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The Chirpsounder / Ionosonde

ionosphereAnybody who has ever set up a working international HF link will know it can be a tricky business. You see there's a pesky movable thing called the ionosphere which is pretty fundamental to the whole business.
Communicating with a point halfway round the planet using HF is like trying to play that old 70's children's game called Rebound. Since radio links are usually close to or distinctly line of sight links, communicating with a point on the other side of a sphere would seem like a fairly insurmountable problem. I'd think the first time this problem was solved using the ionosphere it was probably an accident caused by some early radio pioneers receiving signals for their fellow pioneers some way round the planet and beginning to wonder why and how it was happening.

The reason it was and does happen is because of a thin layer of the Earths atmosphere called the ionosphere. The ionosphere is a region of the upper atmosphere, from about 85 km (53 mi) to 600 km (370 mi) altitude, and includes the thermosphere and parts of the mesosphere and exosphere. It is distinguished because it is ionized by solar radiation. It plays an important part in atmospheric electricity and forms the inner edge of the magnetosphere. It has practical importance because, among other functions, it influences radio propagation to distant places on the Earth. This is the reason we as Telecommunications Engineers are interested in it.

The ionosphere is a layer of electrons and electrically charged atoms and molecules in the upper Earths atmosphere, ranging from a height of about 50 km (31 mi) to more than 1,000 km (620 mi). It exists because of the Suns ultraviolet radiation which causes these gases to ionise and develop a charge. Because of the boundary between this layer and the relatively uncharged layer below, wave diffraction occurs. This phenomenon takes place at different incidences with different frequencies and, with clever utilisation of this property, the ionosphere can be utilized to "bounce" a transmitted signal down to the ground. Transcontinental HF-connections can rely on up to 5 of these bounces, or hops.

aerials-takeoff-anglesIt is the process of determining the appropriate frequencies and their respective bounce points around the planet that is the focus of this post. The applied physics involved in this refraction are beyond the scope of this post but, in a nutshell, what they do produce is a spread of frequencies which bounce at different incident angles to the boundary layer such that different distant points on the surface of the planet can be reached when the bounced radio wave returns to the ground. This is shown more clearly in the diagram on the left.

Unfortunately it is not quite as straightforward as the diagram above suggests as the strength and location of the ionosphere is always changing as day becomes night and also as cosmic radiation from the Sun changes over time. This presents those wishing to use this phenomenon with the constant problem of determining which frequencies are workable and usable between any two given points on the Earth.

The problem of determining these usable frequencies was the driving force behind the invention of the Chirpsounder (also known as an Ionosonde). The Chirpsounder, or rather a pair of Chirpsounders operate in tandem using a Chirp transmitter in one location and a Chirp receiver in another. The job of the transmitter is to transmit a sweep of radio output from one predetermined frequency to another over a given amount of time. A Chirp receiver situated close to the transmitter would, if synchronised to match the sweep timings, receive all of the sweep from the beginning to the end but the same Chirp receiver placed two thousand miles away over the Earths horizon may not fare so well. This is where the technology really comes into its own.

Screenshot_3When a Tx/Rx pair of Chirpsounders are running a synchronised sweep between two distant locations, the receiver will receive from the transmitter only during those parts of the sweep that are conducive to a working link between the two. This information is gathered by the Chirp receiver and is used to provide the user with a graph showing frequency on the x-axis and receive delay on the y-axis. There will also often be a display of receive signal strength incorporated in the output. A sample Chirpsounder output is shown on the right.

As can be seen there are a number of elements shown on the trace and each of these represents a successful reception of the signal from the transmitter. The more solid the line, the more reliable the link and this information, when used in parallel with the received power information can enable telecommunications professionals to choose the most appropriate frequency. Once the decision had been made the operational transmitters and receiver could be set appropriately and the operational radio channel could begin to pass its traffic using the ionospheric bounce. Quite amazing really.

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Configuring 3G Wireless WAN on Modular and Fixed ISRs (HWIC-3G-GSM, HWIC-3G-HSPA, PCEX-3G-HSPA-x)

3gCisco Integrated Services Routers are branch routers which support the new paradigms of network traffic delivery in the cloud and on the move. They provide Internet connectivity to teleworkers, and minor sites supporting less than 20 users. They also support bridging and routing between the LAN and the WAN whilst providing many advanced features such as antivirus protection.


The Third Generation (3G) Wireless High-Speed WAN Interface Card (HWIC) is a multiband, multiservice WAN card for use over WCDMA Radio Access Networks (RAN).


Both the fixed and the modular 3G routers can be used as the primary WAN connectivity and as a backup for critical applications which require a fallback service. 3G WAN is supported on the following modular Cisco ISRs: 800, 1841, 1861, 2800 series, 3800 series, 1900, 2900 and 3900.


One of the first actions required will be to configure a new 3G HWIC data profile.


To configure your 3G HWIC data profile, you will need the following information from your service provider:


Username (if required by your carrier)


Password (if required by your carrier)


Access Point Name (APN)


Once obtained, we can begin to set up the 3G features on the equipment itself by following these procedures:




  1. Data Account Provisioning

  2. Data Call Setup

  3. Voice Initiated Data Callback or Remote Dial-in (Optional)


In order to provision our data account we must have first obtained the key information from the service provider. The next priority is to ensure that we have the necessary service availability and signal strength in order for the connection to work. We need to use the following commands to examine the services available on the 3G network at the location in question.

  1. show cellular network - This displays info about the carrier network.

  2. show cellular radio - This shows the signal strength. We are looking for RSSI of -90dBm for a steady and reliable connection.

  3. show cellular security - This shows SIM lock status and modem lock status.


Once we have determined that the conditions are favourable we can go ahead and set up a modem data profile. To examine the existing data profiles configured on the equipment use the command show cellular profile. 

Assuming the profile we need is not already created we will need to go ahead and create it. In order to do this we use the command cellular gsm profile create . The syntax required is as follows:

cellular <slot/wic/port> gsm profile create <profile number> <apn> <authentication> <username> <password>

for example

cellular 0/0/0 gsm profile create 1 vodafone.apn chap 3guser 3guserpass

The data profile parameters are as follows:

  • apn - Access Point Name - This must be obtained from the service provider

  • authentication - Usually chap or pap

  • username - provided by service provider

  • password - provided by service provider


Once the data profile is properly set we then look to set up the parameters for the correct operation of the data call.

Firstly it is necessary to configure the cellular interface. The steps in summary are as follows:

1. configure terminal


2. interface cellular <slot/wic/port>


3. encapsulation ppp


4. ppp chap hostname <host>


5. ppp chap password 0 <password>


6. asynchronous mode interactive


7. ip address negotiated


The authentication parameters used here must be the same as those configured under the earlier GSM profile.


Once this is configured we need only configure the dialer and the steps for doing this in summary are as follows:



1. configure terminal


2. interface cellular <slot/wic/port>


3. dialer in-band


4. dialer idle-timeout <seconds>


5. dialer string <string>


6. dialer group <number>


7. exit


8. dialer-list <dialer-group> protocol <protocol-name> {permit | deny | list <access-list-number> | access-group}>


9. ip access-list<access list number>permit <ip source address>


10. line <slot/wic/port>


11. script dialer <regexp>


12. exit


13. chat-script <script name> "" "ATDT*98*<profile number>#" TIMEOUT <timeout value> CONNECT


14. interface cellular <slot/wic/port>


So that should be it. Assuming the router is properly configured elsewhere, the traffic should begin to flow using the 3G interface and everything should be working just fine. Of course sometimes things dont work out quite so smoothly and I will publish a post soon detailing the steps needed to troubleshoot these types of connections when they dont work as planned.


I hope this summary is useful and would appreciate your comments using the form provided below.

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