Tutorial: How to Use Cellular as Backup Internet Access When Your DSL, Cable or Fiber Internet Dies

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.

If you want availability, you need two connections to the Internet, so if one fails you are not out of business. We go over this in the lesson “Mature Competitive Carrier Network: Regional Rings, POPs and MANs”, slide 3.17 of Course 101, Telecom Datacom and Networking for Non-Engineers, and mention it in pretty much every other course.

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. I’m posting this tutorial on our our Facebook page, our GoogleMyBusiness page, or our blog; I invite someone better able to tolerate Apple products to leave a comment whether and how the iPhone and MacBook can perform the required functions.

normal office lan and wired internet connection

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.

The solution for restoring Internet access after the CE died is illustrated below.

emergency cut line backup of business internet using cellular

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/Google Nexus 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 5 Mb/s connection to the Internet via cellular. Obviously my cellular service provider limited the connection to 5 Mb/s in software – but who’s complaining? 5 Mb/s is more than three 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.
— EC

Note 1: You must verify your billing plan for “data” on your cellular contract before doing this. I have 40 GB included, which means basically unlimited, and that includes the WiFi hotspot traffic. Make sure you have something similar, to avoid receiving a bill for $10,000 for casual “data” usage.

Note 2: As always, this tutorial is provided as general background information only. We do not guarantee it will work for you. Each situation is unique and requires professional advice to identify and resolve issues including but not limited to performance and security. This tutorial is not professional advice. But I hope you have found it valuable.

Note 3: I might have been able to implement this without the laptop. If you’d like to know that, or what was the other device I could have used to bridge the wired and wireless LAN in-building, or suggest how this could be done with Apple products, please leave a comment on our Facebook page, our GoogleMyBusiness page, or our blog.

Course 2223 Softswitches, SIP, and VoIP Call Setup – Free Preview

CVA – Certified VoIP Analyst
Course 2223 Softswitches, SIP, and VoIP Call Setup
Lesson 1 – What SIP Is and What It Can Do

Click the image to enjoy this free sample from CVA – Certified VoIP Analyst

Link to free sample for SIP and Softswitches

Course 2223 Softswitches, SIP, and VoIP Call Setup

What SIP Is • What It Does • URIs: SIP Phone Numbers • Call Setup Procedure • Call Disposition Rules • How SIP relates to Softswitches and Call Managers

Softswitches, SIP and Call Setup is all about how VoIP phone calls are set up using messages and procedures complying with the standard Session Initiation Protocol.

In this course, you’ll understand what SIP is, how it works, demystify jargon like proxy server and location server, understand how SIP fits in with softswitches and call managers, and trace the establishment of an IP phone call step by step.

Course Lessons
1. Intro + What SIP Is and What It Can Do
2. SIP’s Relationship to Other Protocols
3. SIP URIs: Telephone Numbers
4. Register: Update Your Location
5. INVITE: Dialing
6. Location Service: Finding the Far End
7. The SIP Trapezoid
8. SIP Messages and the Session Description Protocol
9. How SIP Relates to Softswitches and Call Managers

Based on Teracom’s famous Course 130, tuned and refined over the course of over 20 years of instructor-led training, you will gain career- and productivity-enhancing knowledge of how SIP is used to set up a VoIP phone call end-to-end, and how SIP fits in with call managers and softswitches.

This is just a small sample of the vast online telecommunication training and certification available through Teracom Training.

Get more info

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Six courses covering everything VoIP and SIP.

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The foundations of telecommunications plus everything VoIP and SIP.

Tutorial: Optical Ethernet

From the new textbook Telecom 101 Fifth Edition: 2020

SFP optical transceivers

Figure 111. SFP Optical Transceivers

10.5 Optical Ethernet

Optical Ethernet is signaling MAC frames (Section 4.4) from one device to another by flashing a light on and off; light on represents a 1 and light off represents a 0 in many systems.

The light, called a wavelength or lamda – λ in Greek – is as close to one single pure frequency as possible, in the infra-red, lower frequencies than what our eyes detect.

In sophisticated systems, the wavelength might be modulated with QAM (Section 3.4) to increase the bit rate.

Normally, Optical Ethernet is implemented as point-to-point connections: from a hardware port on one switch or router to a hardware port on another switch or router in a different building. This includes connections between core routers in cities, connections between routers and switches within a city, and connections from carriers to customers.

10.5.1 SFP Modules and Connectors

The light is generated by a laser controlled by pulses of electricity at the transmitter.  The intensity and sometimes phase of the light is modulated, i.e. changed in discrete steps, to represent bits optically based on the pulses of electricity. Up to 80 km (50 miles) away at the other end of a tube of glass thinner than one of your hairs, a photodetector at the receiver measures the received light and decides what bits are being represented, and transmits them onward as pulses of electricity.

As illustrated in Figure 111, most systems use two fibers, one for each direction. A device combining the transmitter and detector functions is called an optical transceiver.

This device has metal connectors on one side to plug into a slot on a router or switch, and optical connectors on the other side, either factory- or field-installed on the fibers plugged into the transceiver.

These transceivers are typically implemented as Small Form-factor Pluggable (SFP) modules, which are hot-swappable in the terminating equipment at each end.

100 Gb/s being communicated through this transceiver is the high end of commercially-deployed technology in 2020.

In some cases, the SFP modules are embedded in the terminating equipment, meaning the fibers are plugged into the terminating equipment. This allows re-use of existing fiber. In other cases, the SFP modules are attached to fiber cables by the fiber cable manufacturer, meaning the SFP module is plugged into the terminating equipment. This ensures the fiber and transceiver technology are matched and the optical connection is a high-quality “factory” connection.

The SFP module format is not the subject of a standard, but rather described in industry Multiple Sourcing Agreements (MSA).

10.5.3 IEEE Standards

There are many technologies for transceivers implemented in the SFP module. Some are proprietary; many are standardized by the IEEE. In practice, the same manufacturer’s product is used at both ends of the fiber to ensure compatibility. The table in Figure 112 lists current IEEE standards. More will be published in the future.

SFP optical transceivers

Figure 112. IEEE Optical Ethernet Standards

Most technologies use one fiber for each direction. Some, like for fiber to the home, use two wavelengths for two directions on one fiber. The bitrate of the standards beginning with 1000 is 1,000 Mb/s, or 1 Gb/s. A G at the beginning means Gigabits/second.  40 and 100 Gb/s technologies split the bitstream into subrates and transmit them in parallel on different wavelengths called paths or lanes.

The reach is the maximum length of fiber between devices.  Single-mode and multimode are designations for different qualities of fiber.  Most if not all builds today use single-mode fiber.

Source: Telecom 101 textbook / reference book, Fifth Edition: 2020 available in print and eBook.

Tutorial: 5G Wireless

One place 5G base stations will be deployed is on streetlights

Now that 4G cellular mobile is settled, talk is now turning to 5G.

The first thing to know about 5G is that there are currently no standards, no detailed agreement on what exactly it will be. But we have a number of general indicators to guide the discussion:

1. 5G will employ radio frequencies well above what is currently used for cellular.
The current frequency bands for 3G/4G cellular top out at about 2.6 GHz. Proposals for frequency bands for 5G include “millimeter wave” bands, that is, wavelengths varying between 1 and 10 mm, which correspond to frequencies between about 30 and 300 GHz. No doubt, in the future, there will be unified 5G systems with variations operating in all frequency bands; but the current emphasis is on new technology in the millimeter wave bands.

2. 5G will provide very high bit rates.
With carrier frequencies at 30 GHz and above, very wide frequency bands around those center frequencies can be employed, allowing the radio frequency modems to achieve high numbers of bits per second. In addition, Multiple-Input, Multiple-Output (MIMO) designs can implement massive parallel communications, radically increasing the capacity available to a user. Initial designs and trials have measured 5 Gb/s (5,000 Mb/s). No doubt, this will be pushed beyond 10 Gb/s.

3. Initially, 5G will not be a replacement for 4G.
At millimeter wave frequencies, in-building penetration and refraction around obstacles is poor, and the atmosphere attenuates (diminishes) the signal to the point that line-of-sight between the antennas is necessary, and useful transmission range is measured in the hundreds of meters (yards). This means that the first deployments of 5G will be in environments where base stations can be closely spaced.

One application for all this bandwidth is traffic control: going beyond today’s standalone self-driving vehicles to vehicles communicating with each other and with traffic control systems, with base stations deployed on street lights as suggested by the picture.

We’ll be covering 5G on the last day of BOOT CAMP.

more info

Tutorial: SIP Trunking

One of the newest service offerings from carriers is SIP Trunking.

Like many, many other pieces of jargon in the business, many people would like to understand just what exactly it is.

SIP is an acronym for Session Initiation Protocol. This is a standards-based method of setting up Voice over IP telephone calls.

A key thing to know about Voice over IP phone calls is that once the call is set up and two people are talking, their telephones exchange IP packets with digitized speech in them directly.

One person’s telephone creates an IP packet and puts the IP address of the other person’s telephone in the destination address field. This packet is forwarded by routers directly to the other person’s phone.

To be able to do this, it is necessary to know what the other person’s IP address is!

This is the main function performed by SIP: it is an assistant to enable a caller to find out the IP address of the called party’s telephone, so they can send packets with digitized speech to that person’s phone.

Trunking is a term that has been generally used in the telecom business in the past to mean communication between telephone switches. Trunks connect CO switches, toll centers and other switches in the PSTN.

PBX trunks connect an organization’s private switch to a CO switch.

SIP Trunking is a term invented by the marketing department to mean to mean “native communication of SIP call setup messages and Voice over IP traffic between an organization’s locations, with a Service Level Agreement and transmission characteristics sufficient to guarantee the sound quality.”  And a gateway service thrown in.

Native means carrying the IP packets without converting them to an old-fashioned telephone call. The IP packets in question are at first carrying SIP call setup messages, then once the call is set up, the IP packets each contain typically 20 ms of digitized speech.

SIP Trunking replaces the previous architecture of PBX trunks.

It would be more accurate to refer to this new service as “SIP and Voice over IP Trunking” – but “SIP Trunking” rolls off the tongue better…

To learn more, attend BOOT CAMP March 27-31, or just the last two days of BOOT CAMP, which are Course 130 Understanding Voice over IP March 30-31.

New video!

New video posted!  This is part of the introductory lesson of CTNS Course 2206 Wireless Telecommunications.

The length of the shadow behind an object is proportional to the frequency of the energy.

Click to watch on YouTube

For more information:

Course page: https://www.teracomtraining.com/online-courses-certification/teracom-overview-l2106.htm

CTNS Certification page:

Wireless Telecommunications is a comprehensive course on wireless, mobile telecommunications plus wireless LANs and satellites.

We begin with basic concepts and terminology including base stations and transceivers, mobile switches and backhaul, handoffs, cellular radio concepts and digital radio concepts.

Then, we cover spectrum-sharing technologies and their variations in chronological order: GSM/TDMA vs. CDMA for second generation, 1X vs. UMTS CDMA for third generation along with their data-optimized 1XEV-DO and HSPA, how Steve Jobs ended the standards wars with the iPhone and explaining the OFDM spectrum-sharing method of LTE for 4G.

This course is completed with a lesson on WiFi, or more precisely, 802.11 wireless LANs, and a lesson on satellite communications.

You’ll gain a solid understanding of the key principles of wireless and mobile networks:
• Coverage, capacity and mobility
• Why cellular radio systems are used
• Mobile network components and operation
• Registration and handoffs
• Digital radio
• “Data” over cellular: Internet access
• Cellular technologies: FDMA, TDMA, CDMA, OFDM
• Generations: 1G, 2G, 3G, 4G
• Systems: GSM, UMTS, 1X, HSPA, LTE
• WiFi, 802.11 wireless LANs
• Satellite communications

Tutorial: How do IP packets and MAC Frames go together?

It is important to understand how packets and frames are related, and in particular, IP packets vs. Ethernet or MAC frames.

Simple network example. Routers move packets from one circuit to another.

Packets are for networks. A packet is a block of user data, such as a piece of an e-mail message, with a network address on the front. The network address is the final destination. The standard for network addresses is IP.

Network equipment like routers receive an IP packet on an incoming circuit, examine the indicated destination IP address, use it to make a route decision, then implement the decision by forwarding the packet to the next router, on a different circuit.

A frame is a lower-level idea. Frames are used to communicate between stations on the same circuit. The circuit may have multiple stations physically connected onto it, like a wireless LAN, a few stations connected by a LAN switch, or only two stations like a point-to-point LAN cable. Each station has a Media Access Control (MAC) address, sometimes called a hardware address, link address or Layer 2 address.

A frame has framing to mark the beginning and end, sender and receiver MAC addresses to indicate the stations on the circuit, control information, a payload and an error detection mechanism.

The frame is transmitted on the circuit, and all stations on the circuit receive it. If an error is detected at a receiving station, the frame is discarded and might have to be retransmitted somehow.

If no errors are detected, the receiver compares the destination MAC address on the received frame to its own MAC address, and if they are the same, processes the frame, extracting the data payload and passing it to higher level software on the receiver.

If the MAC addresses are not the same, the receiver ignores it and waits for the next one.

The end result is that the payload in the frame is communicated to the correct station on the same circuit, with no errors.

Packet with its IP address vs. frame and its MAC address

The main purpose of packets is to append an IP address to your data. The IP address is used by network equipment to make route decisions: to relay the packet from one circuit to a different circuit. This is accomplished by receiving the packet then transmitting it to a different machine, usually the next router in the chain.

To actually transmit a packet to another router, the packet is inserted as the payload in a frame, then the frame is broadcast on the circuit that connects to the next router.

Notice that there are two addresses: the IP network address and the MAC address.

The IP address on the packet is the final destination, and so does not change. The MAC address on the frame indicates the destination on the current circuit, and so is changed as the data is forwarded from one circuit to another.


This and related topics are covered in:

CTNS Certification Package

CIPTS Certification Package

Instructor-Led Course 101
Telecom, Datacom and Networking for Non-Engineering Professionals

DVD-Video V3 Fundamentals of Datacom and Networking

Telecom 101 Textbook

Tutorial: Packetized Voice

This is Section 2.9.1 of the new Telecom 101, 4th edition
print and ebook available January 2016



Brought to you by BOOT CAMP
January 25-29 2016 – Silicon Valley


– – –

This diagram provides a very high-level block diagram view of the processes involved in communicating speech in IP packets from one person to another:

Voice in IP Packets
Voice in IP Packets End-to-End

Starting on the left, commands from the speaker’s brain cause combinations of lungs, diaphragm, vocal cords, tongue, jaw and lips to form sounds.

A microphone is positioned in front of the mouth and acts as a transducer, creating a fluctuating voltage which is an analog or representation of the sound pressure waves coming out of the speaker’s throat.

This is fed into a codec, which digitizes the voltage analog by taking samples of it 8,000 times per second and coding the value of sample into binary 1s and 0s. Typically, the value of each sample is represented with a byte, meaning 64 kb/s to be transmitted.

Approximately 20 ms worth of coded speech is taken as a segment and placed or encapsulated in an IP packet.

The IP packet begins with a header, which is control information, the most interesting part being the IP address of the source telephone and IP address of the destination telephone.

IP packets are moved from the source to the destination over a sequence of links.

The links are connected with routers, which relay the packets from one link to the next.

Lower level functions such as framing and link addressing are usually performed following the IEEE Ethernet and MAC standards.

At the lowest level, the links are physcially implemented with Category 6 LAN cables, DSL modems, Cable modems, fiber optics and radio systems.

At the destination, the bits are extracted from the IP packet and fed into a codec, which re-creates the analog voltage.

This voltage drives a speaker, which re-creates the sound pressure waves, which travel down the ear canal to the inner ear, causing hairs on the cochlea to vibrate, triggering neural impulses to the brain, making the listener think they are hearing something.

– – –

It’s important to note that the voice packets are communicated directly from one telephone to the other over the IP network.
The packets do not pass through a CO telephone switch, for example.

– – –

For a full explanation of all of the technologies mentioned in this tutorial, register for BOOT CAMP in Sunny Silicon Valley in January!


is Core Training Course 101 Telecom, Datacom and Networking for Non-Engineering Professionals
and VoIP Course 130 Understanding Voice over IP
in one week at a discounted price.

Get this career-enhancing boost.  Check it out!