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.
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.
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.
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.
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
Course 2223 Softswitches, SIP, and VoIP Call Setup
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
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In this course, you’ll understand what SIP is, how it works,
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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
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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
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.
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
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.
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…
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