RF Principle in Wireless Network

RF Principle in Wireless Network

  • Wireless network use RF signals
  • RF is electromagnetic waves
  • Spectrum defines wave’s size, grouped by categories
  • Wireless N/W radio range is in the microwave segment

RF Signals



  • The signal generated in the transmitter is sent to the antenna
  • The elections movement generates an electric field which is a wave
  • The size of the cycle pattern is called wavelength

Radio waves repeat their pattern over time (at a given point in space), but also over space. The physical distance from one point of the cycle to the same point in the next cycle is called a wavelength, which is usually represented by the Greek symbol λ (lambda). The wavelength is the physical distance covered by the wave in one cycle.



  • The frequency determines how often a signal is seen
  • 1 cycle per second in 1 Hertz
  • Low frequency travel farther in the air than higher frequencies



  • Amplitude is the vertical distance, or height, between crests
  • For the same wavelength and frequency, different amplitude can exists
  • It represents the quality of energy injected in the signal
  • This value is usually regulated as it can affect the receiver

Free Path Loss

Free Path Loss

  • As the wave spreads away from the emitter, it gets weaker
  • The quantity of energy declines as the distance increases: the available quantity of energy available on each point of the circle is less as the circle is larger; the receiver catches only part of this energy
  • Determining a range is determining the energy loss depending on the distance



  • Absorption takes energy from the wave
  • This energy is dissipated in heat in the obstacle
  • When 100% of the energy is taken, the wave stops
  • The effect of absorption is to reduce amplitude
  • The signal is therefore less powerful, but keeps the same wavelength and frequency



  • Part of the energy is reflected
  • Part may be transmitted
  • The angle of reflection is the same as the initial angle
  • Reflection depends on the material roughness relative to the wavelength and the angle
  • Amplitude has no impact



  • Occurs when the signal reflects on surface and arrives to the receiver at different times
  • Delayed multiple copies of the same signal hit the receiver
  • Depends on the wavelength and the position of the receiver

Multipath: Phase


  • 2 signals are in phase when their cycle crests coincide
  • Being out of phase weakens both signal or cancels them if amplitude and wavelength are the same



  • Occurs when micro particle deviate the wave in multiple directions
  • Affects shorter wavelengths more than longer ones
  • Can weaken the signal or block it



  • Occurs when the wave passes from one medium to another: direction change
  • Minor effects on indoor networks
  • Can have high impacts on outdoor long range links

RF Calculation:



  • RSSI is the signal strength indicator
  • dBm value transformed from a vendor dependant grading coefficient
  • Usually negative value, the closer to 0 is better
  • SNR is signal strength relative to noise level
  • The higher the better

The dB scale is also used to compare the relative power (called gain) of antennas.

dBi is the most common scale for antenna gains, but some wireless professionals prefer to use an existing antenna as the reference.

The antenna chosen is the simplest possible antenna, called a dipole antenna. This comparison is expressed in dBd.


dBi = dBd + 2.14

dBd = dBi – 2.14

Covert mW to dBm


With the multiple possible combinations of AP transmitter power level, cables and antenna, you need a way to determine how much energy is actually radiated from the antenna toward the main beam. This measure is called the Effective Isotropic Radiated Power (EIRP). In simple terms, the EIRP, expressed in dBm, is simply the amount of power emitted by the transmitter plus the gain (in dBi) of the antenna (and any amplifier on the path). We also must remove the power lost in cable or attenuators:

EIRP = Tx power (dBm) + antenna gain (dBi) – cable loss (dB)

The EIRP is very important. Most countries allow a maximum Tx power of the transmitter and a final maximum EIRP value. When designing networks with specific antennas, you must know your system EIRP and make sure it complies with local regulations.

Autonomous AP and Migration to CAPWAP

  • Autonomous APs use a firmware/software which offers wireless service without a controller.
  • We can configure/manage each autonomous AP using the CLI via a console port or Telnet/SSH.
  • Default credentials are none/Cisco (means no username is required, and default password is Cisco).
  • Once we connect a Autonomous AP(Without any configuration done from our side) to LAN then it tries to get a IP from DHCP server indefinitely.
  • Autonomous AP is easier to config. from web interface mode
  • Better to configure a static IP to AP via CLI ad then open a web browser to access via web interface.
  • This static IP address is assigned to the AP bridge interface, which is a virtual interface, also shared by all radios and Ethernet interfaces.
  • Basic configuration can be done by express setup but there are many limitation of this setup.
    1. Cannot edit SSIDs (delete SSIDs and re-create them).
    2. Cannot assign SSIDs to specific radio interfaces (default enabled on all radio interfaces).
    3. Cannot configure multiple WEP keys.
    4. Cannot assign an SSID to a VLAN that is already configured on the AP.
    5. Cannot configure combinations of authentication types such as MAC address authentication and EAP authentication on the same SSID.
  • Migration of autonomous APs to CAPWAP by uploading a CAPWAP firmware image to the AP(k9w8 – CAPWAP code for an AP). The conversion can be done several ways:
    1. 1st Way: Use an IOS-to-CAPWAP upgrade tool running on Windows. The autonomous AP must be running Cisco IOS Software version 12.3(7) JA or higher, and the WLC should be running version 3.1 or later.  PC with the Lightweight upgrade tool is also required. In the IOS-to-CAPWAP upgrade tool, we would input a text file (containing each AP to convert IP address, and telnet and privilege mode credentials) and would provide the information needed for the conversion process (controller details, TFTP server IP address, and CAPWAP firmware filename). The tool would then connect to each AP and run the conversion routine.
    2. 2nd Way: We can also convert the AP from the Cisco WCS interface. First add the autonomous AP to the WCS (from Configure > Access Points > Add Autonomous AP). After the autonomous AP is added to WCS, go to Configure > Migration Templates and create a template to convert the autonomous AP to CAPWAP.
    3. 3rd Way: Convert the AP directly from the CLI with the command archive download software ftp|tftp://<address and name of the minimal CAPWAP file to use>.
  • We can convert back the AP from CAPWAP to Cisco IOS(Autonomous)
    1. Download the Cisco IOS firmware image for specific AP. Put this image in a TFTP server (k9w7 – IOS version of the AP code). Then we must associate the CAPWAP AP to the controller, and run, from the controller CLI (migration is not available from the controller web interface), the command config ap tftp-downgrade tftp-server-ip-address filename access-point-name.

Roaming / Mobility Basics

Mobility, or roaming, is a wireless LAN client’s ability to maintain its association seamlessly from one access point to another securely and with as little latency as possible. In This post I will try to elaborate how mobility works when controllers are included in a wireless network.


In wireless networiking , roaming means to the ability to move from one AP coverage area to another without interruption in service or loss in connectivity. This is the key component in wireless network deployment.

Mobility Groups:

A mobility group is a set of Wireless LAN Controllers, by the same mobility group name, that defines of seamless roaming for wireless clients. By creating a mobility group, we can enable multiple WLCs in a network to dynamically share information and forward data traffic when inter-controller or inter-subnet roaming occurs. Controllers in the same mobility group can share the context and state of client devices as well as their list of access points so that they do not consider each other’s access points as rogue devices. With this information, the network can support inter-controller wireless LAN roaming and controller redundancy.

Roaming is the action for a Wireless client to move from one AP to another AP while actively transmitting data without any interruption.

For Voice WLAN: It very common to have roaming.

For Data WLAN: For data deployment, it can be nomadic; user can get slight interruption while moving from one coverage area to another without impacting the user experience.

When a wireless client connects and authenticates to an AP, the AP´s controller (where AP is connected) put an entry for that client in its database, which contains the information of client like: Mac address, IP address, WLAN associate with and the AP where client is connected…etc.

If we have the controller based deployments then Roaming can be of three types.

  1. Intra(All AP on same controller)
  2. Inter or layer 2(Different controller with same network means same subnet)
  3. Layer 3 (Different controller with totally different subnets)


 Intra Controller Roaming:

Intra Controller Roaming

  • Intra controller roaming is the roaming between LAP’s managed by the same WLC, obviously in the same IP subnet.
  • It is necessarily about the subnets that the clients are using that are serviced by the APs.  So, let’s say we have 2 APs, both of which are servicing a specific IP subnet.  When client roam from one AP to the other, which is layer 2 roaming.  The client still maintains its IP address.
  • Wireless Clients move from one AP to another AP in same controller.
  • When client moves its association from one access point to another, the controller simply updates the client database with the newly associated access point.
  • This entry includes the client’s MAC and IP addresses, security context and associations, quality of service (QoS) contexts, the WLAN, and the associated AP.
  • Sometimes it does also establish the new security context means if during roaming a clients session timeout or key change occurs then this information should pass to WLC. If we have open authentication then WLC don’t need to establish or update security Context.

AP: Encryption and Decryption

WLC: Mobility, QoS and Security Management

  • This Roaming process take less then 10ms (It almost seamless).

Lab result and logs will follow soon……………………………………………..

Layer 2 – Inter Controller Roaming:

 L2 - Inter Controller Roaming

Now when we do a layer to roam and multiple controllers are involved, then both controllers need to service the same subnet.  This would mean that we have a dynamic interface in subnet x configured on WLC1 and another interface that is also configured on subnet x on WLC2.  This would be Inter controller roaming (Layer 2).

  •  The wireless user moves from one AP to another AP connected to another controller in the same subnet (as the first controller).
  • It means that the client is roaming between two different controllers, but, these controllers can be part of the same Mobility Group and the same subnet.
  • When the client tries to join the new AP, both controllers exchange the client details (database entry and credentials).
  • The new WLC exchange mobility message with the original WLC and the client entry is moved to new WLC.  This entry includes the client’s MAC and IP addresses, security context and associations, quality of service (QoS) contexts, the WLAN, and the associated AP.
  • Client database entry is updated for the new access point. This process takes less then 20ms and remains transparent to the user.

Lab result and logs will follow soon……………………………………………

Layer 3 – Inter Controller Roaming:

 L3 - Inter Controller Roaming

The wireless user moves from one AP to another AP connected to another controller in a different subnet or if the clients roam between APs registered to different controllers and the client WLAN on the two controllers is on different subnet, then it is called inter-controller L3 roam.

  • It’s similar to inter subnet roaming.
  • Controllers exchange mobility messages on the client roam. However, instead of moving the client database entry to the new controller, the original controller marks the client with an “Anchor” entry in its own client database.
  • The database entry is copied to the new controller client database and marked with a “Foreign” entry in the new controller.
  • The main advantage is clients maintain its original IP address even after changing the controller.
  • The process takes less than 30 ms.
  • The roam remains transparent to the wireless client. 

Lab result and logs will follow soon……………………………………………

Wireless Network Types

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. Consume more power but extend the connection to about 300 feet (100 meters).

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. Have a short range (up to 20–30 feet/7–10 meters), commonly use the 802.15 family of specifications to connect two or a few devices with low power consumption. Bluetooth is an example of WPAN protocol.

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. Extend the range to a larger geographic area, such as a city or suburb. Applications vary from point-to-point or point-to-multipoint links to multiuser coverage. WMANs typically use licensed frequencies (a fee has to be paid for permission to use the frequency), although implementations in the ISM bands can also be found. WiMAX is an example of WMAN protocol (most WiMAX implementations use licensed bands).

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. Provide connectivity over a wide geographical area. Usually, WWANs are networks used for mobile phone and data service and are operated by carriers. WWANs typically use licensed frequencies.


Beginning of CCIE Wireless…

After working 2 years in networking field(Specially Routing and Wireless), now I  decided to go in wireless technology. Even though I  am working on WLC based solution with Cisco ISE but still I am missing basic concepts.(This is a bitter truth)…..this was my past….let bygones be bygones 🙂


Since I did not have any certification in Cisco Wireless (CCNA, CCNP…or CCIE). I decided to read Jerome Henery quick reference guides to get the basics of technology. I will not go for CCNA Wireless exam but still I want to gain the knowledge . I will read these books first(Atleast twice) and then will come back for new post.

CCNA (640-722-IUWNE) Quick Reference by Jerome Henry  (http://www.ciscopress.com/bookstore/product.asp?isbn=1587143089)

CCNP (642-732 -CUWSS) Quick Reference ,2nd Edition by Jerome Henry (http://www.ciscopress.com/bookstore/product.asp?isbn=1587143100)

CCNP Wireless (642-737 IAUWS) Quick Reference by Jerome Henry (http://www.ciscopress.com/bookstore/product.asp?isbn=1587143127)

CCNP Wireless (642-747 IUWMS) Quick Reference, 2nd Edition by Jerome Henry (http://www.ciscopress.com/bookstore/product.asp?isbn=1587143097)

CCNP Wireless (642-742 IUWVN) Quick Reference by Jerome Henry (http://www.ciscopress.com/bookstore/product.asp?isbn=1587143119)

CCIE Wireless Exam (350-050) Quick Reference (http://www.ciscopress.com/bookstore/product.asp?isbn=0132168170)


Hope is one of those no-win-no-fee things, and although it needs some encouragement to survive, its existence doesn’t necessarily prove anything.
Rachel Cusk