Wireless Tutorial (CCNA)

In this article we will discuss about Wireless technologies mentioned in CCNA.
Wireless LAN (WLAN) is very popular nowadays. Maybe you have ever used some wireless applications on your laptop or cellphone. Wireless LANs enable users to communicate without the need of cable. Below is an example of a simple WLAN:

Each WLAN network needs a wireless Access Point (AP) to transmit and receive data from users. Unlike a wired network which operates at full-duplex (send and receive at the same time), a wireless network operates at half-duplex so sometimes an AP is referred as a Wireless Hub.

The major difference between wired LAN and WLAN is WLAN transmits data by radiating energy waves, called radio waves, instead of transmitting electrical signals over a cable.
Also, WLAN uses CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) instead of CSMA/CD for media access. WLAN can’t use CSMA/CD as a sending device can’t transmit and receive data at the same time. CSMA/CA operates as follows:
+ Listen to ensure the media is free. If it is free, set a random time before sending data
+ When the random time has passed, listen again. If the media is free, send the data. If not, set another random time again
+ Wait for an acknowledgment that data has been sent successfully
+ If no acknowledgment is received, resend the data

IEEE 802.11 standards:

Nowadays there are three organizations influencing WLAN standards. They are:
+ ITU-R: is responsible for allocation of the RF bands
+ IEEE: specifies how RF is modulated to transfer data
+ Wi-Fi Alliance: improves the interoperability of wireless products among vendors
But the most popular type of wireless LAN today is based on the IEEE 802.11 standard, which is known informally as Wi-Fi.
* 802.11a: operates in the 5.7 GHz ISM band. Maximum transmission speed is 54Mbps and approximate wireless range is 25-75 feet indoors.
* 802.11b: operates in the 2.4 GHz ISM band. Maximum transmission speed is 11Mbps and approximate wireless range is 100-200 feet indoors.
* 802/11g: operates in the 2.4 GHz ISM band. Maximum transmission speed is 54Mbps and approximate wireless range is 100-200 feet indoors.
ISM Band: The ISM (Industrial, Scientific and Medical) band, which is controlled by the FCC in the US, generally requires licensing for various spectrum use. To accommodate wireless LAN’s, the FCC has set aside bandwidth for unlicensed use including the 2.4Ghz spectrum where many WLAN products operate.
Wi-Fi: stands for Wireless Fidelity and is used to define any of the IEEE 802.11 wireless standards. The term Wi-Fi was created by the Wireless Ethernet Compatibility Alliance (WECA). Products certified as Wi-Fi compliant are interoperable with each other even if they are made by different manufacturers.
Access points can support several or all of the three most popular IEEE WLAN standards including 802.11a, 802.11b and 802.11g.

WLAN Modes:

WLAN has two basic modes of operation:
* Ad-hoc mode: In this mode devices send data directly to each other without an AP.

* Infrastructure mode: Connect to a wired LAN, supports two modes (service sets):
+ Basic Service Set (BSS): uses only a single AP to create a WLAN
+ Extended Service Set (ESS): uses more than one AP to create a WLAN, allows roaming in a larger area than a single AP. Usually there is an overlapped area between two APs to support roaming. The overlapped area should be more than 10% (from 10% to 15%) to allow users moving between two APs without losing their connections (called roaming). The two adjacent APs should use non-overlapping channels to avoid interference. The most popular non-overlapping channels are channels 1, 6 and 11 (will be explained later).

Roaming: The ability to use a wireless device and be able to move from one access point’s range to another without losing the connection.
When configuring ESS, each of the APs should be configured with the same Service Set Identifier (SSID) to support roaming function. SSID is the unique name shared among all devices on the same wireless network. In public places, SSID is set on the AP and broadcasts to all the wireless devices in range. SSIDs are case sensitive text strings and have a maximum length of 32 characters. SSID is also the minimum requirement for a WLAN to operate. In most Linksys APs (a product of Cisco), the default SSID is “linksys”.
In the next part we will discuss about Wireless Encoding, popular Wireless Security Standard and some sources of wireless interference.

Wireless Encoding

When a wireless device sends data, there are some ways to encode the radio signal including frequency, amplitude & phase.
Frequency Hopping Spread Spectrum(FHSS): uses all frequencies in the band, hopping to different ones after fixed time intervals. Of course the next frequency must be predetermined by the transmitter and receiver.

The main idea of this method is signals sent on different frequencies will be received at different levels of quality. By hopping to different frequencies, signals will be greatly improved the possibility that most of it will get through. For example, suppose there is another device using the 150-250 kHz range. If our device transmits in this range then the signals will be significantly interfered. By hopping at different frequencies, there is only a small interference while transmitting and it is acceptable.
Direct Sequence Spread Spectrum (DSSS): This method transmits the signal over a wider frequency band than required by multiplying the original user data with a pseudo random spreading code. The result is a wide-band signal which is very “durable” to noise. Even some bits in this signal are damaged during transmission, some statistical techniques can recover the original data without the need for retransmission.
Note: Spread spectrum here means the bandwidth used to transfer data is much wider than the bandwidth needs to transfer that data.
Traditional communication systems use narrowband signal to transfer data because the required bandwidth is minimum but the signal must have high power to cope with noise. Spread Spectrum does the opposite way when transmitting the signal with much lower power level (can transmit below the noise level) but with much wider bandwidth. Even if the noise affects some parts of the signal, the receiver can easily recover the original data with some algorithms.

Now you understand the basic concept of DSSS. Let’s discuss about the use of DSS in the 2.4 GHz unlicensed band.
The 2.4 GHz band has a bandwidth of 82 MHz, with a range from 2.402 GHz to 2.483 GHz. In the USA, this band has 11 different overlapping DSSS channels while in some other countries it can have up to 14 channels. Channels 1, 6 and 11 have least interference with each other so they are preferred over other channels.

Orthogonal Division Multiplexing (OFDM): encodes a single transmission into multiple sub-carriers to save bandwidth. OFDM selects channels that overlap but do not interfere with each other by selecting the frequencies of the subcarriers so that at each subcarrier frequency, all other subcarriers do not contribute to overall waveform.
In the picture below, notice that only the peaks of each subcarrier carry data. At the peak of each of the subcarriers, the other two subcarriers have zero amplitude.

Below is a summary of the encoding classes which are used popularly in WLAN.

Encoding	Used by
FHSS	The original 802.11 WLAN standards used FHSS, but the current standards (802.11a, 802.11b, and 802.11g) do not
DSSS	802.11b
OFDM	802.11a, 802.11g, 802.11n

WLAN Security Standards

Security is one of the most concerns of people deploying a WLAN so we should grasp them.
Wired Equivalent Privacy (WEP)
WEP is the original security protocol defined in the 802.11b standard so it is very weak comparing to newer security protocols nowadays.
WEP is based on the RC4 encryption algorithm, with a secret key of 40 bits or 104 bits being combined with a 24-bit Initialisation Vector (IV) to encrypt the data (so sometimes you will hear “64-bit” or “128-bit” WEP key). But RC4 in WEP has been found to have weak keys and can be cracked easily within minutes so it is not popular nowadays.
The weak points of WEP is the IV is too small and the secret key is static (the same key is used for both encryption and decryption in the whole communication and never expires).
Wi-Fi Protected Access (WPA)
In 2003, the Wi-Fi Alliance developed WPA to address WEP’s weaknesses. Perhaps one of the most important improvements of WPA is the Temporal Key Integrity Protocol (TKIP) encryption, which changes the encryption key dynamically for each data transmission. While still utilizing RC4 encryption, TKIP utilizes a temporal encryption key that is regularly renewed, making it more difficult for a key to be stolen. In addition, data integrity was improved through the use of the more robust hashing mechanism, the Michael Message Integrity Check (MMIC).
In general, WPA still uses RC4 encryption which is considered an insecure algorithm so many people viewed WPA as a temporary solution for a new security standard to be released (WPA2).
Wi-Fi Protected Access 2 (WPA2)
In 2004, the Wi-Fi Alliance updated the WPA specification by replacing the RC4 encryption algorithm with Advanced Encryption Standard-Counter with CBC-MAC (AES-CCMP), calling the new standard WPA2. AES is much stronger than the RC4 encryption but it requires modern hardware.

Standard	Key Distribution	Encryption
WEP	Static Pre-Shared	Weak
WPA	Dynamic	TKIP
WPA2	Both (Static & Dynamic)	AES

Wireless Interference

The 2.4 GHz & 5 GHz spectrum bands are unlicensed so many applications and devices operate on it, which cause interference. Below is a quick view of the devices operating in these bands:
+ Cordless phones: operate on 3 frequencies, 900 MHz, 2.4 GHz, and 5 GHz. As you can realize, 2.4 GHz and 5 GHz are the frequency bands of 802.11b/g and 802.11a wireless LANs.
Most of the cordless phones nowadays operate in 2.4 GHz band and they use frequency hopping spread spectrum (FHSS) technology. As explained above, FHSS uses all frequencies in the the entire 2.4 GHz spectrum while 802.11b/g uses DSSS which operates in about 1/3 of the 2.4 GHz band (1 channel) so the use of the cordless phones can cause significant interference to your WLAN.

An example of cordless phone

+ Bluetooth: same as cordless phone, Bluetooth devices also operate in the 2.4 GHz band with FHSS technology. Fortunately, Bluetooth does not cause as much trouble as cordless phone because it usually transfers data in a short time (for example you copy some files from your laptop to your cellphone via Bluetooth) within short range. Moreover, from version 1.2 Bluetooth defined the adaptive frequency hopping (AFH) algorithm. This algorithm allows Bluetooth devices to periodically listen and mark channels as good, bad, or unknown so it helps reduce the interference with our WLAN.
+ Microwaves (mostly from oven): do not transmit data but emit high RF power and heating energy. The magnetron tubes used in the microwave ovens radiate a continuous-wave-like at frequencies close to 2.45 GHz (the center burst frequency is around 2.45 – 2.46 GHz) so they can interfere with the WLAN.
+ Antenna: There are a number of 2.4 GHz antennas on the market today so they can interfere with your wireless network.
+ Metal materials or materials that conduct electricity deflect Wi-Fi signals and create blind spots in your coverage. Some of examples are metal siding and decorative metal plates.
+ Game controller, Digital Video Monitor, Wireless Video Camera, Wireless USB may also operate at 2.4 GHz and cause interference too.

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How to Configure WDS Bridging

uitable for: N750 Wireless Dual Band Routers, N600 Wireless Dual Band Routers

A Wireless Distribution System (WDS) is a system that enables the wireless interconnection of access points in an IEEE 802.11 network. It allows a wireless network to be expanded using multiple access points without the need for a wired backbone to link them, as is traditionally required. For more information about WDS, . The following chart is an example of WDS bridging.

 

Note:  

1.     LAN IP of extended router should be different but in the same subnet of the root router;

2.     The DHCP Server on extended router should be disabled;

3.     WDS bridging only requires the WDS setting on either the root router or the extended router, on either 2.4GHz or 5GHz; NO need to setup on both side or both band.

 

Please follow this guide to setup the WDS bridging:

Step 1:

Open the web browser and type in http://tplinklogin.net or the IP of the router (Default it is 192.168.0.1 / 192.168.1.1) to login the Web Management Page of the extended router. The username and password are both "admin". If you are not sure about how to do this, please click here.

 

Step 2:

Go to Wireless 2.4GHz -> Wireless Settings. Check Enable WDS Bridging. Then the page will show like below.

Note: Here we use 2.4GHz wireless network as an example, if you want to WDS bridging the 5GHz wireless network, please go to Wireless 5GHz accordingly.

 

Step 3:

Click Survey. On the pop-up window, find the SSID of your root router and click Connect.

 

Step 4:

The root router’s SSID and BSSID have been filled automatically. Then please configure the security settings (Key type and Password) to match the ones on the root router.

 

Click Save button,  then a window will pop out to inform you to change the channel. Click OK, it will change automatically. Please do NOT reboot the router.

 

Step 5:

Go to DHCP-> DHCP Settings page. Select Disable DHCP Server, and click Save button but do NOT reboot the router.

 

Step 6:

Go to Network -> LAN and change the LAN IP to a different IP address but still in the same subnet as the root router. Save the setting and reboot the extended router.

 

Step 7:

All settings required by WDS function is completed. You can make a simple check by the Ping utility. If the ping proceeds successfully (which means the WDS performs properly), you will see the similar screen as below: (in this example, 192.168.0.1 is the IP address of the root router)

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CCNA: Cisco IOS An Introduction

Cisco IOS (short for Internetwork Operating System) is the software used on a majority of Cisco Systems routers and switches.  IOS consists of routing, switching, internetworking and telecommunications functions in a multitasking operating system. Cisco IOS has uses command line interface (CLI), and provides a fixed set of multiple-word commands. A Cisco IOS command line interface can be accessed through either a console connection, modem connection, or a telnet session. The set of commands available at any particular level is determined by the "mode" and the privilege level of the current user. Cisco IOS follows a command hierarchy, with each level offering different set of commands  All commands are assigned a privilege level, from 0 to 15, and can only be accessed by users with the necessary privilege. Through the CLI, the commands available to each privilege level can be defined.

Some of the widely used command hierarchy levels are given below:
User EXEC level: This is the level that a connected user is allowed initially. User EXEC allows access to a limited set of basic monitoring commands. A ">" sign denotes User EXEC mode.
Privileged EXEC level: Privileged EXEC level allows access to all router commands including router configuration and management commands. This level is usually password protected for security reasons. A "#"sign denotes privileged EXEC mode.
When a user is connected to a Cisco IOS, a User EXEC prompt appears. Now, the user can enter privileged EXEC mode by typing the password shown as below:
Router> enable
Password: [enable password]
Router# configure terminal
Router(config)#
Global configuration mode: "Global configuration mode" provides commands to change the system's configuration. This is typically represented by "(config)#" sign as shown in the above example.
Interface configuration mode: "Interface configuration mode" provides commands to change the configuration of a specific interface of the router.  An interface configuration mode is denoted by "(config-in)#".
A summary of Cisco IOS router command prompt is given below:

Prompt	Explanation
Router>	User EXEC mode
Router#	Privileged EXEC mode
Router(config)#	Global configuration mode. # sign indicates this is only accessible at privileged EXEC mode.
Router(config-if)#	Interface level configuration mode.
Router(config-router)#	Routing engine level within configuration mode.
Router(config-line)#	Line level (vty, tty, async) within configuration mode.

Context Sensitive Help

Cisco IOS CLI offers context sensitive help. At any time during an EXEC session, a user can type a question mark (?) to get help.
Two types of context sensitive help are available:

• Word help and
• Command syntax help.

Word help: Word help can be used to obtain a list of commands that begin with a given character string. To use word help, type in the characters in question followed immediately by the question mark (?).  The following is an example of word help:

Router# co?
configure connect copy

Command syntax help: Command syntax help can be used to obtain a list of commands, keyword, or argument options that are available starting with the keywords that the user had already entered. To use command syntax help, enter a question mark (?) after hitting a space.  The router will then display a list of available command options with <cr> standing for carriage return. The following is an example of command syntax help:

Router# configure ?
memory Configure from NV memory
network Configure from a TFTP network host
terminal Configure from the terminal
<cr>

Cisco IOS also allows abbreviated commands support. For example, consider the following:

Router#configure terminal
Router(config)#
Router#config term
Router(config)#

Both the above commands to the same job. The IOS correctly interprets the full command words. However, if there is any ambiguity, an error message is generated as below:

Router(config)#c
% Ambiguous command: "c"

Checkout a ccna router simulator available from certexams.com.
Example Question: 
What is the command used to add a banner to a Cisco router configuration?
A. add banner
B. banner motd #
C. motd banner #
D. add banner #

Correct answer: B
Explanation:
The banner is displayed whenever anyone logs in to your Cisco router. The syntax is
"banner motd # 
MOTD stands for "Message Of The Day".
# symbol signifies the start of the banner message to the router. You will be prompted for the
message to be displayed. You need to enter "#" symbol at the end of the message, signifying
that the msg has ended.
Alternatively, you can enter the banner in the same line as below:
"banner motd # your message here#

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CCNA: Routing Fundamentals

When IP packets travel over the Internet, routing information is exchanged between the devices that control the flow of information over the Internet. These devices are known as routers, and they use the IP address as the basis for controlling the traffic. These devices need to talk the same language to function properly, though they belong to different administrative domains. For example, one router may be in Newyork(US), and the receiving router may be in London (UK). It is necessary that a routing protocol is followed for smooth flow of traffic. Given below are the widely used routing protocols for routing Internet traffic: RIP v1 RIP v2 OSPF IGRP EIGRP BGP Notations used: Routing Information Protocol (RIP), Open Shortest Path First (OSPF), Interior Gateway Routing Protocol (IGRP), Enhanced Interior Gateway Routing Protocol (EIGRP), and Border Gateway Protocol (BGP).

One often get confused between a routing protocol and a routed protocol. A routing protocol such as RIP is used to route information packets over the Internet, where as a routed protocol such as IP (or IPX) is the payload (contains data) that get routed from source to the destination.
Routing protocols are primarily distinguished into three types:

• Distance Vector Protocols
• Link State Protocols
• Hybrid Protocols

RIP is an example of distance vector protocol. IS-IS is an example of Hybrid protocol, and OSPF is an example of Link State Protocol.
The table below provides the routing protocol used with different routed protocols:

Routing Protocol	Routed Protocol
RIP, OSPF,IS-IS, BGP,EIGRP	IP
RIP, NLSP, EIGRP	IPX
RTMP, EIGRP	AppleTalk

The list of routed, and routing protocols given in the above table is not complete, and given to serve as an example only.
Routing Metric: This is a fundamental measure that routing protocols use for determining appropriate route to deliver packets. Each routing protocol uses its own measure of metric, and a sample of routing metrics used by different routing protocols is given below:

Routing Protocol	Metric
RIPv2	Hop count
EIGRP	Bandwidth, Delay, Load, Reliability, and MTU
OSPF	Cost (Higher bandwidth indicates lower cost)
IS-IS	Cost

The best route in RIP is determined by counting the number of hops required to reach the destination. A lower hop count route is always preferred over a higher hop count route. One disadvantage of using hop count as metric is that if there is a route with one additional hop, but with significantly higher bandwidth, the route with smaller bandwidth is taken. This is illustrated in the figure below:

The RIP routed packets take the path through 56KBPS link since the destination can be reached in one hop. Though, the alternative provides a minimum bandwidth of 1MBPS (though using two links of 1MBPS, and 2MBPS each), it represents 2 hops and not preferred by the RIP protocol.

Link State vs. Distance Vector

Distance Vector routing protocols usually send their entire routing table to their nearest neighbors at regular intervals. A router that receives several such routing tables filter the routes and arrive at its own and retransmits it to its neighbouring routers. There will some period of time where different routers hold non-optimized routes initially. After some time, known as convergence time, a final routing table is arrived at by all the routers. A faster convergence time results in a stable network. 
RIP, as mentioned earlier uses hop count as the metric for computing a route to a given destination. Other Distance Vector routing protocols, such as IGRP, improve on this by using hop count, bandwidth, current load, cost, and reliability to determine the best path.
Link State routing protocols usually send only the routing changes to every other router within their area. Unlike Distance Vector, routers using Link State routing protocols maintain a picture of the entire network. A router can use this network wide information to determine the best route for traffic.
Example Question:
What is true about IP routing?
A. The frame changes at each hop
B. The source IP address changes at each hop
C. The destination IP address changes at each hop
D. The hardware interface addresses remain constant

Correct answer: A
Explanation:
IP Packets are transported from source network to the destination network by what is known as routing. Hop-by-hop routing model is used by the Internet for delivery of packets. At each hop, the destination IP address is examined, the best next hop is determined by the routing protocol (such as RIP, OSPF or BGP) and the packet is forwarded by one more hop through this route. The same process takes place at the next hop. During this process, the logical addresses remain same. In an IP network, the logical addresses are IP addresses. The hardware interface addresses, such as MAC address change with each hop.

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