cwna certified wireless network administrator official study guide phần 7 ppsx

MAC and Physical Layers CWNA Exam Objectives Covered: Understand and apply the following concepts surrounding wireless LAN Frames: The difference between wireless LAN and Ethernet frame

MAC and Physical Layers

CWNA Exam Objectives Covered:

Understand and apply the following concepts surrounding

wireless LAN Frames:

The difference between wireless LAN and Ethernet frames

Layer 3 Protocols supported by wireless LANs

Specify the modes of operation involved in the movement of

data traffic across wireless LANs:

Distributed Coordination Function (DCF)

Point Coordination Function (PCF)

CSMA/CA vs CSMA/CD

Interframe spacing

RTS/CTS

Dynamic Rate Selection

Modulation and coding

We mentioned earlier in this book how most of the technology in any wireless LAN is the same, but that manufacturers approach and utilize that technology differently In this chapter we will discuss some of the MAC and Physical layer characteristics of wireless LANs that are common to all wireless LAN products, regardless of manufacturer We will explain the difference between Ethernet and wireless LAN frames and how wireless LANs avoid collisions We’ll walk through how wireless LAN stations communicate with one another under normal circumstances, then how collision handling occurs in a wireless LAN

It is important for you as a wireless LAN administrator to know this level of detail in order to be able to properly configure and administer an access point, as well as to be able

to diagnose and solve problems that are common to wireless LANs

How Wireless LANs Communicate

In order to understand how to configure and manage a wireless LAN, the administrator must understand communication parameters that are configurable on the equipment and how to implement those parameters In order to estimate throughput across wireless LANs, one must understand the affects of these parameters and collision handling on system throughput This section conveys a basic understanding of many configurable parameters and their affects on network performance

Wireless LAN Frames vs Ethernet Frames

Once a wireless client has joined a network, the client and the rest of the network will communicate by passing frames across the network, in almost the same manner as any other IEEE 802 network To clear up a common misconception, wireless LANs do NOT

use 802.3 Ethernet frames The term wireless Ethernet is somewhat of a misnomer

Wireless LAN frames contain more information than common Ethernet frames do The actual structure of a wireless LAN frame versus that of an Ethernet frame is beyond the scope of both the CWNA exam as well as a wireless LAN administrator’s job

Something to consider is that there are many types of IEEE 802 frames, but there is only one type of wireless frame With 802.3 Ethernet frames, once chosen by the network administrator, the same frame type is used to send all data across the wire just as with wireless Wireless frames are all configured with the same overall frame format One similarity to 802.3 Ethernet is that the payload of both is a maximum of 1500 bytes Ethernet's maximum frame size is 1514 bytes where 802.11 wireless LANs have a maximum frame size of 1518 bytes

There are three different categories of frames generated within the confines of this overall frame format These three frame categories and the types within each category are: Management Frames

o Association request frame

o Association response frame

o Reassociation request frame

o Reassociation response frame

o Probe request frame

o Probe response frame

o Request to send (RTS)

o Clear to send (CTS)

o Acknowledgement (ACK)

o Power-Save Poll (PS Poll)

o Contention-Free End (CF End)

o CF End + CF Ack Data Frames

Certain types of frames (listed above) use certain fields within the overall frame type of a wireless frame What a wireless LAN administrator needs to know is that wireless LANs support practically all Layer 3-7 protocols – IP, IPX, NetBEUI, AppleTalk, RIP, DNS, FTP, etc The main differences from 802.3 Ethernet frames are implemented at the Media Access Control (MAC) sub layer of the Data Link layer and the entire Physical layer Upper layer protocols are simply considered payload by the Layer 2 wireless frames

Collision Handling

Since radio frequency is a shared medium, wireless LANs have to deal with the possibility of collisions just the same as traditional wired LANs do The difference is that, on a wireless LAN, there is no means through which the sending station can

determine that there has actually been a collision It is impossible to detect a collision on

a wireless LAN For this reason, wireless LANs utilize the Carrier Sense Multiple

Access / Collision Avoidance protocol, also known as CSMA/CA CSMA/CA is

somewhat similar to the protocol CSMA/CD, which is common on Ethernet networks The biggest difference between CSMA/CA and CSMA/CD is that CSMA/CA avoids collisions and uses positive acknowledgements (ACKs) instead of arbitrating use of the medium when collisions occur The use of acknowledgements, or ACKs, works in a very simple manner When a wireless station sends a packet, the receiving station sends back

an ACK once that station actually receives the packet If the sending station does not receive an ACK, the sending station assumes there was a collision and resends the data CSMA/CA, added to the large amount of control data used in wireless LANs, causes overhead that uses approximately 50% of the available bandwidth on a wireless LAN This overhead, plus the additional overhead of protocols such as RTS/CTS that enhance collision avoidance, is responsible for the actual throughput of approximately 5.0 - 5.5 Mbps on a typical 802.11b wireless LAN rated at 11 Mbps CSMA/CD also generates overhead, but only about 30% on an average use network When an Ethernet network becomes congested, CSMA/CD can cause overhead of up to 70%, while a congested wireless network remains somewhat constant at around 50 - 55% throughput

The CSMA/CA protocol avoids the probability of collisions among stations sharing the

medium by using a random back off time if the station's physical or logical sensing

mechanism indicates a busy medium The period of time immediately following a busy medium is when the highest probability of collisions occurs, especially under high utilization At this point in time, many stations may be waiting for the medium to become idle and will attempt to transmit at the same time Once the medium is idle, a random back off time defers a station from transmitting a frame, minimizing the chance that stations will collide

Fragmentation

Fragmentation of packets into shorter fragments adds protocol overhead and reduces protocol efficiency (decreases network throughput) when no errors are observed, but reduces the time spent on re-transmissions if errors occur Larger packets have a higher probability of collisions on the network; hence, a method of varying packet fragment size

is needed The IEEE 802.11 standard provides support for fragmentation

By decreasing the length of each packet, the probability of interference during packet transmission can be reduced, as illustrated in Figure 8.1 There is a tradeoff that must be made between the lower packet error rate that can be achieved by using shorter packets, and the increased overhead of more frames on the network due to fragmentation Each fragment requires its own headers and ACK, so the adjustment of the fragmentation level

is also an adjustment of the amount of overhead associated with each packet transmitted Stations never fragment multicast and broadcast frames, but rather only unicast frames in order not to introduce unnecessary overhead into the network Finding the optimal fragmentation setting to maximize the network throughput on an 802.11 network is an important part of administering a wireless LAN Keep in mind that a 1518 byte frame is the largest frame that can traverse a wireless LAN segment without fragmentation

FCS

FCS FCS

Data-1

Data-2

FCS Data-3

1 Decreased chance of collision

2 More overhead

1 Increased chance of collision

2 Less overhead header

One way to use fragmentation to improve network throughput in times of heavy packet errors is to monitor the packet error rate on the network and adjust the fragmentation level manually As a recommended practice, you should monitor the network at multiple times throughout a typical day to see what impact fragmentation adjustment will have at various times Another method of adjustment is to configure the fragmentation threshold

If your network is experiencing a high packet error rate (faulty packets), increase the fragmentation threshold on the client stations and/or the access point (depending on which units allow these settings on your particular equipment) Start with the maximum value and gradually decrease the fragmentation threshold size until an improvement shows If fragmentation is used, the network will experience a performance hit due to the overhead incurred with fragmentation Sometimes this hit is acceptable in order to gain more throughput due to a decrease in packet errors and subsequent retransmissions

Dynamic Rate Shifting (DRS)

Adaptive (or Automatic) Rate Selection (ARS) and Dynamic Rate Shifting (DRS) are both terms used to describe the method of dynamic speed adjustment on wireless LAN clients This speed adjustment occurs as distance increases between the client and the access point or as interference increases It is imperative that a network administrator understands how this function works in order to plan for network throughput, cell sizes, power outputs of access points and stations, and security

Modern spread spectrum systems are designed to make discrete jumps only to specified data rates, such as 1, 2, 5.5, and 11 Mbps As distance increases between the access point and a station, the signal strength will decrease to a point where the current data rate cannot be maintained When this signal strength decrease occurs, the transmitting unit will drop its data rate to the next lower specified data rate, say from 11 Mbps to 5.5 Mbps

or from 2 Mbps to 1 Mbps Figure 8.2 illustrates that, as the distance from the access point increases, the data rate decreases

11 Mbps 11-5.5 Mbps 5.5-2 Mbps 2-1 Mbps

A wireless LAN system will never drop from 11 Mbps to 10 Mbps, for example, since 10 Mbps is not a specified data rate The method of making such discrete jumps is typically called either ARS or DRS, depending on the manufacturer Both FHSS and DSSS implement DRS, and the IEEE 802.11, IEEE 802.11b, HomeRF, and OpenAir standards require it

Distributed Coordination Function

Distributed Coordination Function (DCF) is an access method specified in the 802.11 standard that allows all stations on a wireless LAN to contend for access on the shared transmission medium (RF) using the CSMA/CA protocol In this case, the transmission medium is a portion of the radio frequency band that the wireless LAN is using to send data Basic service sets (BSS), extended service sets (ESS), and independent basic service sets (IBSS) can all use DCF mode The access points in these service sets act in the same manner as IEEE 802.3 based wired hubs to transmit their data, and DCF is the mode in which the access points send the data

Point Coordination Function

Point Coordination Function (PCF) is a transmission mode allowing for contention-free frame transfers on a wireless LAN by making use of a polling mechanism PCF has the advantage of guaranteeing a known amount of latency so that applications requiring QoS (voice or video for example) can be used When using PCF, the access point on a wireless LAN performs the polling For this reason, an ad hoc network cannot utilize PCF, because an ad hoc network has no access point to do the polling

The PCF Process

First, a wireless station must tell the access point that the station is capable of answering

a poll Then the access point asks, or polls, each wireless station to see if that station needs to send a data frame across the network PCF, through polling, generates a significant amount of overhead on a wireless LAN

When using PCF, only one access point should be on each non-overlapping channel to avoid much degraded performance due to co-channel interference

DCF can be used without PCF, but PCF cannot be used without DCF We will explain how these two modes co-exist as we discuss interframe spacing DCF is scalable due to its contention-based design, whereas PCF, by design, limits the scalability of the wireless network by adding the additional overhead of polling frames

Interframe Spacing

Interframe spacing doesn’t sound like something an administrator would need to know; however, if you don’t understand the types of interframe spacing, you cannot effectively grasp RTS/CTS, which helps you solve problems, or DCF and PCF, which are manually configured in the access point Both of these functions are integral in the ongoing communications process of a wireless LAN First, we will define each type of interframe space (IFS), and then we will explain how each type works on the wireless LAN

As we learned when we discussed beacons, all stations on a wireless LAN are synchronized All the stations on a wireless LAN are effectively ‘ticking’ time in sync with one another Interframe spacing is the term we use to refer to standardized time spaces that are used on all 802.11 wireless LANs

time-Three Types of Spacing

There are three main spacing intervals (interframe spaces): SIFS, DIFS, and PIFS Each type of interframe space is used by a wireless LAN either to send certain types of messages across the network or to manage the intervals during which the stations contend for the transmission medium Figure 8.3 illustrates the actual times that each interframe space takes for each type of 802.11 technology

There is a fourth interframe space called the Extended Interframe Space (EIFS), which

is not covered on the CWNA exam EIFS is a variable length space used as a waiting period when a frame transmission results in a bad reception of the frame due to an incorrect FCS value EIFS is not a main focus of this section and an in-depth understanding of its functionality is not essential knowledge to a wireless network administrator

is synchronized and all stations and access points use standard amounts of time (spaces)

to perform various tasks Each node knows these spaces and uses them appropriately A set of standard spaces is specified for DSSS, FHSS, and Infrared as you can see from Figure 8.3 By using these spaces, each node knows when and if it is supposed to perform a certain action on the network

Short Interframe Space (SIFS)

SIFS is the shortest fixed interframe space SIFS are time spaces before and after which the following types of messages are sent The list below is not an exhaustive list

RTS - Request-to-Send frame, used for reserving the medium by stations CTS - Clear-to-Send frame, used as a response by access points to the RTS frame generated by a station in order to ensure all stations have stopped transmitting ACK - Acknowledgement frame used for notifying sending stations that data arrived in readable format at the receiving station

SIFS provide the highest level of priority on a wireless LAN The reason for SIFS having the highest priority is that stations constantly listen to the medium (carrier sense) awaiting a clear medium Once the medium is clear, each station must wait a given amount of time (spacing) before proceeding with a transmission The length of time a station must wait is determined by the function the station needs to perform Each function on a wireless network falls into a spacing category Tasks that are high priority fall into the SIFS category If a station only has to wait a short period of time after the medium is clear to begin its transmissions, it would have priority over stations having to wait longer periods of time SIFS is used for functions requiring a very short period of time, yet needing high priority in order to accomplish the goal

Point Coordination Function Interframe Space (PIFS)

A PIFS interframe space is neither the shortest nor longest fixed interframe space, so it gets more priority than DIFS and less than SIFS Access points use a PIFS interframe

space only when the network is in point coordination function mode, which is manually

configured by the administrator PIFS are shorter in duration than DIFS (see Figure 8.3),

so the access point will always win control of the medium before other contending stations in distributed coordination function (DCF) mode PCF only works with DCF, not as a stand-alone operational mode so that, once the access point is finished polling, other stations can continue to contend for the transmission medium using DCF mode

Distributed Coordination Function Interframe Space (DIFS)

DIFS is the longest fixed interframe space and is used by default on all 802.11-compliant stations that are using the distributed coordination function Each station on the network using DCF mode is required to wait until DIFS has expired before any station can contend for the network All stations operating according to DCF use DIFS for transmitting data frames and management frames This spacing makes the transmission

of these frames lower priority than PCF-based transmissions Instead of all stations assuming the medium is clear and arbitrarily beginning transmissions simultaneously after DIFS (which would cause collisions), each station uses a random back off algorithm

to determine how long to wait before sending its data

The period of time directly following DIFS is referred to as the contention period (CP) All stations in DCF mode use the random back off algorithm during the contention period During the random back off process, a station chooses a random number and multiplies it by the slot time to get the length of time to wait The stations count down these slot times one by one, performing a clear channel assessment (CCA) after each slot time to see if the medium is busy Whichever station's random back off time expires first, that station does a CCA, and provided the medium is clear, it then begins transmission

Once the first station has begun transmissions all other stations sense that the medium is busy, and remember the remaining amount of their random back off time from the previous CP This remaining amount of time is used in lieu of picking another random number during the next CP This process assures fair access to the medium among all stations

Once the random back off period is over, the transmitting station sends its data and receives back the ACK from the receiving station This entire process then repeats It stands to reason that most stations will chose different random numbers, eliminating most collisions However, it is important to remember that collisions do happen on wireless LANs, but they cannot directly be detected Collisions are assumed by the fact that the ACK is not received back from the destination station

Slot Times

A slot time, which is pre-programmed into the radio in the same fashion as the SIFS, PIFS, and DIFS timeframes, is a standard period of time on a wireless network Slot times are used in the same method as a clock's second hand is used A wireless node ticks slot times just like a clock ticks seconds These slot times are determined by the wireless LAN technology being utilized

FHSS Slot Time = 50uS DSSS Slot Time = 20uS Infrared Slot Time = 8uS Notice the following:

PIFS = SIFS + 1 Slot Time DIFS = PIFS + 1 Slot Time Also notice that FHSS has noticeably longer slot times, DIFS times, and PIFS times than DSSS These longer times contribute to FHSS overhead, which decreases throughput

The Communications Process

When you consider the PIFS process described above, it may seem as though the access

point would always have control over the medium, since the access point does not have to

wait for DIFS, but the stations do This would be true, except for the existence of what is

called a superframe A superframe is a period of time, and it consists of three parts:

Contention-Free Period (PCF Mode) Beacon Contention Period(DCF Mode)

Superframe

Again, remember that PIFS, and hence the superframe, only occurs when

1 The network is in point coordination function mode

2 The access point has been configured to do polling

3 The wireless clients have been configured to announce to the access point that they are pollable

Therefore, if we start from a hypothetical beginning point on a network that has the access point configured for PCF mode, and the some of the clients are configured for polling, the process is as follows

1 The access point broadcasts a beacon

2 During the contention free period, the access point polls stations to see if any station needs to send data

3 If a station needs to send data, it sends one frame to the access point in response

to the access point’s poll

4 If a station does not need to send data, it returns a null frame to the access point

in response to the access point’s poll

5 Polling continues throughout the contention free period

6 Once the contention free period ends and the contention period begins, the access point can no longer poll stations During the contention period, stations using DCF mode contend for the medium and the access point uses DCF mode

7 The superframe ends with the end of the CP, and a new one begins with the following CFP

Think of the CFP as using a "controlled access policy" and the CP as using a "random access policy." During the CFP, the access point is in complete control of all functions

on the wireless network, whereas during the CP, stations arbitrate and randomly gain control over the medium The access point, in PCF mode, does not have to wait for the DIFS to expire, but rather uses the PIFS, which is shorter than the DIFS, in order to capture the medium before any client using DCF mode does Since the access point captures the medium and begins polling transmissions during the CFP, the DCF clients sense the medium as being busy and wait to transmit After the CFP the CP begins, during which all stations using DCF mode may contend for the medium and the access point switches to DCF mode

Figure 8.5 illustrates a short timeline for a wireless LAN using DCF and PCF modes

Poll

DIFS PIFS ContentionPeriodtime

Access point seizes control of medium here

Stations in DCF mode would normally contend for access here

The process is somewhat simpler when a wireless LAN is only in DCF mode, because there is no polling and, hence, no superframe This process is as follows:

1 Stations wait for DIFS to expire

2 During the CP, which immediately follows DIFS, stations calculate their random back off time based on a random number multiplied by a slot time

3 Stations tick down their random time with each passing slot time, checking the medium (CCA) at the end of each slot time The station with the shortest time gains control of the medium first

4 A station sends its data

5 The receiving station receives the data and waits a SIFS before returning an ACK back to the station that transmitted the data

6 The transmitting station receives the ACK and the process starts over from the beginning with a new DIFS

Figure 8.6 illustrates a timeline for a DCF mode wireless LAN Keep in mind that this timeline is a few milliseconds long The whole process happens many times every second

Request to Send/Clear to Send (RTS/CTS)

There are two carrier sense mechanisms used on wireless networks The first is physical

carrier sense Physical carrier sense functions by checking the signal strength, called the

Received Signal Strength Indicator (RSSI), on the RF carrier signal to see if there is a

station currently transmitting The second is virtual carrier sense Virtual carrier sense

works by using a field called the Network Allocation Vector (NAV), which acts as a timer on the station If a station wishes to broadcast its intention to use the network, the station sends a frame to the destination station, which will set the NAV field on all stations hearing the frame to the time necessary for the station to complete its transmission, plus the returning ACK frame In this way, any station can reserve use of the network for specified periods of time Virtual carrier sense is implemented with the RTS/CTS protocol

The RTS/CTS protocol is an extension of the CSMA/CA protocol As the wireless LAN administrator, you can take advantage of using this protocol to solve problems like Hidden Node (discussed in Chapter 9, Troubleshooting) Using RTS/CTS allows stations

to broadcast their intent to send data across the network

As you can imagine by the brief description above, RTS/CTS will cause significant network overhead For this reason RTS/CTS is turned OFF by default on a wireless LAN If you are experiencing an unusual amount of collisions on your wireless LAN (evidenced by high latency and low throughput) using RTS/CTS can actually increase the traffic flow on the network by decreasing the number of collisions Use of RTS/CTS should not be done haphazardly RTS/CTS should be configured after careful study of the network's collisions, throughput, latency, etc

Some manufacturers do not allow administrators to change a station's RTS/CTS settings (and many other settings) unless they obtain the special password from the manufacturer By default, an administrator is locked out of those features of the station's driver software Normally, getting this password will not be easy These manufacturers require the administrator to take their 1-2 day product seminar before they will allow the administrator to fill out a series of paperwork to obtain the necessary password(s)

Figure 8.7 illustrates the 4-way handshake process used for RTS/CTS In short, the transmitting station broadcasts the RTS, followed by the CTS reply from the receiving station, both of which go through the access point Next, the transmitting station sends its data payload through the access point to the receiving station, which immediately replies

with an acknowledgement frame, or ACK This process is used for every frame that is sent across the wireless network

Access Point Sending

However, the “on with threshold” setting allows the administrator to control which packets (over a certain size - called the threshold) are announced and cleared to send by the stations Since collisions affect larger packets more than smaller ones, you can set the RTS/CTS threshold to work only when a node wishes to send packets over a certain size This setting allows you to customize the RTS/CTS setting to your network data traffic and optimize the throughput of your wireless LAN while preventing problems like Hidden Node

Figure 8.8 depicts a DCF network using the RTS/CTS protocol to transmit data Notice that the RTS and CTS transmissions are spaced by SIFS The NAV is set with RTS on all nodes, and then reset on all nodes by the immediately following CTS

FIGURE 8.8 RTS/CTS data transmission in DCF mode

sender receiver

other stations

DIFS RTS

CTS

data

ACK

data DIFS

Defer access

contention

NAV (RTS) NAV (CTS)

t

Modulation

Modulation, which is a Physical Layer function, is a process in which the radio transceiver prepares the digital signal within the NIC for transmission over the airwaves Modulation is the process of adding data to a carrier by altering the amplitude, frequency,

or phase of the carrier in a controlled manner Knowing the many different kinds of modulations used with wireless LANs is helpful when trying to build a compatible network piece-by-piece

Complimentary Code Keying (CCK) are the types of spreading codes used in 802.11 and 802.11b wireless LANs

As higher transmission speeds are specified (such as when a system is using DRS), modulation techniques change in order to provide more data throughput For example, 802.11g and 802.11a compliant wireless LAN equipment specify use of orthogonal frequency division multiplexing (OFDM), allowing speeds of up to 54 Mbps, which is a significant improvement over the 11 Mbps specified by 802.11b Figure 8.10 shows the modulation types used for 802.11a networks The 802.11g standard provides backwards compatibility by supporting CCK coding and even supports packet binary convolution coding (PBCC) as an option Bluetooth and HomeRF are both FHSS technologies that use GFSK modulation technology in the 2.4 GHz ISM band

Coding Technique

Modulation Technology

Data Rate

Key Terms

Before taking the exam, you should be familiar with the following terms:

ACK beacons bit error rate contention free period contention period DIFS

PIFS polling probe frame SIFS superframe

A PCF has a lower overhead than using DCF

B PCF can be used in and IBSS while DCF cannot

C PCF uses CSMA/CA while DCF does not

D PCF provides a given level of QoS

5 After a client station sends a data packet to another client station, the receiving station replies with an acknowledgement after which interframe space?

A IFS

B SIFS

C PIFS

D DIFS

6 Why is the CSMA/CA protocol used in order to avoid collisions in a wireless LAN?

A PCF mode requires use of a polling mechanism

C All clients must acknowledge packets received while they're asleep

B Superframe

D Request to send

8 PIFS are only used during the communications of a wireless LAN when which of the following have occurred?

B The access point has been configured to use RTS/CTS

D The network is configured for fragmentation

9 You have just finished installing your first wireless LAN with 802.11b equipment rated at 11 Mbps After testing the throughput of the clients, you find your actual throughput is only 5.5 Mbps What is the likely cause of this throughput?

B Wireless LANs use the CSMA/CA protocol

D DRS has caused all of the clients to decrease their data rates

10 You have just finished installing your first wireless LAN with 802.11b equipment rated at 11 Mbps After testing the throughput of the clients you find your actual throughput is only 5.5 Mbps What can you change to get 11Mbps throughput?

B Move all of the clients closer to the access point

B The overhead of sending acknowledgements is high

D It is not possible to detect collisions on a wireless LAN

7 End stations will broadcast a _ when actively scanning for access points on the network

A Beacon management frame

C Probe request frame

A The network is in point coordination function mode

C The access point has been configured to use CSMA/CD

A Wireless LANs use RTS/CTS by default

C Use of PCF is reducing network throughput

A Turn off RTS/CTS

C Turn up the power on the access point

D Purchase another access point and co-locate both together

11 802.11b devices use what type of modulation at 11 Mbps?

A BPSK

B DPSK

12 802.11a devices use what type of modulation at 24 Mbps?

D CCK

B The receiving station is a hidden node

14 Modulation is which of one of the following?

A The receiving station is sleeping

C There was a collision

D That RTS/CTS is turned on

A The process by which digital data is modified to become RF data

B The process of adding data to a carrier by altering the amplitude, frequency, or phase of the carrier in a controlled manner

C The process of propagating an RF signal through the airwaves

D The means by which RF signals are received and processed by RF antennas

15 Which one of the following is not part of a superframe?

B Beacon Free Period

D Contention Period