WDM : Data is most easily multiplexed on fiber-optic cable by using time-division multiplexing (TDM), as in the T1 system or in the SONET system described later in this chapter. However, developments in optical components make it possible to use frequency- division multiplexing (FDM) on fiber-optic cable (called wavelength-division multiplexing, or WDM), which permits multiple channels of data to operate over the cable’s light wave bandwidth.
Wavelength-division multiplexing, another name for frequency-division multiplexing, has been widely used in radio, TV, and telephone systems. The best example today is the multiplexing of dozens of TV signals on a common coaxial cable coming into the home.
In WDM, different frequencies or “colors’’ of infrared light are employed to carry individual data streams. These are combined and carried on a single fiber. Although frequency as a parameter is more widely used to distinguish the location of wireless signals below 300 GHz, at light frequencies the wavelength parameter is the preferred measure.
Remember that the relationship between wavelength λ in meters and frequency f is f = c/λ, where c is the speed of light in a vacuum or 2.998 x 108 m/s. The speed of light in fiber cable is a bit less than that, or about 2.99 x 108 m/s. Optical wavelength is usually expressed in nanometers or micrometers. Optical frequencies are expressed in terahertz (THz), or 1012 Hz.
Data to be transmitted in a fiber-optic network is used to modulate (by OOK or ASK) a laser-generated infrared light. Infrared signals best match the light-carrying characteristics of fiber-optic cable, which has an attenuation response to infrared light such that the lowest attenuation (about 0.2 dB/km) occurs in two narrow bands of frequencies, one centered at 1310 nm and the other at 1550 nm.
Coarse Wavelength-Division Multiplexing
The first coarse WDM (CWDM) systems used two channels operating at 1310 and 1550 nm. Later, four channels of data were multiplexed. Fig. 19-42 illustrates a CWDM system. A separate serial data source controls each laser. The data source may be a single data source or a multiple TDM source. Current systems use light in the 1550-nm range. A typical four-channel system uses laser wavelengths of 1534, 1543, 1550, and 1557.4 nm. Each laser is switched off and on by the input data. The laser beams are then optically combined and transmitted over a single-fi ber cable. At the receiving end of the cable, special optical filters are used to separate the light beams into individual channels. Each light beam is detected with an optical sensor and then fi ltered into the four data streams.
Dense Wavelength-Division Multiplexing
Dense wavelength-division multiplexing (DWDM) refers to the use of 8, 16, 32, 64, or more data channels on a single fiber. Standard channel wavelengths have been defined by the International Telecommunications Union (ITU) as between 1525 and 1565 nm with a 100-GHz (approximately 0.8-nm) channel spacing.
The block of channels between about 1525 and 1565 nm is called the C or conventional band. Most DWDM activity currently occurs in the C band. Another block of wavelengths from 1570 to 1610 nm is referred to as the long-wavelength band, or L band. Wavelengths in the 1525- to 1538-nm range make up the S-band.
Current DWDM systems allow more than 160 individual data channels to be carried simultaneously on a single fiber at data rates up to 40 Gbps, giving an overall capacity of 160 x40, or 6400, Gbps (6.4 Tbps). The potential for future systems is over 200 channels per fiber at a data rate of 40 Gbps. Even more, channels can be transmitted on a single fiber as better filters and optical components called splitters become available to permit 50-, 25-, or even 12.5-GHz channel spacing.
The key components in a DWDM system are the multiplexer and demultiplexer and the multiwavelength lasers discussed elsewhere in this chapter. Numerous methods have been developed over the years to add and separate optical signals. Optical couplers can be used for multiplexing, and optical filters such as fiber Bragg gratings or thin films can be implemented for demultiplexing. But the one method that appears to be emerging as the most popular is the arrayed waveguide grating (AWG). This device is an array of optical waveguides of different lengths made with silica (SiO2) on a silicon chip, and it can be used for both multiplexing and demultiplexing.
Fig. 19-43 shows the concept of an AWG. The multiple inputs are fed into a cavity or coupler region that acts as a lens to equally divide the inputs to each of the waveguides. Every waveguide in the grating has a length L that differs from its neighbor by ¢L. This produces a phase difference in the beams coming out of the gratings into an output cavity.
The output cavity acts as a lens to refocus the beams from all the grating waveguides onto the output waveguide array. Any output contains the multiplexed inputs. For demultiplexing, the single multiwavelength input signal is applied to any of the inputs where the signals of different wavelengths propagate through the grating. The grating acts as multiple filters to separate the signals into individual paths that appear at the multiple outputs.
AWGs are popular because they are relatively easy to produce with standard semiconductor processes, making them very inexpensive. They have very low insertion loss and low cross-talk. A typical unit has 32 or 40 inputs and 32 or 40 outputs with 100-GHz spacing on ITU grid wavelengths. Some products have up to 64 or 80 input and output channels, and devices with higher channel counts are under development. AWG insertion loss is in the 2- to 4-dB range with 20 to 30 dB of adjacent channel cross-talk attenuation.
Passive Optical Networks
The primary applications for fiber-optic networks are in wide-area networks such as long-distance telephone service and the Internet backbone. As speeds have increased and prices have declined, fiber-optic technology has been adopted into metropolitan-area networks, storage-area networks (SANs), and local-area networks. The most widely used of these technologies—SONET, Ethernet, and Fibre Channel—have been discussed.
A newer and growing fiber-optic system is the passive optical network (PON), a type of metropolitan-area network technology. This technology is also referred to as fiber to the home (FTTH). Similar terms are fiber to the premises or fiber to the curb, designated as FTTP or FTTC. The term FTTx is used, where x represents the destination. PONs are already widely used in Japan and Korea. Here in the United States, both AT&T (formerly SBC) and Verizon already have PONs in place with more on the way.
The PON Concept
Most optical networking uses active components to perform optical-to-electrical and electrical-to-optical (OEO) conversions during transmission and reception. These conversions are expensive because each requires a pair of transceivers and the related power supply. Over long distances, usually 10 to 40 km or more, repeaters or optical amplifiers are necessary to overcome the attenuation, restore signal strength, and reshape the signal. These OEO repeaters and amplifiers are a nuisance as well as expensive and power-hungry.
One solution to this problem is to use a passive optical network. The term passive implies no OEO repeaters, amplifiers, or any other device that uses power. Instead, the transmitter sends the signal out over the network cable, and a receiver at the destination picks it up.
There are no intervening repeaters or amplifiers. Only passive optical devices such as splitters and combiners are used. By using low-attenuation fiber-optic cable, powerful lasers, and sensitive receivers, it is possible to achieve distances of up to about 20 km without intervening active equipment. This makes PONs ideal for metropolitan-area networks.
The PON method has been adopted by telecommunications carriers as the medium of choice for their very high-speed broadband Internet connections to consumers and businesses. These metropolitan networks cover a portion of a city or a similar-size area. PONs will be competitive with cable TV and DSL connections but faster than both. Using optical techniques, the consumer can have an Internet connection speed of 100 Mbps to 1 Gbps or higher. This is much faster than the typical 1- to 20-Mbps cable TV and DSL connections. And PONs make digital TV distribution more practical. Furthermore, they provide the extra bandwidth to carry Internet phone calls (VoIP).
There are several different types of PONs and standards. The earliest standard, called APON, was based upon ATM packets and featured speeds of 155.52 and 622.08 Mbps. A more advanced version, called BPON, features a data rate of up to 1.25 Gbps. The most recent version is a superset of BPON, called GPON, for Gigabit PON. It provides download speeds up to 2.5 Gbps and uploads speed up to 1.25 Gbps. One of the key features of GPON is that it uses encapsulation, a technique that makes it protocol-agnostic. Any type of data protocol including TDM (such as T1 or SONET) or Ethernet can be transmitted.
Fig. 19-44 shows a basic block diagram of a BPON/GPON network. The carrier central office (CO) serves as the Internet service provider (ISP), TV supplier or telephone carrier as the case may be. The equipment at the carrier central offi ce (CO) is referred to as the optical line terminal (OLT). It develops a signal at 1490 nm for transmission (download) to the remote terminals. This signal carries all Internet data and any voice signals as in Voice over IP (VoIP). If TV is transmitted, it is modulated on to a 1550 nm laser. The 1550 and 1490 nm outputs are mixed or added in a passive combiner to create a coarse wavelength division multiplexed (CWDM) signal. This master signal is then sent to a passive splitter that divides the signal into four equal power levels for transmission over the fi rst part of the network. In BPON the data rate is 622 Mbps or 1.25 Gbps but with GPON it is 2.5 Gbps.
Additional splitters are used along the way to further split the signals for distribution to multiple homes. Splitters are available to divide the power by 2:1, 4:1, 8:1, 16:1, 32:1, 64:1, and 128:1. One OLT can transmit to up to 64 destinations up to about 20 km. The upper limit may be 16 devices depending upon the ranges involved. Just remember that each time the signal is split, its power is decreased by the split ratio. The power out of each port on a 4:1 splitter is only one-fourth of the input power. Splitters are passive demultiplexers (DEMUX).
Note also, because the splitters are optical devices made of glass or silicon, they are bidirectional. They also serve as combiners in the opposite direction. In this capacity, they serve as multiplexers (MUX).
At the receiving end, each subscriber has an optical networking unit (ONU) or optical networking terminal (ONT). These boxes connect to your PC, TV set, and/or VoIP telephone. The ONU/ONT is a two-way device, meaning that it can transmit as well as receive. In VoIP or Internet applications, the subscriber needs to transmit voice and dialing data back to the OLT. This is done over a separate 1310-nm laser using the same fiber-optic path. The splitters are bidirectional and also work as combiners or multiplexers. The upload speed is 155 Mbps in BPON and 1.25 Gbps in GPON.
The latest version of GPON is 10-Gigabit GPON, called XGPON or 10G-PON. It can achieve 10 Gbps downstream on 1577 nm and 2.5 Gbps upstream on 1270 nm. These different wavelengths allow it to coexist with existing GPON services. The split ratio is 128:1. Most new installations will use XGPON. Data formatting is the same as for standard GPON.
Another widely used standard is EPON, or Ethernet PON. While BPON and GPON have been adopted as the North American PON standard, EPON is the de facto PON standard in Japan, Korea, and some European nations. EPON is one part of the popular IEEE Ethernet standard and is designated 802.3ah. You will also hear it referred to as Ethernet in the fi rst mile (EFM). The term first mile refers to that distance between the subscriber and any central offi ce. Sometimes it is also called the last mile.
The topology of EPON is similar to that of GPON, but the downstream and upstream data rates are symmetric at 1.25 Gbps. The downstream is on 1490 nm while the upstream is on 1310 nm. Standard Ethernet packets are transmitted with a data payload to 1518 bytes. A real plus for EPON is that because it is Ethernet, it is fully compatible with any other Ethernet LAN.
The latest version of EPON is designated 802.3av. It can achieve 10 Gbps upstream and downstream on wavelengths of 1575-1580 nm and 1260-1280 nm respectively. A 1-Gbps upstream version is available.
While PONs provide the ultimate in bandwidth and data rate for home broadband connections, they are expensive. Carriers must invest in a huge infrastructure that requires rewiring the area served, again. Fiber-optic cables are mostly laid underground, but that process is expensive because right-of-way must be bought and trenches dug. Cables can be carried overhead on existing poles at less expense, but the effect is less aesthetically pleasing.
FTTH is growing in popularity as more TV and video are delivered over the Internet. Verizon was first, with its FiOS service, but now Google, AT&T, and others are installing FTTH and delivering related video services as well as Internet connectivity up to 1 Gbps per user.
While some PON services, such as Verizon’s FiOS service, take the fiber directly to the home, other systems rely on the standard POTS twisted-pair cable that is already in place virtually everywhere. In such cases, the fi ber cable is run to a neighborhood terminal or gateway. Such a gateway may also serve an apartment complex or multiunit dwelling such as a condominium building. Then the signals are distributed over the standard twisted-pair telephone cable that is already in place. An advanced form of digital subscriber line called ADSL2 or ADSL2+ is used with a data rate to 24 Mbps. A more advanced version called VDSL2 is also used in some cases. It supports a data rate to 100 Mbps over shorter runs of twisted-pair cable.
40/100-Gbps Networks and Beyond
As the Internet has grown and the demand for more Internet services has increased, the need for higher network speeds has also become necessary. Most of the demand has come from the massive increase in video over the Internet as well as signifi cant increase in wireless network growth and its attendant demands for higher speeds. Local area networks have kept pace with the Gigabit Ethernet and 10 Gigabit Ethernet standards and equipment. Long-haul fiber networks have also attempted to keep pace with rate increases to 40-Gbps and 100-Gbps systems. Yet many say that the networks are still not fast enough. Work on 400-Gbps and 1-Tbps systems is ongoing. As this is written, 40-Gbps and 100-Gbps systems are emerging and becoming standards to meet the capacity and speed demands of the modern world. Most of these systems are fiber-based.
Earlier we summarized the various forms of 40- and 100-Gbps Ethernet as defined by the IEEE 802.3ba standard. The short-range versions are copper-cable-based. The longer-range versions are fiber-based as summarized in Table 19-1.
The basic format is 4 lanes of 10-Gbps signals for 40 Gbps and 10 lanes of 10 Gbps or 4 lanes of 25 Gbps for 100 Gbps. A lane as defined here refers to either single fiber or one light wavelength (λ) on a single fiber. While some 40-Gbps systems have been implemented, the greatest interest and focus is on 100-Gbps systems. That is the emphasis in this section.
The 100GBASE-SR10 standard defines 10 lanes of 10 Gbps for transmitting and another 10 for receiving. The lanes use OM3 fiber and 850-nm VCSEL lasers for a range of up to 100 meters, or OM4 cable that will achieve a range up to 125 meters.
The 100GBASE-LR4 standard is based upon dense wavelength-division multiplexing (DWDM). It uses four lanes or wavelengths on a pair of single-mode fibers, one for transmit and one for receive. The wavelengths used are 1295, 1300, 1305, and 1310 nm. The net data rate per wavelength is 25 Gbps. The effective range is up to 10 km.
The 100GBASE-ER4 is similar to the LR4 version but includes amplification to achieve the up to 40-km range. A single-lane (λ) version of 100 Gbps is being developed.
Fig. 19-45 shows the basic implementation of a 100GBASE-LR4 transceiver. The data source and destination are the 100 Gigabit Attachment Unit Interface (CAUI). C is the Roman numeral for 100. This is a 10 3 10 interface that links to the equipment being interconnected, such as a server, router, or switch. The upper section is the transmitter. It accepts 10 lanes of 10-Gbps data and translates them into 4 lanes of 25 Gbps with a 10:4 serializer. The 10:4 serializer is a complex circuit made up of shift registers and logic with proper clock timing. The four 25-Gbps signals are then sent to a modulator driver (MD) and then to an externally modulated laser (EML). The four 25-Gbps
signals are then multiplexed on single-mode fiber (SMF).
The lower receiver section takes in the DWDM signal and demultiplexes it into four 25-Gbps streams that are detected by the PIN photodiodes. The signals are amplified in transimpedance amplifiers (TIAs) and then sent to a 4:10 deserializer, which generates the 10 lanes of 10 Gbps for the CAUI interface.
Fig. 19-46 shows a version of a 100-Gbps transceiver using four lanes of 25 Gbps to and from the equipment. The input signals and clock are recovered by the clock and data recovery (CDR) circuit and then sent to a laser driver (LD), which feeds the directly modulated lasers (DML). The DML outputs are multiplexed on a single-mode fiber.
On the receive side, the four 25-Gbps DWDM signals are demultiplexed into four light streams that are detected by the PIN diodes and amplified in the TIA. The 25-Gb signals are then rejuvenated in the CDR before being sent to the equipment interface. A similar arrangement is being proposed for a 400-Gbps system using 16 lanes of 25-Gbps signals (16 x 25= 400).
Optical Transport Technology
The OTN standard does not define a specific physical layer method. Researchers have found that the off-on switching intensity modulation with the direct detection (IMDD) method of most optical systems is not the best for OTN. Instead a new coherent system for OTN transmission on SMF, a single wavelength, has been developed. It uses not only the optical signal amplitude but also its phase and polarization. This coherent method is called dual polarization—quadrature phase-shift keying (DP-QPSK). It uses a combination of technologies to transmit 100 Gbps with a 25-Gbaud rate. Two parallel serial bitstreams can be transmitted over a single fiber on a single wavelength of light by using two different light polarizations, one vertical and the other horizontal. The 90° of separation prevents one stream from interfering with the other and makes optical recovery easy. The result is a 2 bits/Hz data rate.
The two polarized streams are then processed with QPSK to produce another 2 bits/Hz result. The combination of dual-polarization plus the QPSK produces 4 bits/Hz. This allows electrical signals at the 25-Gbps rate to be used to generate a 100-Gbps signal on SMF.
Fig. 19-47 shows a transmitter for DP-QPSK. The 100-Gbps data is divided into four 25-Gbps data streams and applied to DACs to convert 2-bit pairs into four levels. These four levels modulate the single-wavelength laser beam. The laser beam, usually at 1550 nm, is equally split and sent to the four Mach-Zehnder (MZ) modulators along with the DAC outputs. Note that the output of one of the MZ modulators is delayed by 90° (π/2). The MZ modulator outputs are then combined to create the in-phase (I) and quadrature (Q) data streams. A polarization rotator shifts the upper stream by 90°, making the upper and lower streams in quadrature. A balanced beam combiner creates a single optical signal that is applied to the SMF cable. Note that all of the modulation process takes place optically. A variation of this method creates the quadrature polarization of the laser beam prior to the application to the MZ modulators.
A coherent receiver is shown in Fig. 19-48. The DP-QPSK signal from the fiber is applied to a balanced beamsplitter. The upper output goes to a device that rotates the polarization by 90°. The two polarized signals are then applied to two 90° optical hybrids. The other input to the hybrids is a laser beam at 1550 nm that is equally split and serves as the local oscillator (LO) for demodulation.
The 90° optical hybrids are the optical equivalents of electrical quadrature demodulators. The laser LO and polarized input signals are mixed, and four separate output streams are developed. These are applied to PIN photo diodes to recreate the original 25-Gbps data signals. These four signals are then digitized in fast ADCs, and the resulting digital signals are then processed in DSP circuits to recover the data as well as to perform chromatic and polarization mode dispersion compensation as required.
This coherent method may also be used to produce higher rates, such as 200 Gbps, 400 Gbps, or even 1 Tbps using QAM instead of QPSK.
100-Gbps MSA Modules
Pluggable modules for connecting equipment like servers, routers, and switches were discussed earlier in Sec. 19-4. These modules covered standards and data rates to 10 Gbps and 40 Gbps. Now, 100-Gbps multisource agreements (MSAs) are available. These include CFP, CXP, and CDFP. The CFP modules use WDM and are available in several forms, including a 10 x 10 Gbps for 100 Gbps, a 4 x 25 Gbps for 100 Gbps, and a 4 x 10 Gbps for 40 Gbps. Newer versions CFP2 and CFP4 use 8 x 25 Gbps and 4 x 25 Gbps lanes, respectively, for 100 Gbps. The CXP module uses parallel fibers of 10 x 10 Gbps for 100 Gbps. These modules primarily support the IEEE 802.3ba Ethernet standards 100GBASE-LR4/ER4/SR10 but can also handle OTN OTU4. The CDFP MSA module is defined for 400 Gbps. It is available in three basic formats: 16 x 25 Gbps, 8 x 50 Gbps, or 4 x100 Gbps.
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