Tutorial: Mobile Operators, MVNOs and Roaming

Extracted from Chapter 9 of the Telecom 101 reference book.
Note: acronyms and abbreviations used below are explained in lessons leading up to this one.

9.7 Mobile Operators, MVNOs and Roaming

9.7.1 Mobile Network Operator

Mobile Network Operator (MNO) is the term usually used to refer to a facilities-based carrier, i.e. a company that owns base stations, a mobile switch, backhaul between them, and spectrum licenses, and sells services to the public… and to other carriers.

The MNO implements external links to other carriers for PSTN phone calls and for Internet traffic.

For PSTN phone calls, the MNO implements a fiber optic connection to a building traditionally called a Toll Center or Class 4 switching office. The termination of their fiber in that building is called a POP. It is their physical point of presence in the building.

Many other carriers have POPs in the building, including the ILEC, IXCs, CATV companies, other mobile carriers, and any other company that wants to connect phone calls to a phone on the MNO’s network.

The operator of the toll center, usually the ILEC, provides a switch in the Toll Center to switch phone calls from one carrier’s POP to a different carrier’s POP.

For Internet access, the MNO implements a fiber optic connection to one or more Internet Exchange buildings, where they pay the operator of the IX to route packets to other carriers with whom the MNO has established IP packet transit and peering arrangements.

9.7.2 Mobile Virtual Network Operator

Mobile Virtual Network Operator (MVNO) is the term used to refer to a non-facilities-based carrier… one that does not own the hardware or spectrum licenses or POPs.

Instead, the MVNO enters into a long-term contract with one or more facilities-based carriers to have them supply a “white label” service that the MVNO sells.

Typically the MVNO will develop a unique branding and sell smartphones and tablets to go along with its service.

When the MVNO deals exclusively with one carrier, the MVNO bill to the customer would be typically generated by the facilities-based carrier as a white-label service.

If the MVNO is very large and deals with multiple carriers, the MVNO may operate their own billing system, which is a significant investment.

The facilities-based carrier charges to the MVNO includes a volume-discount rate for IP addresses and Internet traffic, voice-minute airtime and switched access to the POP for PSTN phone calls.

The MVNO also has to pay for connectivity from the POP to other toll centers for “long-distance” connections, and the switched-access charge at the far end.

The rate plan the MVNO pays could be a mix of fixed-rate leases and usage-based billing.

Unless the MNO is obliged to sell capacity to MVNOs through regulations and tariffs, the nature of the plan is confidential business information.

9.7.3 Roaming

Roaming service is very similar to the service provided to MVNOs, in that it is the MNO that is providing the airlink, base stations, backhaul, mobile switch and connections to the PSTN and Internet.

In the case of roaming, the visitor uses their own phone, and billing is usage-based.

Roaming is an important feature for smaller players: they are facilities-based in selected cities, but to offer a national and international service to their customers, they must have roaming agreements in place with MNOs in other locations.

By denying roaming service to smaller or startup carriers, or charging an exorbitant price for roaming, an incumbent carrier can erect a barrier against competition.

In many countries, the right to roam and the wholesale cost of roaming is regulated to encourage competition.


These topics are covered in:

Tutorial – What Modems Do: Carrier Frequencies, Phase Shifts and QAM

Modulation means producing energy that is vibrating at a single pure frequency, called a carrier frequency or subcarrier, and changing aspects of it in discrete steps to represent bits. 

The device that performs this function is called a modulator.  A demodulator is required at the far end to interpret the carrier frequency and decide what bits it is representing at any given time.  Clearly, we want devices to do both functions to implement two-way communications, so they are called modulator/demodulators or modems for short.

One aspect of the carrier than can be changed to represent bits is the volume or amplitude of the carrier: changing the amplitude of the carrier in discrete steps makes changes that represent bits.

Another aspect is the phase of the carrier: when the peak of the cycle is happening, in time, with respect to other carriers. Changing the time of the peak so it happens a bit earlier than others, or making it happen a bit later is making changes to the phase of the carrier that can represent bits (Figure 29).

Combinations of phase and amplitude shifting is called Quadrature Amplitude Modulation (QAM). QAM-64 means 64 possible different combinations of 8 different phases and 8 different amplitudes.

Each combination, also called a symbol or signal, is assigned a number. Binary numbers 6 bits long are required to give binary numbers to each of the 64 combinations.

7.3.2 Communicating Six Bits: Sending One of 64 QAM Signals

To communicate six bits in one fell swoop on a carrier, the transmitter generates electricity vibrating at the carrier frequency with the phase and amplitude corresponding to the combination indicated by the six-bit number.

The electricity is communicated on coaxial cables to Cable modems, on twisted pair to DSL modems, turned into radio by antennas for communication through space, or turned into light for communication in tubes of glass in very high capacity fiber transmission systems.

When the receiver detects energy at that single pure carrier frequency, it measures the phase and amplitude, and once it has decided, spits out the six-bit number of the combination it is hearing, and Bob’s your uncle.

7.3.3 Baud Rate

To get many bits per second, the procedure has to be repeated often!

Repeating it once per second yields 6 bits per second; the combination of phase and amplitude of the carrier is maintained for one second then changed to a different combination representing the next six bits.

The rate at which the procedure is repeated is called the baud rate, signaling rate and symbol rate.

The baud rate, how often a new combination can be applied to the carrier to communicate another 6 bits, is limited by interference called harmonics, where energy gets spread into adjacent frequencies, and interferes with communications on other carriers.

7.3.4 Orthogonal Frequency Division Multiplexing (OFDM)

When there are multiple carriers (called subcarriers) each running a modem, and the baud rate is the same as the subcarrier spacing, the harmonics from all subcarriers cancel out.

Eliminating this source of interference allows successful data transmission in parallel on closely spaced subcarriers.

This is a prime design characteristic of Orthogonal Frequency Division Multiplexing (OFDM), used on LTE, 5G, Wi-Fi, cable modems and DSL, and is the sweet spot for baud rate in terms of efficiency.

Source: CTNS Study Guide 2021, Course 2206, Section 7.3

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Digitally-Signed Email: Authentication and Digital Signatures

E-mail was one of the first “killer apps” on the Internet, and has been a major contributor to increases in productivity over the past ten years. Of course, along with email came the scourge of spam. Criminals infect computers with trojan horse programs, creating collections of machines they control remotely to send millions of unsolicited offers for fake watches, pirated software, phony medications and ecard invitations to infect your computer.

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Continue reading “Digitally-Signed Email: Authentication and Digital Signatures”