Sep-03 ©Cisco SystemsCCNA Semester 1 Version 3 Comp11 Mod2 – St. Lawrence College – Cornwall...

Sep-03 ©Cisco Systems CCNA Semester 1 Version 3 Comp11 Mod2 – St. Lawrence College – Cornwall Campus – Parisien slide 1

802.11


Equipment that has been tested to comply with the 802.11 standard is said to be Wi-Fi certified (like Hi-Fi, but Wireless Fidelity).


Wireless LANsLicense Free Spread SpectrumThe FCC has set standards under Part 15 of the Rules and Regulations for equipment used in the 2.4 GHz band. (The exact spectrum is 2400 to 2483.5 MHz). If the equipment uses Spread Spectrum techniques, then effective radiated transmit powers up to 64 watts can be used!

There are two types of Spread Spectrum techniques used: Frequency Hopping Spread Spectrum (FHSS) and Direct Sequence Spread Spectrum (DSSS). This enables many radios to operate in this band with minimum interference – up to a point.

With FHSS, a data packet is first sent on a random channel in the band with the next packet sent, after a pause of a few milliseconds, on another random channel in the band. With 80 channels or more available (one channel per MHz, e.g., 2401, 2402, 2403, etc) signals from multiple radios "hop" around each other. This is how they can operate with other radios in the same band with minimal interference.

DSSS radios operate on a fixed radio channel, but the signal is "spread" on that channel by mixing the signal with a Pseudo-Noise (PN) code. This spreading causes the radio signal with the data on it to occupy a much wider band, and looks more like noise to receivers not designed to "de-spread " that signal.


802.0 SEC

802.1 High Level Interface (HILI)

802.2 Logical Link Control (LLC)

802.3 CSMA/CD Working Group

802.4 Token Bus

802.5 Token Ring

802.6 Metropolitan Area Network (MAN)

802.7 BroadBand Technical Adv. Group (BBTAG)

802.8 Fiber Optics Technical Adv. Group (FOTAG)

802.9 Integrated Services LAN (ISLAN)

802.10 Standard for Interoperable LAN Security (SILS)

801.11 Wireless LAN (WLAN)

802.12 Demand Priority

802.14 Cable-TV Based Broadband Communication Network

802.15 Wireless Personal Area Network (WPAN)

802.16 Broadband Wireless Access (BBWA)

RPRSG Resilient Packet Ring Study Group (RPRSG)

IEEE 802.11

IEEE 802.11a

IEEE 802.11b WiFi

IEEE 802.11g

IEEE 802.15.1 Bluetooth

IEEE 802.11e

IEEE 802.11f

IEEE 802.11h

IEEE 802.11i Security 2004

IEEE 802.15 TG2

IEEE 802.15 TG3

IEEE 802.15 TG4

IEEE 802 Committees


IEEE 802.11Standard for WLAN operations at data rates up to 2 Mbps in the 2.4 GHz ISM band. FHSS or DSSS modulation.

IEEE 802.11a

Standard for WLAN operations at data rates up to 54 Mbps in the 5 GHz band. OFDM Modulation. Proprietary “rate doubling" has achieved 108 Mbps. Realistic rating is 20-26 Mbps.

IEEE 802.11b

Wi-Fi™ or “high-speed wireless” 1, 2, 5.5 and 11 Mbps in the 2.4 GHz band. All 802.11b systems are backward compliant. Realistic rating is 2 to 4 Mbps. DSSS modulation.

IEEE 802.11g802.11a backward compatible to the 802.11b 2.4 GHz band using OFDM.

Direct Sequence Spread Spectrum

Orthogonal Frequency Division Multiplexing

Radio Licenses NOT required in these bands: 2.4 GHz, 5 GHz

Frequency Hop Spread Spectrum


ISM BandThe Industrial, Scientific and Medical (ISM) radio bands were originally reserved internationally for non-commercial use of RF electromagnetic fields for industrial, scientific and medical purposes.

The ISM bands are defined by the ITU-T in S5.138 and S5.150 of the Radio Regulations. Individual countries' use of the bands designated in these sections may differ due to variations in national radio regulations.

In recent years they have also been used for license-free error-tolerant communications applications such as wireless LANs and Bluetooth:

•900 MHz band (33.3 cm)

•2.45 GHz band (12.2 cm) (2.4 - 2.4835 GHz range)

•5.150-5.250 GHz, 5.250-5.350 GHz and 5.725-5.825 GHz bands

IEEE 802.11b wireless Ethernet also operates on the 2.45 GHz band and 802.11a and 802.11g operate in the 5.xxx GHz bands. The use of the spectrum in the bands 5.150-5.250 GHz, 5.250-5.350 GHz and 5.725-5.825 GHz for LE-LAN devices is on the basis that such devices cannot claim protection from other radio systems and cannot cause harmful interference into other radio services in these bands. (LE-LAN = License Exempt LAN)


BlueToothBluetooth® wireless technology enables links between mobile computers, mobile phones, portable handheld devices, and connectivity to the Internet.

Hardware that complies with the Bluetooth wireless specification ensures communication compatibility worldwide.

Unlike many other wireless standards, the Bluetooth wireless specification includes both link layer and application layer definitions for product developers which supports data, voice, and content-centric applications.

Radios that comply with the Bluetooth wireless specification operate in the unlicensed, 2.4 GHz radio spectrum(2.4 - 2.4835 GHz) ensuring communication compatibility worldwide.

These radios use a spread spectrum, frequency hopping, full-duplex signal at up to 1600 hops/sec. The signal hops among 79 frequencies at 1 MHz intervals to give a high degree of interference immunity. Up to seven simultaneous connections can be established and maintained.


Standard Data RateModulation

SchemePros/Cons

802.11 ≤ 2Mbps 2.4GHz

FHSS or DSSS

This specification has been extended into 802.11b.

802.11a ≤ 54Mbps 5GHz

OFDM

"Wi-Fi Certified." 8 available channels. Less potential for RF interference than 802.11b and 802.11g. Better than 802.11b at supporting multimedia voice, video and large-image applications in densely populated user environments. Relatively shorter range than 802.11b. Not interoperable with 802.11b.

802.11b ≤ 11Mbps 2.4GHz

DSSS with CCK

"Wi-Fi Certified." 14 channels available. Not interoperable with 802.11a. Requires fewer access points than 802.11a for coverage of large areas. High-speed access to data at up to 300 feet from base station.

802.11g ≤ 54Mbps 2.4GHz

OFDM > 20Mbps DSSS + CCK <

20Mbps

"Wi-Fi Certified." 14 channels available. May replace 802.11b. Improved security enhancements over 802.11. Compatible with 802.11b.

Bluetooth

Up to 2Mbps

2.45GHzFHSS

No native support for IP, so it does not support TCP/IP and wireless LAN applications well. Best suited for connecting PDAs, cell phones and PCs in short intervals. Adaptive


802.11

A key technology contained within the 802.11 standard is Direct Sequence Spread Spectrum (DSSS).

DSSS applies to wireless devices operating within a 1 to 2 Mbps range. A DSSS system may operate at up to 11 Mbps but will not be considered compliant above 2 Mbps.

The next standard approved was 802.11b, which increased transmission capabilities to 11 Mbps.

Even though DSSS WLANs were able to interoperate with the Frequency Hopping Spread Spectrum (FHSS) WLANs, problems developed prompting design changes by the manufacturers. In this case, IEEE’s task was simply to create a standard that matched the manufacturer’s solution.


802.11b802.11b may also be called Wi-Fi™ or high-speed wireless and refers to DSSS systems that operate at 1, 2, 5.5 and 11 Mbps.

All 802.11b systems are backward compliant in that they also support 802.11 for 1 and 2 Mbps data rates for DSSS only. This backward compatibility is extremely important as it allows upgrading of the wireless network without replacing the NICs or access points.

802.11b devices achieve the higher data throughput rate by using a different coding technique from 802.11, allowing for a greater amount of data to be transferred in the same time frame.

The majority of 802.11b devices still fail to match the 11 Mbps throughput and generally function in the 2 to 4 Mbps range.


802.11a

802.11a covers WLAN devices operating in the 5 GHZ transmission band.

Using the 5 GHZ range disallows interoperability of 802.11b devices as they operate within 2.4 GHZ.

802.11a is capable of supplying data throughput of 54 Mbps and with proprietary technology known as "rate doubling" has achieved 108 Mbps.

In production networks, a more standard rating is 20-26 Mbps.


802.11g802.11g provides the same throughout as 802.11a but with backwards compatibility for 802.11b devices using Orthogonal Frequency Division Multiplexing (OFDM) modulation technology.

Cisco has developed an access point that permits 802.11b and 802.11a devices to coexist on the same WLAN. The access point supplies ‘gateway’ services allowing these otherwise incompatible devices to communicate.


www.wi-fi.com

Excellent Products listings


Access Points (APs)20-30% overlap

91.44 to 152.4 meters


D-Link - 22Mbps 802.11b+ Wireless Router and PC Card Bundle (Refurbished) $49.99


When a source node sends a frame, the receiving node returns a positive acknowledgment (ACK). This can consume 50% of the available bandwidth. This overhead, combined with the collision avoidance protocol (CSMA/CA) reduces the actual data throughput to a maximum of 5.0 to 5.5 Mbps on an 802.11b wireless LAN rated at 11 Mbps.


802.11g

Adaptive


802.11b

Not Adaptive


D-Link - DWL-800AP+ - Enhanced Wireless 2.4GHz Range Extender $69.99

The D-Link AirPlus DWL-800AP+ is an enhanced 802.11b Wireless Range Extender that can operate as an Wireless Access Point or Wireless Repeater.


Netgear - WGR614 – Wireless 54 Mbps Cable/DSL Router "G“ $69.99

Electromagnetic radiation


Electromagnetic Radiation


In a transformer, the idea is to hold the energy inside a ferromagnetic material.

If there is no ferromagnetic material, the energy will

radiate.

Here the Tx (L1) & Rx (L2) coils are closely coupled.



Here the Tx & Rx coils …

… are farther apart.


And the induced voltage is less


And between them is an electromagnetic field.

At some point they are no longer coils, they are

ANTENNAS.



FloydElectronics Fundamentals, Circuits, Devices and Applications, Fifth Edition

Primitive coil antenna.

Dipole antenna.Ground plane

Dipole antenna.

Antenna Evolution


+

-

½ Wave Length Dipole

E

H


The electric field (E), the magnetic field (H), and the direction of propagation (Z) are all at 90 degree to each other



An Electromagnetic wave carries the data stream between the Access Point and the Node.


Omni directional is used to cover an area.

Radiation Patterns


•There are some applications where you do not want an omni directional pattern. •You want a directional pattern.


Reflector Element

Active Dipole

3 Directors

Adding extra elements will focus the pattern

Yagi antenna


Then you can add a Parabolic Reflector to focus the pattern even more.

Making a parabolic antenna


Parabolic antenna pattern

12 dB yagi gain becomes 24 dB for a 1 meter dish.


Various Antenna designs

16 dB Panel 2.4

Ghz

6dB indoor omni 2.4 Ghz

6 dB omni 2.4 Ghz

24 dB solid dish 2.4 Ghz


D-Link - DWL-R60AT Indoor 6 dBi Microstrip Antenna $34.99

D-Link Ant24-0801 8.5 DBI Pico Cell Patch Antenna $139.99

Typical indoor directional WLAN antennas


Most suppliers will have a complete family.


Low speed Cell phone connections


Low speed Cell phone Availability


Radio WavesComputers send data signals electronically.

Radio transmitters convert these electrical signals to radio waves.

Changing electric currents in the antenna of a transmitter generates the radio waves.

These radio waves radiate out in straight lines from the antenna.

However, radio waves attenuate as they move out from the transmitting antenna.

In a WLAN, a radio signal measured at a distance of just 10 meters (30 feet) from the transmitting antenna would be only 1/100th of its original strength.

Like light, radio waves can be absorbed by some materials and reflected by others. When passing from one material, like air, into another material, like a plaster wall, radio waves are refracted. Radio waves are also scattered and absorbed by water droplets in the air.


Modulation Schemes

How does the bit stream become an electromagnetic wave ?

The purpose of a radio is to convert a baseband signal (bit stream) into a modulated electromagnetic signal. A modulation scheme is selected that is appropriate for the particular electromagnetic spectrum. For Wireless LANs there are two main issues…•Interference•Multi-path distortion


Types of Modulation


ModulationThe process of altering the carrier signal that will enter the antenna of the transmitter is called modulation.

There are three basic ways in which a radio carrier signal can be modulated.

•Amplitude Modulated (AM) radio stations modulate the height (amplitude) of the carrier signal.

•Frequency Modulated (FM) radio stations modulate the frequency of the carrier signal as determined by the electrical signal from the microphone.

•In WLANs, a third type of modulation called Phase Modulation is used to superimpose the data signal onto the carrier signal that is broadcast by the transmitter. In this type of modulation, the data bits in the electrical signal change the phase of the carrier signal.


Signals and Noise on a WLANNarrowband is the opposite of spread spectrum technology. As the name implies narrowband does not affect the entire frequency spectrum of the wireless signal. One solution to a narrowband interference problem could be simply changing the channel that the AP is using. Actually diagnosing the cause of narrowband interference can be a costly and time-consuming experience. To identify the source requires a spectrum analyzer and even a low cost model is relatively expensive.

All band interference can affects the entire spectrum range. Bluetooth™ technologies hops across the entire 2.4 GHz many times per second and can cause significant interference on an 802.11b network. It is not uncommon to see signs in facilities that use wireless networks requesting that all Bluetooth™ devices be shut down before entering. Leakage from a microwave of as little as one watt into the RF spectrum can cause major network disruption. Wireless phones operating in the 2.4GHZ spectrum can also cause network disorder.


Generally the RF signal will not be affected by even the most extreme weather conditions.

However, fog or very high moisture conditions can and do affect wireless networks.

Lightning can also charge the atmosphere and alter the path of a transmitted signal.

The first and most obvious source of a signal problem is the transmitting station and antenna type. A higher output station will transmit the signal further and a parabolic dish antenna that concentrates the signal will increase the transmission range.

In a SOHO environment most access points will utilize twin omnidirectional antennae that transmit the signal in all directions thereby reducing the range of communication.


Wireless Allocations in CanadaThis color means ‘Fixed Service’

5 Ghz Band 2.4 Ghz Band


5150-5250 MHz, 5250-5350 MHz and 5725-5825 MHz.

The band is shared with some pretty noisy services.


The use of the spectrum in the bands 5150-5250 MHz,5250-5350 MHz and 5725-5825 MHz for LE-LAN devices is on the basis that such devices cannot claim protection from other radio systems and cannot cause harmful interference into other radio services in these bands. (LE-LAN = License Exempt LAN)1 W transmitter output power; a power spectral density of 17 dBm in any 1 MHz band; a maximum 4 W EIRP; and fixed, point-to-point LE-LAN devices operating in this band may employ transmitting antennas with directional gain up to 23 dBi.

The modulation scheme must take this into consideration. Spectrum efficiency is NOT an issue.

Note: Reference Antenna = an antenna measured at Central StatesdBd = dB of Gain over a DipoledBi = dB of Gain over an Isotropic Radiator

EIRP is the equivalent isotropically radiated power. EIRP represents the total effective transmit power of a radio, including gains that the antenna provides and losses from the antenna cable. You must take all of these into account when calculating the EIRP for a specific radio.


The 2.4 Ghz Band is similar.


Then there is the problem of Multi-path distortion interference.

Which will distort a nice clean pulse.


Standard Data RateModulation

SchemePros/Cons

802.11 ≤ 2Mbps 2.4GHz

FHSS or DSSS

This specification has been extended into 802.11b.

802.11a ≤ 54Mbps 5GHz

OFDM

"Wi-Fi Certified." 8 available channels. Less potential for RF interference than 802.11b and 802.11g. Better than 802.11b at supporting multimedia voice, video and large-image applications in densely populated user environments. Relatively shorter range than 802.11b. Not interoperable with 802.11b.

802.11b ≤ 11Mbps 2.4GHz

DSSS with CCK

"Wi-Fi Certified." 14 channels available. Not interoperable with 802.11a. Requires fewer access points than 802.11a for coverage of large areas. High-speed access to data at up to 300 feet from base station.

802.11g ≤ 54Mbps 2.4GHz

OFDM > 20Mbps DSSS + CCK <

20Mbps

"Wi-Fi Certified." 14 channels available. May replace 802.11b. Improved security enhancements over 802.11. Compatible with 802.11b.

Bluetooth

Up to 2Mbps

2.45GHzFHSS

No native support for IP, so it does not support TCP/IP and wireless LAN applications well. Best suited for connecting PDAs, cell phones and PCs in short intervals.


Spread Spectrum modulation schemes ease address problems, each in their own way.

•DSSS Direct Sequence Spread Spectrum

•OFDM Orthogonal Frequency Division Multiplexing

•FHSS Frequency Hopping Spread Spectrum


FHSS vs DSSSWith FHSS, a data packet is first sent on a random channel in the band with the next packet sent, after a pause of a few milliseconds, on another random channel in the band. With 80 channels or more available (one channel per MHz, e.g., 2401, 2402, 2403, etc) signals from multiple radios "hop" around each other. This is how they can operate with other radios in the same band with minimal interference.

DSSS radios operate on a fixed radio channel, but the signal is "spread" on that channel by mixing the signal with a Pseudo-Noise (PN) code. This spreading causes the radio signal with the data on it to occupy a much wider band, and looks more like noise to receivers not designed to "de-spread " that signal.



•In DSSS individual pulses are increased to a much higher frequency by multiplying them with a code that is unique to

each WLAN. All the stations know the code.

The result is a string of chips.



DSSS has good interference rejection.


OFDMOrthogonal Frequency Division Multiplexing (OFDM) is a special form of multi-carrier modulation, initially proposed in the 1970s. It is particularly suited for transmission over a dispersive (i.e., frequency selective) channel.

In a multipath channel, most conventional modulation techniques are sensitive to interference. OFDM is significantly less sensitive to interference, because a special set of signals is used to build the composite transmitted signal. The basic idea is that each bit occupies a frequency-time window which ensures little or no distortion of the waveform. In practice, it means that bits are transmitted in parallel over a number of frequency-nonselective channels. Applications of OFDM are found in

•Digital Audio Broadcasting (DAB) and

•Digital Video Broadcasting over the terrestrial network: Digital Terrestrial Television Broadcasting (DTTB). In the DTTB OFDM transmission standard, about 2,000 to 8,000 subcarriers are used.

•Wireless LANs, 802.11a and 802.11g

http://www.wireless.per.nl:202/telelearn/index.htm


OFDM Orthogonal Frequency Division Multiplexing

In OFDM, a serial bit stream of 10 bits are converted into 10 parallel bits, each of which modulates its own radio carrier. Each carrier is now carrying a bit rate that is 1/10th the bit rate of the original. A reflected signal path needs to be 10 times longer to cause the same interference. Longer paths are more attenuated so the strength of the interference is also less.

Direct signal.

Original reflected signal.

Longer reflected signal.


OFDM has good multi-path rejection.

OFDM Orthogonal Frequency Division Multiplexing.

Where does orthogonal come from ?If the individual Radio Carriers are separated by exactly the bit rate…

…then they will always be zero at the adjacent carrier frequency and there will be no interference between

them.


FHSS Frequency Hopping Spread Spectrum

FHSS also uses many frequencies, but only one at a time. The baseband jumps around very rapidly from one frequency to the next according to a predetermined pattern that is unique to each WLAN. Any interfering signal strong enough to cause an error will only affect the particular packet on that frequency. The transport layer would just resend that packet.


WLAN authentication occurs at Layer 2. It is the process of authenticating the device not the user. There three states….

Layer 2


Unauthenticated and unassociated The node is disconnected from the network and not associated to an access point.

Authenticated and unassociated The node has been authenticated on the network but has not yet associated with the access point.

Authenticated and associated The node is connected to the network and able to transmit and receive data through the access point.

Layer 2 - Authentication


Service Set Identifier (SSID)

Active scanning causes a probe request to be sent from the wireless node seeking to join the network. The probe request will contain the Service Set Identifier (SSID) of the network it wishes to join. When an AP with the same SSID is found, the AP will issue a probe response.


Service Set Identifier (SSID)

Passive scanning nodes listen for beacon management frames (beacons), which are transmitted by the AP (infrastructure mode) or peer nodes (ad hoc). When a node receives a beacon that contains the SSID of the network it is trying to join, an attempt is made to join the network. Passive scanning is a continuous process and nodes may associate or disassociate with APs as signal strength changes.


Open Authentication. This is an open connectivity standard in which only the SSID must match. This may be used in a secure or non-secure environment although the ability of low level network ‘sniffers’ to discover the SSID of the WLAN is high. Shared Key. This process requires the use of Wireless Equivalency Protocol (WEP) encryption. WEP is a fairly simple algorithm using 64 and 128 bit keys. The AP is configured with an encrypted key and nodes attempting to access the network through the AP must have a matching key. Statically assigned WEP keys provide a higher level of security than the open system but are definitely not hack proof.

Wi-Fi Protected Access (WPA).

Subset of 802.11i and backward compatible. Now mandatory for certification. Usually requires a server.

Authentication


The payload of wireless and 802.3 frames is 1500 bytes; however, an Ether frame may not exceed 1518 bytes whereas a wireless frame could be as large as 2346 bytes. Usually the WLAN frame size will be limited to 1518 bytes as it is most commonly connected to a wired Ethernet network.


The basic access method for 802.11 is the Distributed Coordination Function (DCF) which uses Carrier Sense Multiple Access / Collision Avoidance (CSMA / CA). This requires each station to listen for other users. If the channel is idle, the station may transmit. However if it is busy, each station waits until transmission stops, and then enters into a random back off procedure. This prevents multiple stations from seizing the medium immediately after completion of the preceding transmission.

SIFS Short Inter-Frame SpacingPIFS PCF Inter-Frame Spacing = SIFS + slot timeDIFS DCF Inter-Frame Spacing = SIFS + 2*slot time

– 61 % OH @ 54 Mbps link sending 512 byte data => ~ 21

Mbps useful..

WLAN Access


1. The station listens before it sends. 2. If someone is already transmitting, wait for a

random period and try again (as normal). 3. If no one is transmitting then it sends a short

message. This message is called the Ready To Send message (RTS).

4. This message contains the destination address and the duration of the transmission. Other stations now know that they must wait that long before they can transmit.

5. The destination then sends a short message which is the Clear To Send message (CTS). This message tells the source that it can send without fear of collisions.

6. Each packet is acknowledged. If an acknowledgement is not received, the MAC layer retransmits the data. This entire sequence is called the 4-way handshake as shown by figure 7 below.

The hidden node problem

– 72 % OH @ 54 Mbps link sending 512 byte data => ~ 15 Mbps useful.


VPN - Using an integrated server VPN technology creates a tunnel on top of an existing protocol such as IP. This is a Layer 3 connection as opposed to the Layer 2 connection between the AP and the sending node.

EAP-MD5 Challenge – Extensible Authentication Protocol is the earliest authentication type, which is very similar to CHAP password protection on a wired network.

LEAP (Cisco) – Lightweight Extensible Authentication Protocol is the type primarily used on Cisco WLAN access points. LEAP provides security during credential exchange, encrypts using dynamic WEP keys, and supports mutual authentication.

User authentication – Allows only authorized users to connect, send and receive data over the wireless network.

Encryption – Provides encryption services further protecting the data from intruders.

Data authentication – Ensures the integrity of the data, authenticating source and destination devices.

Power over Ethernet - Support staff can disable a PoE-enabled access point by shutting off its power after detecting a breach of security.


Wireless Security – Home users1.Change the default SSID (network name). 2.Disable the SSID broadcast option. 3.Change the default password needed to access a wireless device. 4.Enable MAC address filtering.

SSID (service set identifier) a 32-character unique identifier attached to the header of packets sent over a WLAN that acts as a password when a mobile device tries to connect to the BSS. The SSID differentiates one WLAN from another, so all access points and all devices attempting to connect to a specific WLAN must use the same SSID. A device will not be permitted to join the BSS unless it can provide the unique SSID. Because an SSID can be sniffed in plain text from a packet it does not supply any security to the network. An SSID is also referred to as a network name because essentially it is a name that identifies a wireless network.


END


•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.

CCK

Sep-03 ©Cisco SystemsCCNA Semester 1 Version 3 Comp11 Mod2 – St. Lawrence College – Cornwall...

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Transcript of Sep-03 ©Cisco SystemsCCNA Semester 1 Version 3 Comp11 Mod2 – St. Lawrence College – Cornwall...