Radio Spectrum and Radio Bands. by Hoang Lihuo

 Radio spectrum is the part of the electromagnetic spectrum with frequencies from 30 Hz to 300 GHz.Electromagnetic waves in this frequency range, called radio waves, are widely used in modern technology, particularly in telecommunication. To prevent interference between different users, the generation and transmission of radio waves is strictly regulated by national laws, coordinated by an international body, the International Telecommunication Union (ITU).

   

    Radio band is a small contiguous section of the radio spectrum frequencies, in which channels are usually used or set aside for the same purpose. To prevent interference and allow for efficient use of the radio spectrum, similar services are allocated in bands. For example, broadcasting, mobile radio,

or navigation devices, will be allocated in non-overlapping ranges of frequencies.

    Every country has the sovereign right to manage energy at radio frequencies in its territory.

    When the airspace through which the radio waves travel is public property (which is most of the time), and there is contention for its use (which is all of the time), regulation is required to allow the rational use of the shared resource.

    In the United States, the Federal Communications Commission (FCC) regulates spectrum for use by other than the Federal government.

    Industry Canada, which includes the former Department of Communications, regulates the airwaves in Canada.

Canada, Mexico and the USA coordinate radio frequency usage due to geographic proximity.

    The International Telecommunication Union (ITU) in Geneva facilitates coordination between countries, particularly outside of North America.

    The range of radio frequencies, called the radio spectrum, is divided into blocks of frequencies, allocated for particular different services.

    Allocations are divided into allotments, which are bands of frequencies assigned to specific users or to the public.

    A license to emit radio-frequency energy at the specified frequencies in a specified area is issued by government to record the assignment of the allotment and the conditions for its use.

    The diagram lists some of the bands that are currently in use, beginning with old-fashioned analog AM radio, CB radio and the first generations of in-home analog cordless phones.

    Analog TV channels 2-13, interrupted by a band for analog FM radio is next, followed by the Maritime VHF radio band and the UHF television channels from 14-51.

    The 700-MHz band used to be for UHF channels 52 and up, recently reallocated for new applications including cellular and WiFi-like services.

    Google's involvement in the US auction for allotments in the 700-MHz band in the US caused the inclusion of conditions in the licenses requiring openness: use of any phone and roaming on any carrier.

    The 800-MHz band was initially allocated for the first generation of mobile cellular radio in North America.

    At 900 MHz is an unlicensed band, meaning allocated to the public, and a license to emit energy at that frequency is not required. These bands are also called Industrial, Scientific and Medical (ISM) bands.

    The 900-MHz band was used for cordless phones in North America.

    In the rest of the world (called "Europe" in the telecom business), the 900-MHz and 1800-MHz bands were allocated for second generation mobile called GSM: the Global System for Mobile Communications.

    In North America, the 1900-MHz or 1.9-GHz band was initially allocated for the second generation of cellular, called Personal Communications Services.

    New bands are allocated, and older bands are re-allocated to new 4G and 5G cellular systems on an ongoing basis. These bands lie in the frequencies below 2.4 GHz.

    At 2.4 GHz is another ISM band. This band is used by numerous technologies including Bluetooth, WiFi, cordless phones, private point-to-point transmissions and … microwave ovens, to avoid the need for a license.

    This makes the 2.4 GHz band noisy.

    WiMax, like WiFi on steroids requires a license, typically at 2.5 GHz.

    The 5 GHz ISM band is used for WiFi and point-to-point systems.

    Satellite and other line-of-sight communication systems use frequencies above 5 GHz; but as with everything else, there are tradeoffs.

    It turns out that the capacity, the widths of the bands, is very good at high frequencies, but transmission characteristics like penetration through walls are not so good.

    Lower frequencies experience much better transmission characteristics but have lower capacity.

    The sweet spot appears to be in the range 500 MHz – 2500 MHz with current technologies.

How to Use Cellular as Backup Internet Access When Your DSL, Cable or Fiber Internet Dies. by Hoang Lihuo

    The Internet connection at your office dies. Lights on your modem are flashing in a strange pattern. You call the ISP, and they quickly diagnose that the modem power supply has failed, and they will overnight you a replacement. Presumably you are not the first person to have this problem with that modem. 
    So how do you continue to operate while you are waiting for the replacement power supply? It's hard to run your business without e-mail and ordering and administration systems, which are all accessed via the Internet.
     A large business will be a station on a Metropolitan Area Network, which is a ring, meaning two connections to the Internet for that business and automatic reconfiguration in the case of one failing. But this is expensive... the second connection is not free. 
    Small and medium businesses usually have a single DSL or cable modem connection to the Internet. When that fails, connectivity to email, ordering and administration servers is impossible, and many businesses these days would be "dead in the water" until the ISP fixes the problem with their hardware. 
Unless you have an Android smartphone, a good "data" plan and a laptop with WiFi running Windows. 

    The scenario described happened at our office last week. Since many of our customers might find themselves in a similar situation - even at home - I thought I'd share the quick and painless solution I came up with. Even if you're not likely to need this solution, understanding how it works will no doubt sharpen your understanding of the devices involved and their functions. 

    In this tutorial, I will use the technology in our office: 50 Mb/s DSL, Android smartphone and Windows laptop. The solution is equally applicable to an Internet connection using a cable modem or if you are one of the lucky few, an Internet connection via fiber. 

    For the smartphone and laptop, there may be equivalent functions on Apple products, but as I am allergic to Apples, we don't have any in the office.

    The diagram above illustrates the normal network setup in our office, a typical configuration for networking at a small or medium business. On the left is the access circuit to the Internet Service Provider (ISP), terminating on a modem in our office.

    The modem is contained in a box that also includes a computer and an Ethernet switch. This box is more properly called the Customer Edge (CE). The computer in the CE runs many different computer programs performing various functions: Stateful Packet Inspection firewall, DHCP server offering private IP addresses to the computers in-building, DHCP client obtaining a public IP address from the ISP, a Network Address Translation function between the two, routing, port forwarding and more.

    In-building is a collection of desktop computers, servers and network printers. These are connected with Category 5e LAN cables to Gigabit Ethernet LAN switches, one of which is also connected to the CE.

    When a desktop computer is restarted, its DHCP client obtains a private IP address and Domain Name Server (DNS) address from the DHCP server in the CE. The private address of the CE is configured as the "default gateway" for the desktop by Windows.

    When a desktop computer wants to communicate with a server over the Internet, it looks up the server's numeric IP address via the DNS, then creates a packet from the desktop to the Internet server and transmits it to its default gateway, the CE. The NAT function in the CE changes the addresses on the packet to be from the CE to the Internet server and forwards the packet to the ISP via the modem and access circuit. The response from the Internet server is relayed to the CE, where the NAT changes the destination address on the return packet to be the desktop's private address and relays it to the desktop.

    An Android smartphone and a laptop running Windows were used to restore connectivity to the Internet without making any changes to the desktops, servers or network printers.

First, I took my Samsung smartphone running Android out of my pocket and plugged in the charger. Then on its menu under Settings > more > Tethering & portable hotspot > Set up Wi-Fi hotspot, I entered a Network SSID ("TERACOM") and a password, clicked Save, then clicked Portable Wi-Fi hotspot to turn it on.

        The smartphone is now acting as a wireless LAN Access Point, just like any other WiFi AP at Starbucks, in the airport or in your home. At this point, the smartphone is the CE device, performing all of the same functions that the DSL CE device had been before it died: firewall, DHCP client to get a public IP address from the ISP (now via cellular), DHCP server to assign private IP addresses to any clients that wanted to connect (now via WiFi), NAT to translate between the two and router to forward packets.

    Just as the DSL CE equipment "bridged" or connected the DSL modem on the ISP side to the Ethernet LAN in-building, allowing all the devices on the LAN to send and receive packets to/from the Internet via DSL, the smartphone "bridges" or connects the cellular modem on the ISP side to the WiFi wireless Ethernet LAN in-building, allowing all the devices on the wireless LAN to send and receive packets to/from the Internet via cellular radio.

    The remaining problem was that none of the desktops or servers had wireless LAN cards in them, so they could not connect to the smartphone AP and hence the smartphone's cellular Internet connection. What was needed was a device to "bridge" or connect the wired LAN to the wireless LAN in-building.

    By definition, this device would need two LAN interfaces: a physical Ethernet jack to plug into the wired LAN, plus a wireless LAN capability. Looking around the office, I spotted two devices that fit this description.

    One of them was my laptop, with both a LAN jack and wireless LAN. I fired up the laptop, plugged it into an Ethernet switch with a LAN cable, and in the Network and Sharing Center, clicked Change Adapter Settings to get to the Network Connections screen that showed the two LAN interfaces.

    I enabled both the wired and wireless LAN interfaces. Then right-clicking the Wireless Network Connection icon, selected the TERACOM wireless network and entered the password.

    Once that was successfully connected, I selected the two adapters in the Network Connections screen, right-clicked and chose "Bridge Connections". A message saying "Please wait while Windows bridges the connections" appeared, then an icon called "Network Bridge" appeared, and after a few seconds, "TERACOM" appeared as well.

    My laptop was now acting as an Ethernet switch, connecting the wired LAN to the smartphone's wireless LAN.

    Each of the desktops, servers and network printers in the office had to be rebooted so they would run their DHCP client again, obtaining a private IP address and DNS address from the smartphone AP, and be configured so the smartphone was the "default gateway" in Windows.

After rebooting my desktop computer, it had Internet access over the wired LAN, through the wired Ethernet switch to my laptop, to the smartphone via WiFi then to the ISP over cellular. After rebooting the other desktops and servers, all had Internet access again, with no changes to the configuration of the desktops or servers.

    This took about 20 minutes to get up and running, and we were back in business.

Running a bandwidth test on speedtest.net, I found we had exactly 10 Mb/s connection to the Internet via cellular. Obviously my cellular service provider limited the connection to 10 Mb/s in software - but who's complaining? 10 Mb/s is seven times as fast as a T1, which cost $20,000 per month when I first started in this business 20 years ago.

    I hope you found this tutorial useful, either as a template for your own emergency backup Internet connection, or simply as a way of better understanding the devices, their functions and relationships.

Voice Digitization Principles. by Hoang Lihuo

Voice Digitization Principles

    In this tutorial, we understand how voice is digitized - turned into a stream of 1s and 0s to be communicated over the IP packet-based telecommunications network.

    This is a necessary step for carrying Voice over IP, which is taking over the world, and was used on older channelized SONET and T1 trunk carrier systems.

There are three steps in voice digitization: quantization, sampling and coding:

• Quantization: Change from continuous in value to discrete in value 
• Sampling: Change from continuous in time to discrete in time 
• Coding: Code value of sample into 1s and 0s 

  
  

    Quantization is the process of changing from a signal which is continuous in value to a signal which is discrete in value.

    This is accomplished by dividing the possible range of values into a number of "bins" or levels or steps, and assigning a number to each of these levels.

    Then, when asked what the value of the signal is, we say that the signal is "in level #42" rather than measuring its voltage.

    Another example of quantization is sugar cubes. Instead of putting some fractional value of a bag of sugar in your coffee, your choice is "one lump or two".The sugar has been quantized into uniform lumps.

    Sampling is the process of changing the signal from being continuous in time to one that is discrete in time. 

    On a regular basis, we take the value of the signal and record it. The value of the signal is the level number.

    How often do we need to sample the signal?

    A mathematician by the name of Nyquist proved that the signal has to be sampled more than twice as often as the frequency bandwidth of the signal to be able to reproduce it. This is called the Nyquist Rule. 

    Code: The value of the signal taken at each sample (the level number) must be coded into 1's and 0's so that it can be transmitted over a digital carrier system or stored in a computer.

    At the far end, we perform the reverse process: re-creating the analog waveform from the received codes by de-coding the level number, generating a voltage with a value equal to that of the center of the level, and smoothly changing the voltage in this manner as each new code comes down the line.

    The whole point in doing this is to move the analog voice signal from the near-end loop to the far-end loop without adding in any noise.

    There is in fact a small amount of noise added in, up front, as part of the analog-to-digital conversion.

    This is the quantization error, the difference in value between the center of the level, and where the signal actually was.

    How do we make the quantization error smaller on average? Make the levels finer.

    How many levels does the telephone company use? Enough so that a human can't hear the quantization error noise on the line.