN
and stay connected. This is known as association and reassociation. Association is
the process of communicating with the other wireless clients or the AP in order to
join the network. It is done by passive scanning (the client listens to each
available channel for a set of time, about 10 seconds. The clients listens for a
beacon frame transmitted from all available APs, containing information of the
transmission rate, the AP SSID, how often the beacon frames are sent, and the
duration of transmission) or active scanning (the client first sends a probe frame,
then waits for the probe response frame from all available APs that contains the
information needed to start a connection with the AP.). Reassociation is when the
client drops the connection with the AP and reestablishes the connection with
another AP.
20) What are an associate request frame, an associate response frame, a
reassociation request frame, a reassociation response frame, and a
disassociation frame?
An associate request frame is a frame sent by a device to an AP that includes the
device’s own capabilities and supported rates. The AP sends an associate response
frame, which contains the status code and device and the ID number for the
device, as a response to the associate request frame. The reassociation request
frame and reassociation response frame are used when a device wants to drop a
connection with an AP and join another AP in the network when roaming. The
newly joined AP will send a disassociation frame to the old AP through the wired
network, terminating the old AP’s association with the device.
21) How is power management defined in 802.11?
The IEEE 802.11 standard allows the mobile device to be off as much as possible
to conserve battery life but still not miss out on data transmission. Since mobile
WLAN devices depends heavily on battery life as their primary power source, the
ability to go into a sleep mode after a period of time is critical to conserve battery
power. The power management functions can only be used in infrastructure mode.
22) What are the three different types of MAC frame formats?
The three different types of MAC frame format are management frames (used to
setup initial communication between the AP and the device), control frames
(provide assistance in delivering the frames that contain the data- a RTS is a
control frame), and data frames (carry the information to be transmitted to the
destination device.