Wireless 101 - Part 2

Antenna

An antenna is a device to transmit and/or receive electromagnetic waves. Electromagnetic waves are often referred to as radio waves. Most antennas are resonant devices, which operate efficiently over a relatively narrow frequency band. An antenna must be tuned (matched) to the same frequency band as the radio system to which it is connected otherwise reception and/or transmission will be impaired.

Types of antenna

There are 3 types of antennas used with mobile wireless, omnidirectional, dish and panel antennas.
+ Omnidirectional radiate equally in all directions
+ Dishes are very directional
+ Panels are not as directional as Dishes.



Decibels

Decibels (dB) are the accepted method of describing a gain or loss relationship in a communication system. If a level is stated in decibels, then it is comparing a current signal level to a previous level or preset standard level. The beauty of dB is they may be added and subtracted. A decibel relationship (for power) is calculated using the following formula:

“A” might be the power applied to the connector on an antenna, the input terminal of an amplifier or one end of a transmission line. “B” might be the power arriving at the opposite end of the transmission line, the amplifier output or the peak power in the main lobe of radiated energy from an antenna. If “A” is larger than “B”, the result will be a positive number or gain. If “A” is smaller than “B”, the result will be a negative number or loss.

You will notice that the “B” is capitalized in dB. This is because it refers to the last name of Alexander Graham Bell.

Note:

+ dBi is a measure of the increase in signal (gain) by your antenna compared to the hypothetical isotropic antenna (which uniformly distributes energy in all directions) -> It is a ratio. The greater the dBi value, the higher the gain and the more acute the angle of coverage.

+ dBm is a measure of signal power. It is the the power ratio in decibel (dB) of the measured power referenced to one milliwatt (mW). The “m” stands for “milliwatt”.

Example:

At 1700 MHz, 1/4 of the power applied to one end of a coax cable arrives at the other end. What is the cable loss in dB?

Solution:

=> Loss = 10 * (- 0.602) = – 6.02 dB

From the formula above we can calculate at 3 dB the power is reduced by half. Loss = 10 * log (1/2) = -3 dB; this is an important number to remember.

Beamwidth

The angle, in degrees, between the two half-power points (-3 dB) of an antenna beam, where more than 90% of the energy is radiated.

OFDM

OFDM was proposed in the late 1960s, and in 1970, US patent was issued. OFDM encodes a single transmission into
multiple sub-carriers. All the slow subchannel are then multiplexed into one fast combined channel.

The trouble with traditional FDM is that the guard bands waste bandwidth and thus reduce capacity. OFDM selects channels that overlap but do not interfere with each other.



OFDM works because the frequencies of the subcarriers are selected so that at each subcarrier frequency, all other subcarriers do not contribute to overall waveform.

In this example, three subcarriers are overlapped but do not interfere with each other. 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.

Types of network in CCNA Wireless

+ A LAN (local area network) is a data communications network that typically connects personal computers within a very limited geographical (usually within a single building). LANs use a variety of wired and wireless technologies, standards and protocols. School computer labs and home networks are examples of LANs.

+ A PAN (personal area network) is a term used to refer to the interconnection of personal digital devices within a range of about 30 feet (10 meters) and without the use of wires or cables. For example, a PAN could be used to wirelessly transmit data from a notebook computer to a PDA or portable printer.

+ A MAN (metropolitan area network) is a public high-speed network capable of voice and data transmission within a range of about 50 miles (80 km). Examples of MANs that provide data transport services include local ISPs, cable television companies, and local telephone companies.

+ A WAN (wide area network) covers a large geographical area and typically consists of several smaller networks, which might use different computer platforms and network technologies. The Internet is the world’s largest WAN. Networks for nationwide banks and superstore chains can be classified as WANs.

Bluetooth

Bluetooth wireless technology is a short-range communications technology intended to replace the cables connecting portable and/or fixed devices while maintaining high levels of security. Connections between Bluetooth devices allow these devices to communicate wirelessly through short-range, ad hoc networks. Bluetooth operates in the 2.4 GHz unlicensed ISM band.

Note:

Industrial, scientific and medical (ISM) band is a part of the radio spectrum that can be used by anybody without a license in most countries. In the U.S, the 902-928 MHz, 2.4 GHz and 5.7-5.8 GHz bands were initially used for machines that emitted radio frequencies, such as RF welders, industrial heaters and microwave ovens, but not for radio communications. In 1985, the FCC Rules opened up the ISM bands for wireless LANs and mobile communications. Nowadays, numerous applications use this band, including cordless phones, wireless garage door openers, wireless microphones, vehicle tracking, amateur radio…

WiMAX

Worldwide Interoperability for Microwave Access (WiMax) is defined by the WiMax forum and standardized by the IEEE 802.16 suite. The most current standard is 802.16e.

Operates in two separate frequency bands, 2-11 GHz and 10-66 GHz
At the higher frequencies, line of sight (LOS) is required – point-to-point links only
In the lower region, the signals propagate without the requirement for line of sight (NLOS) to customers

Basic Service Set (BSS)

A group of stations that share an access point are said to be part of one BSS.

Extended Service Set (ESS)

Some WLANs are large enough to require multiple access points. A group of access points connected to the same WLAN are known as an ESS. Within an ESS, a client can associate with any one of many access points that use the same Extended service set identifier (ESSID). That allows users to roam about an office without losing wireless connection.

IEEE 802.11 standard

A family of standards that defines the physical layers (PHY) and the Media Access Control (MAC) layer.

* IEEE 802.11a: 54 Mbps in the 5.7 GHz ISM band
* IEEE 802.11b: 11 Mbps in the 2.4 GHz ISM band
* IEEE 802.11g: 54 Mbps in the 2.4 GHz ISM band
* IEEE 802.11i: security. The IEEE initiated the 802.11i project to overcome the problem of WEP (which has many flaws and it could be exploited easily)
* IEEE 802.11e: QoS
* IEEE 802.11f: Inter Access Point Protocol (IAPP)

More information about 802.11i:

The new security standard, 802.11i, which was ratified in June 2004, fixes all WEP weaknesses. It is divided into three main categories:

1. Temporary Key Integrity Protocol (TKIP) is a short-term solution that fixes all WEP weaknesses. TKIP can be used with old 802.11 equipment (after a driver/firmware upgrade) and provides integrity and confidentiality.
2. Counter Mode with CBC-MAC Protocol (CCMP) [RFC2610] is a new protocol, designed from ground up. It uses AES as its cryptographic algorithm, and, since this is more CPU intensive than RC4 (used in WEP and TKIP), new 802.11 hardware may be required. Some drivers can implement CCMP in software. CCMP provides integrity and confidentiality.
3. 802.1X Port-Based Network Access Control: Either when using TKIP or CCMP, 802.1X is used for authentication.

Wireless Access Points

There are two categories of Wireless Access Points (WAPs):
* Autonomous WAPs
* Lightweight WAPs (LWAPs)

Autonomous WAPs operate independently, and each contains its own configuration file and security policy. Autonomous WAPs suffer from scalability issues in enterprise environments, as a large number of independent WAPs can quickly become difficult to manage.

Lightweight WAPs (LWAPs) are centrally controlled using one or more Wireless LAN Controllers (WLCs), providing a more scalable solution than Autonomous WAPs.

Encryption

Encryption is the process of changing data into a form that can be read only by the intended receiver. To decipher the message, the receiver of the encrypted data must have the proper decryption key (password).

TKIP

TKIP stands for Temporal Key Integrity Protocol. It is basically a patch for the weakness found in WEP. The problem with the original WEP is that an attacker could recover your key after observing a relatively small amount of your traffic. TKIP addresses that problem by automatically negotiating a new key every few minutes — effectively never giving an attacker enough data to break a key. Both WEP and WPA-TKIP use the RC4 stream cipher.

TKIP Session Key

* Different for every pair
* Different for every station
* Generated for each session
* Derived from a “seed” called the passphrase

AES

AES stands for Advanced Encryption Standard and is a totally separate cipher system. It is a 128-bit, 192-bit, or 256-bit block cipher and is considered the gold standard of encryption systems today. AES takes more computing power to run so small devices like Nintendo DS don’t have it, but is the most secure option you can pick for your wireless network.

EAP

Extensible Authentication Protocol (EAP) [RFC 3748] is just the transport protocol optimized for authentication, not the authentication method itself:

” EAP is an authentication framework which supports multiple authentication methods. EAP typically runs directly over data link layers such as Point-to-Point Protocol (PPP) or IEEE 802, without requiring IP. EAP provides its own support for duplicate elimination and retransmission, but is reliant on lower layer ordering guarantees. Fragmentation is not supported within EAP itself; however, individual EAP methods may support this.” — RFC 3748, page 3

Some of the most-used EAP authentication mechanism are listed below:

* EAP-MD5: MD5-Challenge requires username/password, and is equivalent to the PPP CHAP protocol [RFC1994]. This method does not provide dictionary attack resistance, mutual authentication, or key derivation, and has therefore little use in a wireless authentication enviroment.
* Lightweight EAP (LEAP): A username/password combination is sent to a Authentication Server (RADIUS) for authentication. Leap is a proprietary protocol developed by Cisco, and is not considered secure. Cisco is phasing out LEAP in favor of PEAP.
* EAP-TLS: Creates a TLS session within EAP, between the Supplicant and the Authentication Server. Both the server and the client(s) need a valid (x509) certificate, and therefore a PKI. This method provides authentication both ways.
* EAP-TTLS: Sets up a encrypted TLS-tunnel for safe transport of authentication data. Within the TLS tunnel, (any) other authentication methods may be used. Developed by Funk Software and Meetinghouse, and is currently an IETF draft.
*EAP-FAST: Provides a way to ensure the same level of security as EAP-TLS, but without the need to manage certificates on the client or server side. To achieve this, the same AAA server on which the authentication will occur generates the client credential, called the Protected Access Credential (PAC).
* Protected EAP (PEAP): Uses, as EAP-TTLS, an encrypted TLS-tunnel. Supplicant certificates for both EAP-TTLS and EAP-PEAP are optional, but server (AS) certificates are required. Developed by Microsoft, Cisco, and RSA Security, and is currently an IETF draft.
* EAP-MSCHAPv2: Requires username/password, and is basically an EAP encapsulation of MS-CHAP-v2 [RFC2759]. Usually used inside of a PEAP-encrypted tunnel. Developed by Microsoft, and is currently an IETF draft.

RADIUS

Remote Authentication Dial-In User Service (RADIUS) is defined in [RFC2865] (with friends), and was primarily used by ISPs who authenticated username and password before the user got authorized to use the ISP’s network.

802.1X does not specify what kind of back-end authentication server must be present, but RADIUS is the “de-facto” back-end authentication server used in 802.1X.

Roaming

Roaming is the movement of a client from one AP to another while still transmitting. Roaming can be done across different mobility groups, but must remain inside the same mobility domain. There are 2 types of roaming:

A client roaming from AP1 to AP2. These two APs are in the same mobility group and mobility domain

Roaming in the same Mobility Group

A client roaming from AP1 to AP2. These two APs are in different mobility groups but in the same mobility domain