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History of Wireless Local Area Networks.

Este es el resumen del primer capítulo del libro CWNA 107, en el encontrarás lo que a mi parecer son las ideas claves del mismo y con ellas podrás responder las preguntas de revisión que encontrarás al final del post.

Wireless networking technology was first used by the U.S. military during World War II to transmit data over an RF medium using classified encryption technology to send battle plans across enemy lines. The spread spectrum radio technologies often used in today’s WLANs were also originally patented during the era of World War II, although they were not implemented until almost two decades later.

In 1970, the University of Hawaii developed the first wireless network, called ALOHAnet, to wirelessly communicate data between the Hawaiian Islands. The network used a LAN com- munication Open Systems Interconnection (OSI) layer 2 protocol called ALOHA on a wireless shared medium in the 400 MHz frequency range.

In the 1990s, commercial networking vendors began to produce low-speed wireless data networking products, most of which operated in the 900 MHz frequency band. The Institute of Electrical and Electronics Engineers (IEEE) began to discuss standardizing WLAN technologies in 1991. In 1997, the IEEE ratified the original 802.11 standard that is the foundation of the WLAN technologies you will be learning about in this book.

This legacy 802.11 technology was deployed between 1997 and 1999 mostly in ware- housing and manufacturing environments for the use of low-speed data collection with wireless barcode scanners. In 1999, the IEEE defined higher data speeds with the 802.11b amendment.

Standards Organizations.

Federal Communications Commission.

To put it simply, the Federal Communications Commission (FCC) regulates communications within the United States as well as communications to and from the United States. The task of the FCC in wireless networking is to regulate the radio signals that are used for wire- less networking. The FCC and the respective controlling agencies in other countries typically regulate two categories of wireless communications: licensed spectrum and unlicensed spectrum. The difference is that unlicensed users do not have to go through the license application procedures before they can install a wireless system. Both licensed and unlicensed communications are typically regulated in the following sic areas:

  1. Frequency.
  2. Bandwidth.
  3. Maximum power of the intentional radiator (IR).
  4. Maximum equivalent isotropically radiated power (EIRP).
  5. Use (indoor and/or outdoor).
  6. Spectrum sharing rules.

Essentially, the FCC and other regulatory bodies set the rules for what the user cando regarding RF transmissions. From there, the standards organizations create the standards to work within these guidelines. These organizations work together to help meet the demands of the fast-growing wireless industry. The FCC rules are published in the Code of Federal Regulations (CFR).

International Telecommunication Union Radiocommunication Sector.

A global hierarchy exists for management of the RF spectrum worldwide. The United Nations has tasked the International Telecommunication Union Radiocommunication Sector (ITU-R) with global spectrum management. The ITU-R strives to ensure interference- free communications on land, sea, and in the skies. The ITU-R maintains a database of worldwide frequency assignments through five administrative regions.

The five administrative regions are broken down as follows:

  • Region A: The Americas.
  • Region B: Western Europe.
  • Region C: Eastern Europe and Northern Asia.
  • Region D: Africa
  • Region E: Asia and Australasia

In addition to the five administrative regions, the ITU-R defines three radio regulatory regions.

  • Region 1: Europe, Middle East, and Africa
  • Region 2: Americas
  • Region 3: Asia and Oceania

Institute of Electrical and Electronics Engineers.

The Institute of Electrical and Electronics Engineers, commonly known as the IEEE, is a global professional society with more than 420,000 members in 160 countries. The IEEE’s mission is to “foster technological innovation and excellence for the benefit of humanity.” To networking professionals, that means creating the standards that we use to communicate. The IEEE is probably best known for its LAN standards, the IEEE 802 project. IEEE projects are subdivided into working groups to develop standards that address specific problems or needs. For instance, the IEEE 802.3 working group was responsible for the creation of a standard for Ethernet, and the IEEE 802.11 working group was respon- sible for creating the WLAN standard. It is important to remember that the IEEE standards, like many other standards, are written documents describing how technical processes and equipment should function.

Internet Engineering Task Force.

The Internet Engineering Task Force, commonly known as the IETF, is an international community of people in the networking industry whose goal is to make the Internet work better. The mission of the IETF, as defined by the organization in a document known as RFC 3935, is “to produce high quality, relevant technical and engineering documents that influence the way people design, use, and manage the Internet in such a way as to make the Internet work better. These documents include protocol standards, best current practices, and informational documents of various kinds.” The results of a working group are usually the creation of a document known as a Request for Comments (RFC). Contrary to its name, an RFC is not actually a request for comments but a statement or definition. Most RFCs describe network protocols, services, or policies and may evolve into an Internet standard. RFCs are numbered sequentially, and once a number is assigned, it is never reused. At the top of the RFC document, it states whether the RFC is updated by another RFC and also if it makes any other RFCs obsolete. Not all RFCs are standards. Each RFC is given a status, relative to its relationship with the Internet standardization process: Informational, Experimental, Standards Track, or Historic. If it is a Standards Track RFC, it could be a Proposed Standard, Draft Standard, or Internet Standard. When an RFC becomes a standard, it still keeps its RFC number, but it is also given an “STD xxxx” label. The relationship between the STD numbers and the RFC numbers is not one-to-one. STD numbers identify protocols, whereas RFC numbers identify documents.

Wi-Fi Alliance.

The Wi-Fi Alliance is a global, nonprofit industry association of more than 550 member companies devoted to promoting the growth of WLANs. One of the primary tasks of the Wi-Fi Alliance is to market the Wi-Fi brand and raise consumer awareness of new 802.11 technologies as they become available. The Wi-Fi Alliance’s main task is to ensure the interoperability of WLAN products by providing certification testing. The Wi-Fi Alliance, originally named the Wireless Ethernet Compatibility Alliance (WECA), was founded in August 1999. The name was changed to the Wi-Fi Alliance in October 2002.

Connectivity

Wi-Fi CERTIFIED b/g: The Wi-Fi Alliance certifies backward compatibility with legacy 802.11b/g devices that operate in the 2.4 GHz frequency band.

Wi-Fi CERTIFIED a: The Wi-Fi Alliance certifies backward compatibility with legacy 802.11a radios that transmit in 5 GHz frequency band.

Wi-Fi CERTIFIED n: The Wi-Fi Alliance certifies the operational capabilities for 802.11n radios for both the 2.4 GHz and 5 GHz frequency bands. 802.11n introduced PHY and MAC layer enhancements to achieve higher data rates. 802.11n requires multiple-input- multiple output (MIMO) radio systems that are backward compatible with 802.11a/b/g technology.

Wi-Fi CERTIFIED ac: The Wi-Fi Alliance certifies the operational capabilities for 802.11ac radios for the 5 GHz frequency band. 802.11ac technology introduced further PHY and MAC layer enhancements to achieve higher data rates beyond 802.11n. 802.11ac radios are backward compatible with 802.11a/n radios.

Wi-Fi Direct: Wi-Fi Direct enables Wi-Fi devices to connect directly without the use of an access point (AP), making it easier to print, share, sync, and display. Wi-Fi Direct is ideal for mobile phones, cameras, printers, PCs, and gaming devices needing to establish a one-to-one connection, or even connecting a small group of devices

Wi-Fi CERTIFIED WiGig: The WiGig certification program is based on technology origi- nally defined in the 802.11ad amendment for directional multi-gigabit (DMG) radios that transmit the 60 GHz frequency band. Multi-band Wi-Fi CERTIFIED WiGig devices can seamlessly transfer between the 2.4, 5, or 60 GHz bands.

Security

The Wi-Fi Protected Access 2 (WPA2) certification is based on robust security network (RSN) capabilities, security mechanisms that were originally defined in the IEEE 802.11i amendment. All Wi-Fi WPA2–certified devices must support CCMP/AES dynamic encryption methods. The Wi-Fi Alliance specifies two methods for user and device authorization for WLANs. WPA2 Enterprise requires support for 802.1X port-based access control security for enterprise deployments. WPA2-Personal uses a less complex passphrase method intended for SOHO environments.

Extensible Authentication Protocol Enterprise devices must support Extensible Authentication Protocol (EAP), the authen- tication protocol used within an 802.1X authorization framework. The Wi-Fi Alliance certification program tests for many variants of EAP, including EAP-TLS, EAP-TTLS, EAP-PEAP, and others.

WPA2 with Protected Management Frames This certification is based upon the IEEE 802.11w-2009 amendment. It is the management frame protection (MFP) amendment, with a goal of delivering certain types of management frames in a secure manner. The intent is to prevent spoofing of certain types of 802.11.

Access

Passpoint Passpoint is designed to revolutionize the end-user experience when connect- ing to Wi-Fi hotspots. This is done by automatically identifying the hotspot provider and connecting to it, automatically authenticating the user to the network using Extensible Authentication Protocol (EAP), and providing secure transmission using WPA2-Enterprise encryption. Passpoint is based on the Hotspot 2.0 technical specification.

Wi-Fi Protected Setup Wi-Fi Protected Setup (WPS) defines simplified and automatic WPA and WPA2 security configurations for home and small-business users. Users can easily configure a network with security protection by using either near field communication (NFC), a personal identification number (PIN), or a button located on the AP and the client device. WPS technology is defined in the Wi-Fi Simple Configuration technical specification.

IBSS with Wi-Fi Protected Setup IBSS with Wi-Fi Protected Setup provides easy con- figuration and strong security for ad-hoc (peer-to-peer) Wi-Fi networks. This is designed for mobile products and devices that have a limited user interface, such as smartphones, cameras, and media players.

Applications and Services

Voice-EnterpriseMany of the mechanisms defined by the IEEE 802.11k, 802.11r and 802.11v amendments are also defined by the Voice-Enterprise certification. Both access point and client devices must support prioritization using Wi-Fi Multimedia (WMM), with voice traffic being placed in the highest-priority queue (Access Category Voice, AC-VO). Voice-Enterprise equipment must also support seamless roaming between APs, WPA2-Enterprise security, optimization of power through the WMM-Power Save mechanism, and traffic management through WMM-Admission Control.

Miracast Miracast seamlessly integrates the display of high-resolution streaming video content between devices. Wireless links are used to replace wired connections. Devices are designed to identify and connect with each other, manage their connections, and optimize the transmission of video content. Miracast is based on the Wi-Fi Display technical specification. The Miracast certification program is for any video-capable device, such as cameras, televisions, projectors, tablets, and smartphones. Paired Miracast devices can stream high-definition (HD) content or mirror displays via a peer-to-peer Wi-Fi connection.

Wi-Fi Aware Wi-Fi Aware-enabled devices use power-efficient discovery of nearby ser- vices or information before making a connection. The neighbor awareness networking (NAN) technical specification defines mechanisms for WLAN devices to synchronize channel and time information to allow for the discovery of services. Wi-Fi Aware does not require the existence of a WLAN infrastructure, and discovery occurs in the background, even in crowded user environments. Prior to establishing a connection, users can find other nearby users for the purposes of sharing media, local information, and gaming opponents.

Wi-Fi Location Wi-Fi Location is based on the Fine Timing Measurement (FTM) protocol defined in the IEEE 802.11-2016 standard. Wi-Fi Location-enabled devices and networks can provide devices with highly accurate indoor location information via the Wi-Fi network without the need for an overlay infrastructure such as iBeacons or a real-time locating system (RTLS). Application and OS developers can create location-based applications and services. Some of the potential uses include asset management, geo-fencing, and hyperlocal marketing.

Optimization

Wi-Fi Multimedia (WMM) is based on the QoS mechanisms that were originally defined in the IEEE 802.11e amendment. WMM enables Wi-Fi networks to prioritize traffic generated by different applications. In a network where WMM is sup- ported by both the AP and the client device, traffic generated by time-sensitive applications, such as voice or video, can be prioritized for transmission on the half-duplex RF medium.

WMM-Power Save WMM-Power Save (WMM-PS) helps conserve battery power for devices using Wi-Fi radios by managing the time the client device spends in sleep mode. Conserving battery life is critical for handheld devices, such as barcode scanners and voice over Wi-Fi (VoWiFi) phones. To take advantage of power-saving capabilities, both the device and the access point must support WMM-PS.

WMM-Admission Control WMM-Admission Control (WMM-AC) allows Wi-Fi net- works to manage network traffic based upon channel conditions, network traffic load, and type of traffic (voice, video, best effort data, or background data). The access point allows only the traffic that it can support to connect to the network, based upon the available network resources.

Wi-Fi CERTIFIED TDLS The IEEE 802.11z-2010 amendment defines a Tunneled Direct Link Setup (TDLS) security protocol. The Wi-Fi Alliance also introduced Wi-Fi CERTIFIED TDLS as a certification program for devices using TDLS to connect directly to one another after they have joined a traditional Wi-Fi network.

Wi-Fi TimeSync Wi-Fi CERTIFIED TimeSync enables sub-microsecond clock synchronization between multiple devices, aiding precise service coordination and accurate representation of audio, video, or data. The technology supports in-room multichannel audio and video capabilities. Uses for Wi-Fi TimeSync technology include home theater systems, recording studios, camera systems, and more.

Wi-Fi Vantage A growing trend in the Wi-Fi industry is for a managed service provider (MSP) to offer “wireless as a service.” Many telecom carrier companies offer MSP services that oversee Wi-Fi operations in airports, stadiums, schools, office buildings, retail and hotel locations, and other venues.

RF Coexistence

Converged Wireless Group-RF Profile Converged Wireless Group-RF Profile (CWG-RF) was developed jointly by the Wi-Fi Alliance and the Cellular Telecommunications and Internet Association (CTIA), now known as The Wireless Association. CWG-RF defines performance and tests metrics for Wi-Fi and cellular radios in a converged handset to help ensure that both technologies perform well in the presence of the other

OSI Model.

The IEEE 802.11-2016 standard defines communication mechanisms only at the Physical layer and MAC sublayer of the Data-Link layer of the OSI model.

Core, Distribution, and Access.

Wireless networking can be implemented as either point-to-point or point-to-multipoint solutions. Most wireless networks are used to provide network access to the individual client stations and are designed as point-to-multipoint networks. This type of implementation is designed and installed on the access layer, providing connectivity to the end user. 802.11 wireless networking is most often implemented at the access layer with WLAN clients communicating through strategically deployed access points.

Wireless bridge links are typically used to provide connectivity between buildings, in the same way that county or state roads provide distribution of traffic between neighborhoods. The purpose of wireless bridging is to connect two separate, wired networks wirelessly. Routing data traffic between networks is usually associated with the distribution layer. Wireless bridge links cannot usually meet the speed or distance requirements of the core layer, but they can be very effective at the distribution layer.

Wireless networking can be implemented as either point-to-point or point-to-multipoint solutions. Most wireless networks are used to provide network access to the individual cli- ent stations and are designed as point-to-multipoint networks. This type of implementation is designed and installed on the access layer, providing connectivity to the end user. 802.11 wireless networking is most often implemented at the access layer with WLAN clients communicating through strategically deployed access points.

Wireless bridge links cannot usually meet the speed or distance requirements of the core layer, but they can be very effective at the distribution layer. An 802.11 bridge link is an example of wireless technology being implemented at the distribution layer.

Communications Fundamentals.

Communication Terminology.

Simplex In simplex communications, one device is capable of only transmitting, and the other device is capable of only receiving. FM radio is an example of simplex communica- tions. Simplex communications are rarely used on computer networks.

Half-Duplex In half-duplex communications, both devices are capable of transmitting and receiving; however, only one device can transmit at a time. Walkie-talkies, or two-way radios, are examples of half-duplex devices.

Full-Duplex In full-duplex communications, both devices are capable of transmitting and receiving at the same time. A telephone conversation is an example of a full-duplex communitations.

Understanding Carrier Signals.

Because data ultimately consists of bits, the transmitter needs a way of sending both

0s and 1s to transmit data from one location to another. An AC or DC signal by itself does not perform this task. However, if a signal fluctuates or is altered, even slightly, the signal can be interpreted so that data can be properly sent and received. This modi- fied signal is now capable of distinguishing between 0s and 1s and is referred to as a carrier signal. The method of adjusting the signal to create the carrier signal is called modulation. Three components of a wave that can fluctuate or be modified to create a carrier signal are amplitude, frequency, and phase.

Amplitude and Wavelength.

RF communication starts when radio waves are generated from an RF transmitter and picked up, or “heard,” by a receiver at another location.

Amplitude Amplitude is the height, force, or power of the wave. If you were standing in the ocean as the waves came to shore, you would feel the force of a larger wave much more than you would a smaller wave. Transmitters do the same thing, but with radio waves. Smaller waves are not as noticeable as bigger waves. A bigger wave generates a much larger electrical signal picked up by the receiving antenna. The receiver can then distinguish between highs and lows.

Wavelength Wavelength is the distance between similar points on two back-to-back waves. When measuring a wave, the wavelength is typically measured from the peak of a wave to the peak of the next wave. Amplitude and wavelength are both properties of waves.

Frecuency.

Frequency describes a behavior of waves. Waves travel away from the source that generates them. How fast the waves travel, or more specifically, how many waves are generated over a 1-second period of time, is known as frequency. If you were to try to count the radio waves that are used in wireless networking, in the time it would take for one wave of water to hit the pier, several billion radio waves would have also hit the pier.

Phase.

Phase is a relative term. It is the relationship between two waves with the same frequency. To determine phase, a wavelength is divided into 360 pieces, referred to as degrees (see Figure 1.7). If you think of these degrees as starting times, then if one wave begins at the 0 degree point and another wave begins at the 90 degree point, these waves are considered to be 90 degrees out of phase.

Understanding Keying Methods

When data is sent, a signal is transmitted from the transceiver. In order for the data to be transmitted, the signal must be manipulated so that the receiving station has a way of distinguishing 0s and 1s. This method of manipulating a signal so that it can represent multiple pieces of data is known as a keying method. A keying method is what changes a signal into a carrier signal. It provides the signal with the ability to encode data so that it can be communicated or transported.

Three types of keying methods are reviewed in the following sections: amplitude-shift keying (ASK), frequency-shift keying (FSK), and phase-shift keying (PSK). These keying methods are also referred to as modulation techniques. Keying methods use the following two different techniques to represent data:

Current State With current state techniques, the current value (the current state) of the signal is used to distinguish between 0s and 1s. The use of the word current in this context does not refer to current as in voltage but rather to current as in the present time. Current state techniques will designate a specific or current value to indicate a binary 0 and another value to indicate a binary 1. At a specific point in time, it is the value of the signal that determines the binary value. For example, you can represent 0s and 1s by using an ordinary door. Once a minute you can check to see whether the door is open or closed. If the door is open, it represents a 0, and if the door is closed, it represents a 1. The current state of the door, open or closed, is what determines 0s or 1s.

State Transition With state transition techniques, the change (or transition) of the signal is used to distinguish between 0s and 1s. State transition techniques may represent a 0 by a change in a wave’s phase at a specific time, whereas a 1 would be represented by no change in a wave’s phase at a specific time. At a specific point in time, it is the presence of a change or the lack of presence of a change that determines the binary value. The upcoming section “Phase-Shift Keying” provides examples of this in detail, but a door can be used again to provide a simple example. Once a minute you check the door. In this case, if the door is moving (opening or closing), it represents a 0, and if the door is still (either open or closed), it represents a 1. In this example, the state of transition (moving or not moving) is what determines 0s or 1s.

Amplitude-Shift Keying

Amplitude-shift keying (ASK) varies the amplitude, or height, of a signal to represent the binary data. ASK is a current state technique, where one level of amplitude can represent a 0 bit and another level of amplitude can represent a 1 bit.

This shifting of amplitude determines the data that is being transmitted. The way the receiving station performs this task is to first divide the signal being received into periods of time known as symbol periods. The receiving station then samples or examines the wave during this symbol period to determine the amplitude of the wave. Depending on the value of the wave’s amplitude, the receiving station can determine the binary value.

Frequency-Shift Keying

Frequency-shift keying (FSK) varies the frequency of the signal to represent the binary data. FSK is a current state technique, where one frequency can represent a 0 bit and another frequency can represent a 1 bit (see Figure 1.9). This shifting of frequency deter- mines the data that is being transmitted. When the receiving station samples the signal during the symbol period, it determines the frequency of the wave, and depending on the value of the frequency, the station can determine the binary value.

The faster frequency wave is interpreted as a binary 1, and the slower frequency wave is interpreted as a binary 0. FSK is used in some of the legacy deployments of 802.11 wireless networks. With the demand for faster communications, FSK techniques would require more expensive technology to support faster speeds, making it less practical.

### Phase-Shift Keying

Phase-shift keying (PSK) varies the phase of the signal to represent the binary data. PSK can be a state transition technique, where the change of phase can represent a 0 bit and the lack of a phase change can represent a 1 bit, or vice versa. This shifting of phase deter- mines the data that is being transmitted. PSK can also be a current state technique, where the value of the phase can represent a 0 bit or a 1 bit. When the receiving station samples the signal during the symbol period, it determines the phase of the wave and the status of the bit.

PSK technology is used extensively for radio transmissions as defined by the 802.11- 2016 standard. Typically, the receiving station samples the signal during the symbol period, compares the phase of the current sample with the previous sample, and deter- mines the difference. This degree of difference, or differential, is used to determine the bit value. More advanced versions of PSK can encode multiple bits per symbol. Instead of using two phases to represent the binary values, you can use four phases. Each of the four phases is capable of representing two binary values (00, 01, 10, or 11) instead of one

(0 or 1), thus shortening the transmission time. When more than two phases are used, this is referred to as multiple phase-shift keying (MPSK).

Review Questions

  1. 802.11 technology is typically deployed at which fundamental layer of network architecture?

A. Core

B. Distribution

C. Access

D. Network

  1. Which organization is responsible for enforcing maximum transmit power rules in an unlicensed frequency band?

A. IEEE

B. Wi-Fi Alliance

C. ISO

D. IETF

E. None of the above

  1. 802.11 wireless bridge links are typically associated with which network architecture layer?

A. Core

B. Distribution

C. Access

D. Network

  1. The 802.11-2016 standard was created by which organization?

A. IEEE

B. OSI

C. ISO

D. Wi-Fi Alliance

E. FCC

  1. What organization ensures interoperability of WLAN products?

A. IEEE

B. ITU-R

C. ISO

D. Wi-Fi Alliance

E. FCC

  1. What type of signal is required to carry data?

A. Communications signal

B. Data signal

C. Carrier signal

D. Binary signal

E. Digital signal

  1. Which keying method is most susceptible to interference from noise?

A. FSK

B. ASK

C. PSK

D. DSK

  1. Which sublayer of the OSI model’s Data-Link layer is used for communication between 802.11 radios?

A. LLC

B. PLCP

C. MAC

D. PMD

  1. While performing some research, Janie comes across a reference to a document titled RFC 3935. Which of the following organization’s website would be best to further research this document?

A. IEEE

B. Wi-Fi Alliance WECA

C. FCC

D. IETF

  1. Wi-Fi Alliance is responsible for which of the following certification programs?

A. 802.11i

B. WEP

C. 802.11-2016

D. WMM

  1. Which wave properties can be modulated to encode data? (Choose all that apply.)

A. Amplitude

B. Frequency

C. Phase

D. Wavelength.

  1. The IEEE 802.11-2016 standard defines communication mechanisms at which layers of the

OSI model? (Choose all that apply.)

A. Network

B. Physical

C. Transport

D. Application

E. Data-Link F. Session

  1. The height or power of a wave is known as what?

A. Phase

B. Frequency

C. Amplitude

D. Wavelength

  1. What are the communication differences between Wi-Fi Direct and Wi-Fi CERTIFIED TDLS devices? (Choose all that apply.)

A. Wi-Fi CERTIFIED TDLS devices never associate to an AP.

B. Wi-Fi Direct devices can communicate with each other without associating to an AP.

C. Wi-Fi CERTIFIED TDLS devices remain associated to an AP while communicating directly with each other.

D. Wi-Fi Direct devices must associate with an AP before they can communicate with each other.

  1. What Wi-Fi Alliance certifications are required before a Wi-Fi radio can be certified as Voice-Enterprise compliant? (Choose all that apply.)

A. WMM-Power Save

B. Wi-Fi Direct

C. WPA2-Enterprise

D. Voice-Personal

E. WMM-Admission Control

  1. Which of the following wireless communications parameters and usage are typically gov-

erned by a local regulatory authority? (Choose all that apply.)

A. Frequency

B. Bandwidth

C. Maximum transmit power

D. Maximum EIRP

E. Indoor/outdoor usage

  1. What type of communications do 802.11 radios use to transmit and receive?

A. Simplex

B. Half-duplex

C. Full-duplex

D. Echo-duplex

  1. A wave is divided into degrees. How many degrees make up a complete wave?

A. 100

B. 180

C. 212

D. 360

  1. What are the advantages of using unlicensed frequency bands for RF transmissions? (Choose all that apply.)

A. There are no governmental regulations.

B. There is no additional financial cost.

C. Anyone can use the frequency band.

D. There are no rules.

  1. The OSI model consists of how many layers?

A. Four

B. Six

C. Seven

D. Nine

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