As mentioned at the end of Fiber Optics Technology 101 - Part I, I will be covering how fiber optics is used to connect various local telephone switching systems together, how it is used in undersea cables, as well as its use in CATV and delve a little bit into Fiber To The Home (or FTTH). Part III will cover Fiber To The Home in more depth.
- Public Switched Telephone Network or PSTN -
While almost everyone's home or business telephone is connected to the central switching office (or CO) by a twisted pair of copper wires, the switching offices themselves connect to each other and to the various long distance carriers via fiber optic cables. Fiber has replaced all of the copper lines and most of the microwave systems that were used in the past to connect CO's together or to carry connections to and from long distance carriers.
As the sophistication of the electronic switching system (or ESS) used has increased, the actual number of them required for phone call routing has decreased. In many cases, where there was once a switching system in every town (or a large number of them in a single city), the new ESS can each handle many more times the number of phones than the older mechanical switching systems or early electronic switches. It's not uncommon to find a single ESS servicing a large number of towns. A fiber optic cable connects the ESS to what is called a concentrator, which is basically a combination optical-to-electronic and electrical-to-optical converter. The concentrator is usually located in the old central office where the mechanical switching systems had been housed. All of the twisted pairs that connect the customer telephones are still there, but now they connect to the concentrator. All of the call switching is done back at the ESS via the concentrator. This reduces the cost of operating the phone system while it adds more features for the customers (Call Waiting, Caller ID, etc.)
While the concentrators are usually connected to the ESS at a CO in a hub and spoke configuration, the CO's themselves are often (though not always) connected together in a loose mesh or ring configuration, allowing for redundant paths between CO's, allowing for quick restoration of service if there is a failure of any fiber connection between them. Depending upon the amount of phone traffic being carried between CO's and to and from the long distance carriers, the fiber links may be on multiple fibers or will use Coarse or Dense WDM to increase traffic capacity.
- Undersea Cables -
Undersea cables to carry messages have been around since the late 1800's. The first intercontinental telegraph cable between the US and England was run along a ridge in the North Atlantic. It carried a single telegraph channel and failed not long after it was completed. The undersea cables used today are now high capacity fiber optic cables which replaced the massive copper cables used by telephone companies for decades. The physical size of the fiber optic cables is a fraction of that of the old copper cables, meaning it is easier to manufacture and transport as well as less expensive to make. Like the old copper cables, the fiber cables require amplifiers along the cable in order to maintain signal strength and fidelity over long distances. The amplifiers, called EDFAs (or Erbium Doped Fiber Amplifiers), are spliced in approximately every 50 kilometers along the cable. Normally amplification is not required after such a short run. Long-haul fibers can go hundreds of kilometers between amplifiers, but due to the harsh conditions and the expense of pulling up undersea cables in order to make repairs, the cables have quite a bit of redundancy built in. With a 50 kilometer spacing, three or four adjacent amplifiers can fail and the system will still remain in operation with no interruption of the data flow.
Undersea fiber cables will have up to a couple of dozen fiber pairs. Each fiber pair is capable of carrying multiple wavelengths using DWDM, giving these cables massive bandwidth capabilities.
Most undersea cables are laid out in a rough 'ring' configuration, allowing for redundant paths in case one cable fails or is removed from service for maintenance, upgrade, or repair.
Not all undersea cables are used for long-haul circuits. Many are run along coastlines to interconnect widely separated coastal cities. This has been done along the coast of South and Central America, Australia, and Africa. It is less expensive to run undersea cable in relatively shallow waters than trying to install terrestrial fiber cables through jungles, rain forests, over mountain ranges, or through deserts. With these shorter span cables little, if any, amplification is required, greatly reducing the cost of the cable. These short span cables can also have a larger number of fiber pairs due to the fact that they usually don't require expensive amplifiers and won't be sitting on the ocean floor hundreds of meters below the surface of the ocean. The short span cables make landfall at a number of places along the coastline where the traffic they carry can be routed to terrestrial fiber cables, telephone, and/or data systems. Or the traffic can continue along the next stretch of undersea cable after being amplified at the landfall facility.
- Cable Television Systems -
Cable TV, or CATV, has been around for decades. Over that time the capabilities of CATV systems has increased dramatically. Where originally CATV was used to bring network television broadcasts to homes that were normally incapable of picking them up due to distance or terrain, CATV systems now carry a couple of hundred channels as well as Internet and telephone services to their customers. Most CATV operators are now called Multiple Services Operators, or MSOs. Comcast, Harron, and Cox are all examples of MSOs.
What makes it possible for MSOs to offer all of these services is fiber optics.
In the past, CATV operators would have antenna arrays or satellite dishes that would receive the various broadcasts. These signals were then fed down a coaxial cable from the CATV head end (where all of the receivers, amplifiers, and control equipment are located). The coaxial cable would then feed other coaxial cables that ran through a community and to each customer's home. Along the way the signals coming down the coaxial cable would be amplified to make up for cable losses and loss each time the signals were divided and fed to other runs of coaxial cable. This type of system required a considerable amount of coaxial cable and a large number of amplifiers to keep the signals strong enough for use by the customers. The downsides to this system were the expense of the copper coaxial cables and amplifiers, the electricity to power the amplifiers, as well as the fact that the systems were for the most part one-way systems. That means that signals came down from the head end to the customer, but not the other way. This limited the types of services that CATV operators could offer in the past.
Today, MSOs use a combination of fiber and coaxial cable to bring services to their customers. These are called Hybrid Fiber/Coaxial systems, or HFC. The addition of fiber to the system allows for two things - low loss connections from the head end to the neighborhood and a return path from the customer to the head end. Fiber between the head end and what is called a node removes the need for expensive coaxial cable and amplifiers while increasing the bandwidth available to carry more TV programming, data for Internet connections and telephone services.
Here is an example of what a traditional Coaxial and Hybrid Fiber-Coaxial systems look like:
A node is the 'black box' that converts the optical signals on the fiber from the head end to an RF signal that is fed into the coaxial cables that run through a neighborhood and to the customers, as well as converting RF signals from the customers back to optical signals that return to the head end on a second fiber. Each node services up to 1000 customers. Depending upon the demand for Internet and phone services, each node can be divided to service a smaller number of customers. All that's required is another pair of fibers between the head end and the node.
For a somewhat more in depth explanation of HFC, click here. It also includes the use of DWDM in HFC systems in order to reduce the number of fibers required to provide services to customers.
- Fiber To The Home -
This is where things get really interesting. While the concept of Fiber To The Home (or FTTH) has been around for a while, it had not been widely used due to the high costs of the supporting equipment. But that has all changed.
One of the big advantages of FTTH is that it provides a large amount of bandwidth to the average consumer as well as providing video services along the lines of a standard CATV system. It helps matters that three the three Regional Bell Operating Companies (or RBOCs) - AT&T, Qwest, and Verizon, have agreed to standards for deployment of FTTH, meaning that equipment manufacturers will be able to provide the necessary equipment at a much lower cost due to high volumes.
So what exactly is Fiber To The Home?
As the name implies, FTTH brings a fiber optic network connection directly to the home. This connection will provide a high speed data link - 1250 Mbit/sec download link and 622 Mbit/sec upload link - for the consumer (different systems use different data speeds, so you mileage may vary). The link will provide both data (Internet) and phone services. There will also be a video downlink that will provide a couple of hundred TV channels in both Standard and High Definition formats.
I won't go into depth on FTTH at this point as it is a rather broad subject. I will cover it in Part III next week.
- Public Switched Telephone Network or PSTN -
While almost everyone's home or business telephone is connected to the central switching office (or CO) by a twisted pair of copper wires, the switching offices themselves connect to each other and to the various long distance carriers via fiber optic cables. Fiber has replaced all of the copper lines and most of the microwave systems that were used in the past to connect CO's together or to carry connections to and from long distance carriers.
As the sophistication of the electronic switching system (or ESS) used has increased, the actual number of them required for phone call routing has decreased. In many cases, where there was once a switching system in every town (or a large number of them in a single city), the new ESS can each handle many more times the number of phones than the older mechanical switching systems or early electronic switches. It's not uncommon to find a single ESS servicing a large number of towns. A fiber optic cable connects the ESS to what is called a concentrator, which is basically a combination optical-to-electronic and electrical-to-optical converter. The concentrator is usually located in the old central office where the mechanical switching systems had been housed. All of the twisted pairs that connect the customer telephones are still there, but now they connect to the concentrator. All of the call switching is done back at the ESS via the concentrator. This reduces the cost of operating the phone system while it adds more features for the customers (Call Waiting, Caller ID, etc.)
While the concentrators are usually connected to the ESS at a CO in a hub and spoke configuration, the CO's themselves are often (though not always) connected together in a loose mesh or ring configuration, allowing for redundant paths between CO's, allowing for quick restoration of service if there is a failure of any fiber connection between them. Depending upon the amount of phone traffic being carried between CO's and to and from the long distance carriers, the fiber links may be on multiple fibers or will use Coarse or Dense WDM to increase traffic capacity.
- Undersea Cables -
Undersea cables to carry messages have been around since the late 1800's. The first intercontinental telegraph cable between the US and England was run along a ridge in the North Atlantic. It carried a single telegraph channel and failed not long after it was completed. The undersea cables used today are now high capacity fiber optic cables which replaced the massive copper cables used by telephone companies for decades. The physical size of the fiber optic cables is a fraction of that of the old copper cables, meaning it is easier to manufacture and transport as well as less expensive to make. Like the old copper cables, the fiber cables require amplifiers along the cable in order to maintain signal strength and fidelity over long distances. The amplifiers, called EDFAs (or Erbium Doped Fiber Amplifiers), are spliced in approximately every 50 kilometers along the cable. Normally amplification is not required after such a short run. Long-haul fibers can go hundreds of kilometers between amplifiers, but due to the harsh conditions and the expense of pulling up undersea cables in order to make repairs, the cables have quite a bit of redundancy built in. With a 50 kilometer spacing, three or four adjacent amplifiers can fail and the system will still remain in operation with no interruption of the data flow.
Undersea fiber cables will have up to a couple of dozen fiber pairs. Each fiber pair is capable of carrying multiple wavelengths using DWDM, giving these cables massive bandwidth capabilities.
Most undersea cables are laid out in a rough 'ring' configuration, allowing for redundant paths in case one cable fails or is removed from service for maintenance, upgrade, or repair.
Not all undersea cables are used for long-haul circuits. Many are run along coastlines to interconnect widely separated coastal cities. This has been done along the coast of South and Central America, Australia, and Africa. It is less expensive to run undersea cable in relatively shallow waters than trying to install terrestrial fiber cables through jungles, rain forests, over mountain ranges, or through deserts. With these shorter span cables little, if any, amplification is required, greatly reducing the cost of the cable. These short span cables can also have a larger number of fiber pairs due to the fact that they usually don't require expensive amplifiers and won't be sitting on the ocean floor hundreds of meters below the surface of the ocean. The short span cables make landfall at a number of places along the coastline where the traffic they carry can be routed to terrestrial fiber cables, telephone, and/or data systems. Or the traffic can continue along the next stretch of undersea cable after being amplified at the landfall facility.
- Cable Television Systems -
Cable TV, or CATV, has been around for decades. Over that time the capabilities of CATV systems has increased dramatically. Where originally CATV was used to bring network television broadcasts to homes that were normally incapable of picking them up due to distance or terrain, CATV systems now carry a couple of hundred channels as well as Internet and telephone services to their customers. Most CATV operators are now called Multiple Services Operators, or MSOs. Comcast, Harron, and Cox are all examples of MSOs.
What makes it possible for MSOs to offer all of these services is fiber optics.
In the past, CATV operators would have antenna arrays or satellite dishes that would receive the various broadcasts. These signals were then fed down a coaxial cable from the CATV head end (where all of the receivers, amplifiers, and control equipment are located). The coaxial cable would then feed other coaxial cables that ran through a community and to each customer's home. Along the way the signals coming down the coaxial cable would be amplified to make up for cable losses and loss each time the signals were divided and fed to other runs of coaxial cable. This type of system required a considerable amount of coaxial cable and a large number of amplifiers to keep the signals strong enough for use by the customers. The downsides to this system were the expense of the copper coaxial cables and amplifiers, the electricity to power the amplifiers, as well as the fact that the systems were for the most part one-way systems. That means that signals came down from the head end to the customer, but not the other way. This limited the types of services that CATV operators could offer in the past.
Today, MSOs use a combination of fiber and coaxial cable to bring services to their customers. These are called Hybrid Fiber/Coaxial systems, or HFC. The addition of fiber to the system allows for two things - low loss connections from the head end to the neighborhood and a return path from the customer to the head end. Fiber between the head end and what is called a node removes the need for expensive coaxial cable and amplifiers while increasing the bandwidth available to carry more TV programming, data for Internet connections and telephone services.
Here is an example of what a traditional Coaxial and Hybrid Fiber-Coaxial systems look like:
A node is the 'black box' that converts the optical signals on the fiber from the head end to an RF signal that is fed into the coaxial cables that run through a neighborhood and to the customers, as well as converting RF signals from the customers back to optical signals that return to the head end on a second fiber. Each node services up to 1000 customers. Depending upon the demand for Internet and phone services, each node can be divided to service a smaller number of customers. All that's required is another pair of fibers between the head end and the node.
For a somewhat more in depth explanation of HFC, click here. It also includes the use of DWDM in HFC systems in order to reduce the number of fibers required to provide services to customers.
- Fiber To The Home -
This is where things get really interesting. While the concept of Fiber To The Home (or FTTH) has been around for a while, it had not been widely used due to the high costs of the supporting equipment. But that has all changed.
One of the big advantages of FTTH is that it provides a large amount of bandwidth to the average consumer as well as providing video services along the lines of a standard CATV system. It helps matters that three the three Regional Bell Operating Companies (or RBOCs) - AT&T, Qwest, and Verizon, have agreed to standards for deployment of FTTH, meaning that equipment manufacturers will be able to provide the necessary equipment at a much lower cost due to high volumes.
So what exactly is Fiber To The Home?
As the name implies, FTTH brings a fiber optic network connection directly to the home. This connection will provide a high speed data link - 1250 Mbit/sec download link and 622 Mbit/sec upload link - for the consumer (different systems use different data speeds, so you mileage may vary). The link will provide both data (Internet) and phone services. There will also be a video downlink that will provide a couple of hundred TV channels in both Standard and High Definition formats.
I won't go into depth on FTTH at this point as it is a rather broad subject. I will cover it in Part III next week.
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