In this post, we take a closer look at the fourth development: a worldwide standard for mobile wireless has finally been achieved with 4G LTE.
Mobility means it is possible to start communicating with a particular radio base station, then when moving physically away, be handed off to another base station down the road to continue communications uninterrupted. In a non-mobile system (like WiFi), communication ceases if you move too far away.
The first generation (1G) of mobile radio was characterized by analog FM on frequency channels. Numerous incompatible systems were deployed: AMPS in North America, TACS in the UK, NMT in Finland and others.
The second generation (2G) was digital, which means modems communicating 1s and 0s between the handset and base station. Again, several incompatible systems were deployed, and two warring factions emerged, which could be called the “GSM/TDMA faction”, and the “CDMA faction”.
By far, the most popular 2G system was GSM, a European technology where a number of users time-share a single radio channel. Another system was IS-136, called “TDMA” in North America, deployed by the company currently known as AT&T Wireless in the US and Rogers in Canada.
A less popular 2G system employed CDMA, using technology patented by American company Qualcomm, and deployed by Verizon, Sprint and Canadian telephone companies.
These 2G systems were totally incompatible. A basic phone from a carrier could not work on another carrier unless they both used exactly the same system.
To try to avoid a repeat of the incompatibility for the third generation, the International Telecommunications Union (ITU) struck a standards committee in year 2000 called IMT-2000, its mission to define a world standard for 3G.
They failed. IMT-2000 instead published a 3G “standard” with five incompatible variations. The two serious variations were both CDMA – but differed on the width of the radio bands, the control infrastructure and synchronization method among other things.
The GSM/TDMA faction favored the deployment of CDMA in a 5 MHz wide band. This was called IMT-DS, Direct Spread, Wideband CDMA and Universal Mobile Telephone Service (UMTS). Its data-optimized version was called HSPA.
The CDMA faction favored a strategy that was a basically a software upgrade from 2G, employing existing 1.25 MHz radio carriers. This is called IMT-MC, CDMA multi-carrier, CDMA2000 and 1X. Its data-optimized version was called 1XEV-DO.
Again, these 3G systems were completely incompatible. A basic UMTS phone could not work on a 1X network.
Market forces finally pushed the two camps together.
The fact that there were far more users in the GSM/TDMA faction meant that their phones were less expensive, had better features and appeared on the market first. This put the carriers in the CDMA/1X faction at a disadvantage. This trend was continuing into 3G, where UMTS phones would have the same advantage over 1X phones.
Then, Steve Jobs invented the world’s most popular consumer electronic product, the iPhone – but only permitted carriers in the GSM/TDMA/UMTS faction to have it. This severely tilted the playing field.
In the face of this, the CDMA/1X faction threw in the towel, and decided to go with the GSM/TDMA/UMTS faction’s proposal for the fourth generation (4G), called LTE, to level the playing field.
And once this was agreed, Steve Jobs allowed the iPhone on all networks. One of the legacies of Steve Jobs will not just be the iPhone, but ending the standards wars by pushing the industry to agree on LTE as a single worldwide standard for mobile communications as of the fourth generation, using the leverage of his iPhone.
Bluetooth is a set of standards for short-range digital radio communication published by a consortium of companies called the Special Interest Group. It was originally developed as a wireless link to replace cables connecting computers and communications equipment.
Bluetooth connections are called piconets and Personal Area Networks since (in theory) up to eight devices can communicate on a channel within a range of 1 to 100 meters depending on the power.
In reality, Bluetooth is mostly used point-to-point with ten meters range.
The first data rate for Bluetooth was 0.7 Mb/s, followed by an enhancement to “3” Mb/s (2.1 Mb/s in practice). A High Speed variation employs collocated Wi-Fi for short high-bitrate transmissions at 24 Mb/s. The Smart or Low Energy variation allows coin-sized batteries on devices like heart-rate monitors.
Applications include wireless keyboard, mouse and modem connections… though today, 2 Mb/s Bluetooth is likely slower than the modem.
Bluetooth is used to replace wires connecting a phone to an earpiece, or to an automobile sound system for hands-free phone calls while driving. In this case, both two-way audio and two-way control messages are transmitted.
Bluetooth is also used to stream music from a smartphone to a receiver connected to an amplifier and speakers in an automobile or in a living room.
In the future, wireless collection of readings from devices like heart-rate monitors will be widespread.
Each of these types of applications corresponds to a Bluetooth profile, which is a specified set of capabilities and protocols the devices must support.
Bluetooth implements frequency-hopping, where the devices communicate at one of 79 carriers spaced at 1 MHz in the 2.4 GHz unlicensed band for 625 microseconds (µs), then hop to a different carrier for 625 µs, then to another, in a repeating pattern known to both devices. A particular hop sequence is called a channel, and is identified by an access code.
This is called Frequency-Hopping Spread Spectrum (FHSS), since hopping between 79 carriers spreads energy across spectrum 79 times wider than one carrier. It has the advantage of reduced sensitivity to noise or fading at any particular carrier.
If different pairs of devices are using different hop sequences, they can communicate at the same time in the same place without interfering. There are security advantages if the hop sequence can not be determined by a third party.
The initiator of communications is called the master. It determines the frequency hopping pattern, when the pattern begins, when a packet begins and when a bit begins. The packet and bit timing is based on the master’s clock, which ticks every 312.5 microseconds. Two ticks make a slot. A slot corresponds to a hop. The master transmits and the slave listens in even-numbered slots; vice-versa in odd-numbered slots.
To establish the channel, the master derives a channel access code from its Bluetooth address, and indicates the code to the slave at the beginning of every packet. Both master and slave use this to determine the actual frequency-hopping sequence.
Data is organized into Bluetooth packets for transmission. Packets can be 1, 3 or 5 slots long. A bit rate of 2 Mb/s would mean Bluetooth packets are about 150, 450 or 750 bytes long.
Discovering other devices means sending requests in packets on pre-defined channels called inquiry scan channels. Making a device discoverable means it listens on the inquiry channels, and responds to inquiries with information like its Bluetooth address, name and capabilities. This results in a list of Bluetooth devices displayed on the discovering device, such as a smartphone.
Connecting to a device means paging the device on its paging channel, a channel with access code derived from the target’s Bluetooth address. Devices listen on their paging channel, and respond to pages to establish a session. Once the session setup protocol is completed on the paging channel, the devices begin communicating on the channel defined by the master.
The frequency hopping pattern can be adapted to skip carriers where the signal to noise ratio is permanently low, to improve overall performance.
I hope you’ve enjoyed this tutorial!
This discussion is covered in the following Teracom training courses:
• DVD-Video Course V6: Understanding Wireless
A volunteer project to set up WiFi in a 150-year-old building with stone walls that I did recently required repeaters, also known as range extenders.
I ended up writing detailed instructions to get a popular WiFi access point / router on Amazon working as a repeater… and thought you might find this useful to extend WiFi coverage in your home or small office.
Even if you don’t need to extend your WiFi coverage, understanding the configuration, including the IP addresses, DHCP, subnets and all the other items covered in this tutorial is career-enhancing knowledge.
The instructions weren’t very complete, so I looked at the product’s Q&A section on Amazon and found instructions.
But those instructions turned out to be not quite right. And being an Engineer, I couldn’t help but proposing correct instructions…
These instructions assume you are connecting the WiFi access point / router pictured, TP-LINK model TL-WR841N, to any WiFi with a working Internet connection.
[example] = example values used during my setup.
Yours might be a bit different.
SOURCE-AP = the access point / router generating the wireless signal you want to repeat. This is often supplied by your ISP.
REPEATER-AP = the access point / router repeating the wireless signal, the one that we are setting up.
SOURCE-NET = the SSID (network name) of the wireless signal you want to repeat.
REPEATER-NET = the SSID (network name) of the repeated wireless signal.
GUI = Graphical User Interface.
This is the access point / router’s control panel.
Before starting, gather the following information:
– The LAN/wireless side IP address of the SOURCE-AP GUI. [192.168.3.1]
– The username and password for the SOURCE-AP GUI.
– The subnet the SOURCE-AP is using on the LAN/wireless side. [192.168.3.x]
– The SOURCE-NET name [GROUND]
– The encryption type and password [WPA-2 PERSONAL, xxxx]
– The channel the wireless signal to be repeated is on. 
If you don’t know the channel, you can find out during the setup below. However, it is preferable to log in to the SOURCE-AP GUI and set the channel to 3 instead of “auto” so it does not change, and uses an unpopular channel likely to have less interference.
To determine the LAN/wireless IP address and subnet of the SOURCE-AP, look at the IP address and default gateway of a device directly connected to the SOURCE-AP. (Open the Network connections folder, click change adapter settings, and view status and then details in Windows). The value in the default gateway field is the IP address of the SOURCE-AP GUI. The part of the address common to the default gateway and the device is the subnet ID.
Do this setup and get it working somewhere comfortable near the SOURCE-AP. Once it’s working, you can place the repeater anywhere near an electrical outlet.
Here we go:
1. Plug the power into the REPEATER-AP. If any settings have already been changed on the device, press and hold the reset button on the back for ten seconds until all lights are illuminated to indicate reset happening. Reset is not necessary if the unit is fresh out of the box.
2. Plug a PC into a LAN port on the REPEATER-AP with the supplied LAN patch cable. I used my laptop. Make sure the LAN adapter is set to get an IP address automatically. (Open the Network connections folder, click change adapter settings, and view properties in Windows). Make sure the LAN adapter is the only one enabled. Disable the wireless adapter.
3. Open a browser and go to http://tplinklogin.net . This gets you to the GUI of REPEATER-AP, initially 192.168.0.1. The default username, password is admin, admin. Don’t do the quick setup.
4. Click “Wireless” on the left column menu.
On the Wireless Settings page that appears:
a. Under the dropdown list for “Channel”, select the channel the wireless signal to be repeated is on.  If you don’t know, skip this step and the unit will force you to select the correct one after the “Survey” step below.
b. Click the “Enable WDS bridging” checkbox.
c. Click “Survey”. A list of SSIDs appears. Click “connect” on the one that is SOURCE-NET. [GROUND] All of the fields are automatically populated except for the password.
d. Enter the password and click Save. Wait ten seconds for the processing to finish.
e. At the top of the page beside Wireless Network Name, enter a name for REPEATER-NET [R1] and click Save.
5. Click “Wireless Security” on the left column menu. Select Personal WPA2-PSK, AES encryption and enter a password for REPEATER-NET.
6. Click “DHCP” on the left column menu. Click the DHCP disable radio button. Click Save. Ignore the reboot warning.
7. Click “Network” on the left column menu.
8. Click LAN. Change the IP address to one in the SOURCE-AP subnet that is not being used by any other device and click Save [192.168.3.200]. A reboot warning will appear. Click OK and let the unit reboot.
9. The address in the browser will magically change to the IP address you entered in the previous step. This is the new IP address for the GUI on REPEATER-AP. You will be prompted to log in again. The status screen will appear. Under Network, click the WAN MAC menu item on the left.
You should also now have Internet through REPEATER-AP!
Open news.google.com in a new tab in your browser to verify.
Wireless devices can now connect to REPEATER-NET.
Wired devices can connect to REPEATER- AP.
Both get Internet access through SOURCE-AP.
Ain’t life grand?
10. To avoid problems with dynamic addresses and timeouts, make the IP address of REPEATER-AP static.
Open a new tab in your browser. Enter the address of the SOURCE-AP GUI [192.168.3.1] and log in. Find the screen that lets you assign static IP addresses. The SOURCE-AP could be any brand of device; it is often supplied by your ISP. The function might be called “DHCP reservations” or “IP address reservation”. Make a new entry, with the WAN MAC address displayed in the REPEATER-AP GUI and the REPEATER-AP IP address you entered in Step 8.
I actually set up a chain of four of these units to provide wireless coverage from the basement to the fourth floor of a 150-year-old building with stone walls. And it worked!
P.S. Don’t forget to go back in to REPEATER-AP and change the password. The menu item is hiding under System Tools on the left.
Notice required by the legal department: This information is provided as general background information only. Design and implementation of a communication system requires professional advice to identify and resolve issues specific to that particular system, including but not limited to performance, availability and security issues. Additionally, while we have strived to be as accurate as possible, we make no representation or warranty that the information provided is 100% accurate. This information is not to be relied upon as professional advice, nor is it to be used as the basis of a design. Users of this information agree to hold the author and Teracom Training Institute Ltd. harmless from any liability or damages. Acceptance and use of this information shall constitute indication of your agreement to these conditions.
In this post, we take a closer look at the third development: MPLS has replaced ATM for traffic management, achieving another long-held goal in the telecommunications business, called convergence or service integration.
A long-held goal in the telecommunications business has been to transport and deliver all types of communications on the same network and access circuit, and in an ideal world, with a single bill to the customer. This idea is sometimes called convergence, though service integration is a more accurate term.
It results in a large cost savings compared to different networks, access circuits and bills for each type of communications.
In days past, this was not the case.
A residence would have at least two entry cables: twisted pair for telephone and coax for television, and separate bills for each.
The situation was even worse and more expensive in the case of a medium or large organization.
At each location, a typical organization would have the requirement to communicate
• Telephone calls to/from the PSTN,
• Telephone calls to/from other locations of the organization,
• Data to/from other locations of the organization, and
• Data, video and possibly voice to/from the Internet.
In days past, the organization might have had four physical access circuits and services – along with four bills:
• ISDN PRI over T1 to a LEC for telephone calls to/from the PSTN,
• Tie lines or a voice VPN with a custom dialing plan from an IXC for telephone calls to/from other locations of the organization,
• Dedicated T1s from an IXC for data to/from other locations of the organization, and
• DSL, Cable or T1 access from an ISP for data, video and possibly voice to/from the Internet.
Not only did this mean four services and four access technologies and four bills for the customer, it also meant the carrier had to implement and support four network technologies… a very expensive situation.
The solution to integrate all of this onto one access circuit and one network is twofold:
At the source,
• Format all types of traffic the same way, and
• Paste an identifier on the front of each piece of traffic, indicating what it is and where it goes.
Then all traffic can be carried interspersed on the same access circuit and in the same network, which results in a huge cost savings for both the customer and the carrier.
The identifier on the traffic is used to both route the traffic to the correct destination, and manage the traffic in the network, performing functions like load balancing, prioritization and restoration.
Starting in the 1980s, telephone companies and equipment manufacturers attempted to implement this with a technology called Asynchronous Transfer Mode (ATM). Literally billions of dollars were spent developing and deploying ATM from 1980 to 2000… but it failed and died, becoming too complex and too expensive, and not used for voice at the big telephone companies.
Multiprotocol Label Switching (MPLS) combined with IP has succeeded where ATM failed and is now universally implemented.
Of course, there is a lot of jargon to learn and many components to the “MPLS” story.
Here is a VERY brief explanation:
• All traffic is formatted into IP packets by the equipment that generates it, for example, a telephone or computer.
• Traffic is categorized into classes. A class of traffic goes from the same place to the same place and experiences the same transmission characteristics like delay and lost packets.
• A packet is identified as belonging to a particular class by pasting a number called a label on the front of the IP packet.
• The device that does the classification and labeling of packets is the ingress device, called a Label Edge Router in MPLS. It is normally Provider Equipment (PE), meaning owned and furnished by the service provider, located at the customer premise.
• Network equipment, called Label Switching Routers in MPLS, use the label number to route and in some cases prioritize the packet.
• Labels can be stacked, meaning one label pasted in front of another. This allows the network to manage similar kinds of traffic as a single entity in network control systems.
Returning to our example illustrated above, the four circuits illustrated at the top of the diagram can be replaced with one access circuit with three traffic classes (three labels). The physical access circuit could be 10 Mb/s to 10 Gb/s Optical Ethernet.
The three traffic classes / labels would be:
• A traffic class for telephone calls. This might be called a “SIP trunking service” by the marketing department. This class will carry VoIP phone calls to the carrier for communication to other locations of the organization, or for conversion to traditional telephony for phone calls to the public telephone network.
• A traffic class for data. This might be called a “VPN service” by the marketing department. This class carries file transfers, client-server database communications and the like securely to other locations of the organization.
• A traffic class for Internet traffic. This class carries anything in IP packets to the Internet.
All of this traffic is IP packets interspersed over the single access circuit.
At the other end of the access circuit, the carrier uses the label to route the traffic onward and possibly prioritize it to assure the appropriate service level.
The result is all of the organization’s traffic carried over a single access circuit, using a single technology.
This is one of the Holy Grails of the telecommunications business, called convergence or service integration, having significant advantages in cost and flexibility.
This is a concise description of a story that has many different facets.
In Teracom training, this discussion comes AFTER many other lessons explaining all of the underlying concepts, related technologies like PRI and SIP trunking and their jargon.
If you would like the whole story, it is currently included in the following training:
In this post, we take a closer look at the second one: Optical Ethernet has replaced SONET for all new core fiber network projects, and is also routinely used for “last mile” connections, achieving a long-held goal in telecommunications: one technology for all parts of the network.
Ethernet was a brand name for the first LAN, developed at Xerox’s Palo Alto Research Center in Silicon Valley. The mouse and the graphical user interface used in Windows and Macs appear to have also been invented there. And people say Xerox never does anything original…
An almost-identical technology was subsequently codified in the 802 series of standards from the Institute of Electrical and Electronic Engineers (IEEE). Products conforming to the IEEE 802 standards ended up dominating the market, and Ethernet no longer exists. When people say “Ethernet” today, they are referring to IEEE 802 standards.
Ethernet moves frames of data between computers that are on the same physical circuit. A frame is a block of data, typically about 1500 bytes, prefaced by the address of the receiver, the address of the sender and control information, followed by an error check.
The addresses are Media Access Control (MAC) addresses, 48-bit numbers identifying the LAN chip in each computer. LAN frames are also called MAC frames.
In the beginning, many computers were connected together by tapping onto a coaxial copper-wire “bus” cable.
Today, one computer is connected with a LAN cable to one port on a LAN switch as illustrated in the diagram. The LAN switch moves frames internally from one port to another, and hence from one computer to another.
Ethernet was developed for communicating data packets between computers inside a building, in a bursty, as-needed manner.
Ethernet then escaped and took over the world of fiber connections between buildings, replacing the previous technology used for fiber backbones called SONET.
SONET carried 64 kb/s streams of bits called DS0 channels on fiber between buildings. It was designed to carry phone calls in these channels. It can also carry data packets on these channels. But using channels for communications is not efficient, since the bits in the channel are reserved whether there is anything to transmit or not, and the channels only go between fixed places.
The new-generation all-IP telecom network does not use channels. Everything is put in IP packets, which are created and transmitted only when there is information to be communicated, and routed one-by-one to different destinations. This is more efficient and much more flexible.
Packets are transmitted from the originating machine in a MAC frame on a physical circuit to a router, then to the next router in another city, to the next router, and finally delivered in a MAC frame on a physical circuit to the destination.
The connections between routers in different cities are LAN cables… but not the familiar blue copper-wire LAN patch cables used in-building. Inter-city LAN cables are made of glass fiber. A MAC frame is signaled from one end to the other by pointing a laser into the fiber and turning it on and off. Light on means “1” and light off means “0”. This is called Optical Ethernet, and allows much higher bit rates and much longer reach than copper wire LAN cables.
Today, Optical Ethernet is not used just for inter-city links, but also for the access circuit, the circuit from the customer to the network, sometimes called the “last mile”.
The use of Ethernet for in-building communications, access circuits and intercity backbones represents the achievement of a long-held goal in the telecommunications business: to save money by using the same technology in all parts of the network.
This is a concise description of a story that has many different facets. If you would like to learn more, for example, the relationship between Ethernet and IP, how packets and frames work together, the difference between a LAN switch and a router, why Ethernet is “Layer 2” and IP is “Layer 3”, about LAN cables and fiber optics, convergence and service integration, those topics and much more are covered in the following Teracom training:
Eight major developments and trends in telecom that you need to know about
Teracom’s training represents the core knowledge set required for the telecom business. We’ve been teaching people the fundamentals of telecom and networking since 1992, so there have been many changes to the core knowledge set, and updates to our training over the years!
For the new school year, we have updated our core training yet again, with some significant shifts. For example, Voice over IP is now part of the fundamentals, and channelized systems like T1 and SONET are now referred to as “legacy technologies” for the first time ever.
Here’s a summary of the recent developments and trends in telecommunications that triggered these updates:
1. All new phone systems are VoIP. SIP trunking services replace PBX / PRI trunks from LECs.
2. Optical Ethernet has replaced SONET for all new core fiber network projects, and is also routinely used for “last mile” connections, achieving a long-held goal in telecommunications: one technology for all parts of the network.
3. MPLS has replaced ATM for traffic management on carrier networks, achieving another long-held goal: convergence and service integration… one network service, one access circuit, one bill for all telecom services.
4. 4G LTE has achieved the goal of a worldwide standard for mobile wireless.
5. “Data” on cellular plans means Internet access. It can be used for phone calls, video on demand, web surfing, real-time traffic on maps or any other application. Cellular data plans can be replaced with WiFi, which is often free.
6. Broadband carriers, also known as Cable TV companies, have evolved into telecom companies, gaining a majority share of residential Internet access in the USA, and providing services to business using both cable modems and fiber.
7. Telephone companies provide Cable TV service using Fiber to the Neighborhood and VDSL over loops in brownfields, and often Fiber to the Premise in greenfields.
8. In the future, the Internet and the telephone network will be the same thing. Basic telephone service will be “IP dial tone”: the ability to send an IP packet to any other point on the network. There will be no such thing as “long distance”.
To explore and understand these developments in more detail, while getting a firm grounding in the fundamentals and installed base…
“I really appreciated the telecommunications training course provided by Teracom Training Institute. I did learn a lot and understand things better, so that I am now able to tie everything together to understand all the facets of Telecommunications. Many of the acronyms, technologies, network designs and services – I would have no idea what they meant if it were not for this class. Thanks, I really enjoyed it.”
— Natasha White, Comcast, West Chester PA
The term “port” crops up in IP networking, particularly in the context of rules in routers and software firewalls. One hears about “opening a port on a firewall” and “TCP ports” and “UDP ports”.
So just what is a “port”, exactly?
Like about 40% of the words in English after the Norman invasion of southern England following the Battle of Hastings in 1066, the English word “port” is French. Une porte is a door.
Of course, the French got it from Latin: porta (gate, door). The Latin word portus (port, harbor, and earlier, entrance, passage) and the Greek word poros (journey, passage, way) are obviously related.
In the computer hardware business, a port is a doorway into the machine: a jack, where a cable can be connected. In days past, there were serial ports and parallel ports on PCs. Today, we have USB ports and LAN ports. Technicians talk about connecting customers to ports on access equipment, for example, equipment with banks of modems.
In the computer software business, a port can be thought of as a doorway into the software running on the machine, a passageway to a specific computer program running on the computer.
Why is this necessary? Since there can be many computer programs (a.k.a. applications, apps) running on the same computer at the same time, when trying to communicate to a particular program, we require a mechanism to identify it, a way of telling the host computer to which program to relay our communications.
For example, we all know that it’s possible to have multiple applications using the Internet connection on a computer at the same time. Think of an Outlook email program and a Chrome browser program running at the same time on a PC connected to the Internet.
When data arrives at this computer, how does the computer know whether this data is for the email program or for the browser program? And how does it convey the data to the correct program?
The answer: every program is assigned a number called a port number. Your browser is assigned port 80, for example.
Here’s how it works: the sending program creates a message and tags it with the port number identifying the program it wishes to communicate with on the destination computer. This is put in a packet that is tagged with the network address (IP address) of the destination host computer and transmitted. When the packet arrives at the destination computer identified by the IP address, this receiving computer looks at the destination port number and parks the message in a memory space associated with that port number. The program on the destination computer assigned that port number is constantly checking that memory space to see if there is anything new waiting for it.
The result is the ability for a computer program running on one computer to communicate with a specific computer program on another computer.
Visiting our warehouse service a couple of weeks ago, I was struck by the analogy possible between the idea of computer ports and a multi-tenant warehouse, so whipped out my Android smartphone and took a picture with the totally cool panoramic feature.
The warehouse is analogous to the host computer. It has a single street address. It handles goods for multiple users. Users have space allocated inside the warehouse. The warehouse has (on this side) six ports, also called loading docks. Each port has a number. A user can be assigned a port, either temporarily or permanently.
To communicate goods to that user, they’re carried in a shipping container (IP packet) on a truck (Ethernet frame) over a road (LAN cable) to the warehouse at its street address (IP address). To get the contents of the shipping container delivered to the correct user, the truck is backed up to the appropriate loading dock (port) identified by its door number (port number) and the contents of the container are unloaded to the space behind that port.
In computer communications today, the port number is 16 bits long, and the source and destination port number are populated at the beginning of the transport layer header, Layer 4 of the OSI model. The world’s most popular standard protocols for implementing the transport layer are the TCP (Transmission Control Protocol) and UDP (User Datagram Protocol).
Hence, one hears of “TCP ports” and “UDP ports”, particularly when configuring rules for packet forwarding on a router or firewall. When one “blocks” a port, that means that communication to a particular computer program is denied. When one “opens” a port, communication to that computer program is being allowed.
Standard practice is to allow communications only to specifically-identified ports and deny all other communications.
The port number of the application and the IP address of the host computer concatenated together is called a socket in UNIX and IP and is called a transport service in the OSI model. The result is the ability to identify the specific source computer program on one computer and the specific desired destination computer program on a different computer.
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: 16 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 Facebook page, our Google+ 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.
Figure 1 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 6 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.
The tutorial is part of the text and one graphic from Lesson 11 “TCP/IP over MPLS”. The Online Course when released at the end of March will have extensive animations following along with a voiceover of the text. Enjoy!
In this course, we cover wireless, concentrating mostly on mobile communications.
We’ll cover the principles of operation, jargon and buzzwords in the mobility business, the idea behind cellular radio systems, and explain the different spectrum-sharing technologies, including 1G analog FDMA, 2G TDMA/GSM vs. CDMA, 3G 1X vs. UMTS CDMA and 4G OFDMA.
We’ll conclude with a lesson on 802.11 wireless LANs (Wi-Fi) and a lesson on satellite communications.