DeKalb Technical College

FILE NAME:

CIS1114-Review2-(08OCT03)-Answers.DOC

Review Questions

Revision: 1

Page 1 of 7

CIS 1114

Date: 08OCT03

 

1) What is the primary function of Cisco Aironet Wireless LAN Adapters, also referred to as client adapters?

 

to transfer data packets through the wireless infrastructure

Chapter 2 Overview

The primary function of the client adapters is to transfer data packets through the wireless infrastructure. Installation, configuration, and monitoring of wireless network interface cards (NICs) will also be covered.

 

2) Please list and briefly describe the four modulation formats and data rates specified for the High Rate PHY?

 

differential binary phase shift keying (DBPSK) modulation for 1 Mbps

packet binary convolutional coding (PBCC) mode for enhanced performance

differential quadrature phase shift keying (DQPSK) modulation for 2 Mbps

complementary code keying (CCK) modulation scheme for 5.5 Mbps and 11 Mbps

 

Four modulation formats and data rates are specified for the High Rate PHY. The basic access rate shall be based on 1 -Mbps differential binary phase shift keying (DBPSK) modulation. The enhanced access rate is based on 2- Mbps differential quadrature phase shift keying (DQPSK). The extended direct sequence specification defines two additional data rates. The High Rate access rates are based on the Complementary Code Keying (CCK) modulation scheme for 5.5 Mbps and 11 Mbps. The optional packet binary convolutional coding (PBCC) mode is also provided for enhanced performance up to 22 Mbps.

Complementary Code Keying (CCK) is used to increase the peak data rate of 802.11b from 2 to 11 Mbps, while still using DQPSK modulation. It does this by first increasing the data clock rate from 1 Mbps to 1.375 Mbps, and then taking data in 8-bit blocks (8*1.375 = 11). Six of the eight bits are used to choose 1 of 64 complementary codes, which are each eight chips long and clocked out at 11 MHz. The other 2 bits are combined with the code in the DQPSK modulator.

 

 

 

 

 

 

 

 

binary phase shift keying

 

quadrature phase shift keying

Remember that baud indicates how fast the analog signal is changing in the Voice Channel. The data is encoded as follows:

DIBIT

Phase Shift

00

+90

01

0

10

180

11

270

For every change in the baud rate (phase shift), we can decode 2 bits. This leads to the following:

2 bits x 600 baud = 1200 bps

Example of Carrier Phase Modulation:

quadrature amplitude modulation

Quadrature Amplitude Modulation refers to QPSK with Amplitude Modulation. Basically, it is a mix of phase modulation and amplitude modulation. QAM phase modulates the carrier and also modulates the amplitude of the carrier.

Phase Modulated and Amplitude Modulated Carrier:

There are two types, 8-QAM and 16-QAM. 8-QAM encodes 3 bits of data (23=8) for every baud and 16-QAM encodes 4 bits of data (24=16) for every baud. Both are used in the V.32 standard for 9600 bps modem (a milestone for communications!). 8-QAM transfers 4800 bps and 16-QAM transfers 9600 bps. The baud rate used with QAM is 2400 baud half-duplex.

16-QAM has 12 phase angles, 4 of which have 2 amplitude values! 16-QAM changes phase with every baud change.

16-QAM Phasor Diagram

Higher transfer rates use much more complex QAM methods. For example, V.32bis (14.4 kbps) uses a 64 point constellation to transfer 6 bits per baud. Compare that to the above 16 point constellation!

complementary code keying (CCK) modulation uses Direct Sequence Spread Spectrum and differential quadrature phase shift keying (DQPSK)

 

Short for Complementary Code Keying, a set of 64 eight-bit code words used to encode data for 5.5 and 11Mbps data rates in the 2.4GHz band of 802.11b wireless networking. The code words have unique mathematical properties that allow them to be correctly distinguished from one another by a receiver even in the presence of substantial noise and multipath interference.

CCK works only in conjunction with the DSSS technology that is specified in the original 802.11 standard. It does not work with FHSS. CCK applies sophisticated mathematical formulas to the DSSS codes, permitting the codes to represent a greater volume of information per clock cycle. The transmitter can then send multiple bits of information with each DSSS code, enough to make possible the 11Mbps of data rather than the 2Mbps in the original standard.

 

 

 

 

DSSS main parameters

DSSS is defined (in IEEE 802.11) in the 2.4 GHz band as operating on one of 14 possible

carriers (country specific bands have different number of frequencies, defined in IEEE 802.11 and IEEE 802.11.d).  The selected carrier (channel) is PSK modulated with a channel width of 22 MHz.  The rates defined in IEEE 802.11 are 1 Mbps and 2 Mbps.

IEEE 802.11.b adds to DSSS the rates of 5.5 Mbps and 11 Mbps (in the 2.4 GHz band),

while keeping the channel width at 22 MHz.

 

3) Please briefly describe the following standards:

 

A)    IEEE 802.16 ~ 10GHz ~ called the next 802.11

Broadband Wireless Access

In recent years there has been increasing interest shown in wireless technologies for subscriber access, as an alternative to traditional twisted-pair local loop.

An 802.16 wireless service provides a communications path between a subscriber site and a core network (the network to which 802.16 is providing access). Examples of a core network are the public telephone network and the Internet. IEEE 802.16 standards are concerned with the air interface between a subscriber's transceiver station and a base transceiver station.

Protocols defined specifically for wireless transmission address issues related to the transmission of blocks of data over a network. The standards are organized into a three-layer architecture.

 

B)    IEEE 802.15

Wireless Personal Area Network (WPAN)

At issue is whether the multiband OFDM (MB-OFDM) proposal for an ultrawideband radio standard (capable of 110 Mbit/s over a distance of 10 meters) put before the 802.15.3a committee by the MultiBand OFDM Alliance (MBOA) last July complies with strict FCC regulations.

 

C)    IEEE 802.11a

- IEEE 802.11.a – Orthogonal Frequency Division Multiplexing (OFDM) operation (PHY) at 6 through 54 Mbps, in the 5 GHz band (also called U-NII - Unlicensed National Information Infrastructure)

Unlicensed National Information Structure

(U-NII) bands

“ 5.15 to 5.25 GHz - 40 mW

“ 5.25 to 5.35 GHz - 200 mW

“ 5.725 to 5.825 GHz - 800 mW

 

D)    IEEE 802.11b

- IEEE 802.11.b – Direct Sequence Spread Spectrum (DSSS) operation (PHY) at 5.5 and 11 Mbps, in the 2.4 GHz band and uses complementary code keying (CCK)

Blessed in September 1999

 

E)     IEEE 802.11e

- IEEE 802.11.e - MAC enhancements for QoS (EDCF - Enhanced DCF; HCF - Hybrid Coordination Function)

 

 

F)     IEEE 802.11g

Operates at the 2.4 GHz band 54 Mbps but with full backward compatibility with 802.11b. Uses Orthogonal Frequency Division Multiplexing (OFDM)

It provides interoperability among and between all WLAN speeds working at the 2.4 GHz frequency eliminating the need for a complete equipment upgrade.

 

G)    IEEE 802.11i

- IEEE 802.11.i - MAC enhancements for enhanced security

 

4) As indicated in your curriculum, there are many benefits in standardizing network functions provide ~ please list five of these benefits.

 

Standardization provides all of the following benefits:

 

5) Please describe in detail the Physical (PHY) Layer of the OSI model as it relates to the IEEE 802.11 standard.  Furthermore, please include a description/purpose of the two physical layer sub layers (Physical Layer Convergence Procedure (PLCP) & Physical Medium-dependent (PMD) System).

 

The basic purpose of the physical layer is to send the signal to the network and receive the signal from the network.  The 802.11 PHY layer is divided into the following two parts Physical Layer Convergence Procedure (PLCP) sublayer & Physical Medium-Dependent (PMD) sublayer.

 

Purposes of the Physical Medium-Dependent (PMD) sublayer:

Actual transmission and reception of the signal.

Modulation and demodulation        ~ quadrature phase shift keying

                                                          ~ differential binary phase shift keying

                                                          ~ quadrature amplitude modulation

 

Frequency used                               ~ 2.4 GHz to 2.485 GHz

                                                          ~ 5.725 GHz to 5.85 GHz

                                                          ~ Infrared ~ note: indoor only

 

Channels being used                        ~ 1 through 11 in the US

                                                          ~ 1 through 14 in Europe and Japan

 

 

Purposes of the Physical Layer Convergence Procedure (PLCP) sublayer:

It prepares the MAC protocol data units (MPDU) for transmission by appending fields to the MPDU.  For example, with DSSS the synchronization field (preamble, alternating 0s and 1s is 128 bits long) whereas with FHSS the synchronization field is 80 bits long.  This modified MPDU is called the PLCP frame.

 

The PLCP listens to the media to determine when data can be sent.

 

6) As indicated in your curriculum, there are many benefits in standardizing network functions provide ~ please list five of these benefits.

Basic Service Set (BSS)
The basic service set (BSS) is the basic building block of an IEEE 802.11 LAN. Figure shows a BSS with three stations that are members of the BSS, in addition to the access point (AP). The BSS covers a single RF area, or cell, as indicated by the circle. As a station moves further from the AP, its data rate will decrease. When it moves out of its BSS, it can no longer communicate with other members of the BSS. A BSS uses infrastructure mode, a mode that needs an AP. All stations communicate by way of the AP, and do not communicate directly. A BSS has one service set ID (SSID).

Independent BSS (IBSS)
The independent basic service set (IBSS) is the most basic type of IEEE 802.11 LAN. A minimum IEEE 802.11 LAN consists of only two stations. In this mode of operation, IEEE 802.11 stations communicate directly. Because this type of IEEE 802.11 LAN is often formed without pre-planning for only as long as the WLAN is needed, it is often referred to as an ad hoc network.

Because an IBSS consists of STAs that are directly connected, it is also called a peer-to-peer network. There is, by definition, only one BSS and there is no Distribution System (DS). An IBSS with four stations is shown in Figure . An IBSS may have an arbitrary number of members. In order to communicate outside of the IBSS, one of the STAs must be acting as a gateway or router.

Distribution System (DS)
PHY limitations determine the station-to-station distances that may be supported. For some networks this distance is sufficient. For other networks, increased coverage is required. Instead of existing independently, a BSS may also form a component of an extended service set (ESS). An ESS is built from multiple BSSs, which are connected through APs. The APs are connected to a common DS as shown in Figure . The DS can be either wired or wireless, LAN or WAN. The IEEE 802.11 WLAN architecture is specified independently of the physical characteristics of the DS.

The DS enables mobile device support by providing the services necessary to handle address to destination mapping and seamless integration of multiple BSSs. Data moves between a BSS and the DS through an AP. Note that all APs are also STAs, which make them addressable entities.

Extended service set (ESS)
An extended service set (ESS) is defined as two or more BSSs connected by a common DS, as illustrated in Figure . This allows for the creation of a wireless network of arbitrary size and complexity. As with a BSS, all packets in an ESS must go through one of the APs.

A key concept is that the ESS network appears the same to the LLC layer as an IBSS or a single BSS network. Stations within an ESS may communicate and mobile stations can move from one BSS to another (within the same ESS), transparently to LLC.

Roaming
Roaming is the process or ability of a wireless client to move from one cell, or BSS, to another, without losing connectivity to the network. Access points hand the client off from one to another and are invisible to the client. The IEEE 802.11 standard does not define how roaming should be performed, but does define the basic building blocks, which include active and passive scanning and a re-association process. Re-association with an AP must occur when a STA roams from one AP to another.

7) Please list and describe the three connection services that the Logical Link Control Layer (LLC, 802.2) provides for the Upper Layer Protocols (ULP).

 

 

Destination Service Access Point

Source Service Access Point

 

 

 

8) Please list and describe the three main types of frames used in the MAC layer.

  1. Data frames
  2. Control frames
  3. Management frames

Data frames are used for data transmission. Control frames, such as Request To Send (RTS), Clear to Send (CTS), and Acknowledgment (ACK), control access to the medium using RTS, CTS, and ACK frames. Management frames, such as beacon frames, are transmitted in the same manner as data frames to exchange management information, but are not forwarded to upper layers.

 

9) Please list and describe the four different types of Interframe spaces (IFS).

 

 

  1. SIFS is the short interframe space

SIFS are found in IEEE 802.11 networks. They are used for the highest priority transmissions enabling Stations with this type of information to access the radio link first. Examples of information which will be transmitted after the SIFS has expired include RTS (Request To Send) and CTS (Clear To Send) messages in addition to positive acknowledgements.

 

  1. PIFS is the PCF interframe space

The PCF is an optional capability provided by IEEE 802.11.  It provides contention-free frame (CF) transfer and is only usable on infrastructure network configurations.  This access method uses a point coordinator (PC), which shall operate at the access point (AP) of the BSS, to determine which STA currently has the right to transmit.  The operation is essentially that of polling, with the PC performing the role of the polling master. The PCF relies on the PC to perform polling, enabling polled stations to transmit without contending for the channel.  The AP within each BSS performs the function of PC.

 

  1. DIFS is the DCF interframe space

The DCF is the fundamental access mechanism used to support asynchronous data transfer on a best effort basis.  All stations must support DCF. The DCF operates solely in the ad-hoc network1 and either operates solely or coexists with the PCF in an infrastructure mode2.  Contention services promote fair access to the channel for all stations.

The DCF is based on carrier sense multiple access with collision avoidance (CSMA/CA). CSMA/CD (collision detection) is not used because a station is unable to listen to the channel for collisions while transmitting.  In IEEE 802.11 carrier sensing is performed at both the air interface, referred to as Physical Carrier sensing, and at the MAC sub layer, referred to as Virtual Carrier sensing.  Physical carrier sensing detects the presence of other IEEE 802.11 WLAN users by analyzing all detected packets.  A source station performs virtual carrier sensing by sending MPDU duration information in the header of request to send (RTS), clear to send (CTS) and data frames.

 

  1. EIFS is the extended interframe space

EIFS are found in IEEE 802.11 networks. EIFS unlike SIFS (Short Interframe Space), PIFS (PCF Interframe Space) and DIFS (DCF Interframe Space) has a variable value and is only used when there has been an error in frame transmission. It is not used to control access onto the radio link.

 

10) Please list and explain the modes of operation for both the green (status) and amber (traffic) light emitting diodes that are located on a Cisco PC card.

 

The green LED on the PC card is the status LED. It has several modes of operation:

 

The amber LED is the RF Traffic LED. It has two primary modes of operation:

 

 

11) In the 802.11 standard, which mechanism provides security services for the WLAN?

 

Wired Equivalent Privacy

 

12) What is the best way to get specific software drivers for Cisco wireless adapter cards?

 

Download them from the CCO site (www.cisco.com)

 

13) What is orthogonal frequency division multiplexing (OFDM) and how is it used in the 802.11a standard?

 

A technique that divides a communications channel into 52 equally-spaced frequency bands and reduces multipath interference.