Methods of Multiplexing Data
1. Conventional optical fiber communications systems employ optical fibers to transport information in optical telecommunication networks. An electrical signal carrying information is used to modulate the light emitted by an optical source, typically a laser diode. The modulated light is then propagated through an optical fiber link comprising, in modern systems, at least one erbium-doped fiber amplifier (EDFA1) and, in some systems, dispersion compensation modules (DCM2). The light emerging from the optical fiber link illuminates an optical detector converting the information encoded on the optical signal back into an electrical signal. In early development stages of optical communication systems, the only way to increase the bit rate was to increase the modulation speed of the laser. In the later evolution of multichannel fiber transmission systems, two distinct methods of multiplexing data have been introduced: wavelength division multiplexing (WDM3) and coherence division multiplexing (CDM4).
2. Currently, WDM communication systems are the only multichannel optical systems deployed commercially. To increase the optical fiber capacity, WDM communication systems employ multiple lasers and wavelength-selective passive components to multiplex and demultiplex a plurality of distinct optical channels onto a single fiber. A plurality of laser sources, each modulated by a single information channel, have distinct frequencies lying on an internationally agreed frequency grid, and are typically separated by 50, 100 or 200 GHz within the transparency range of the optical fiber.
3. A traditional WDM communication system comprises a plurality of WDM transmitters, a wavelength division multiplexer and a wavelength division demultiplexer interconnected by an optical link, and a plurality of WDM optical receivers.
4. Each WDM transmitter operating at a specified distinct wavelength is capable of accepting an electrical input carrying an information channel. If the information channel is coded in an optical domain, then the optical signals have to be converted into an electrical domain by plurality of transponders to drive WDM transmitters. The number of individual information channels in modern WDM communication systems varies from 8 to 128.
5. A conventional optical link comprises one or more spans. Each span customarily comprises at least one optical amplifier (EDFA), a segment of optical fiber, and, optionally, a dispersion compensation module (DCM). The number of spans depends on the WDM system design and length of the transmission line. For a conventional long haul link, each span has a length of between 80 and 120 km. The maximum length of a link, which is determined by the requirement to regenerate the optical signal, is typically about 600 km. The multiplexed optical signal transmitted via the optical link is routed to the wavelength division demultiplexer for demultiplexing back into individual channels. In each individual channel, the optical signal is received and detected by a respective WDM receiver. A number of WDM receivers corresponds to the number of WDM transmitters. Each WDM optical receiver detects the respective optical signal and processes it to provide recovered clock and data for the subsequent system electronics.
6. WDM communication systems though significantly enhancing capacity of communication networks have certain technical limitations. To add more WDM channels to the system, one has to broaden the optical bandwidth determined by the spectral band of the optical amplifier, or reduce spacing between the adjacent channels. Broadening the spectral band of the optical amplifier requires new types of amplifiers operating in a wider band than conventional EDFAs. To reduce spacing between WDM channels, new WDM transmitters, multiplexers and demultiplexers should be used with narrower transmission band and tight performance specifications which are not commercially available at this time. In multichannel WDM systems, a substantial inventory of spare parts is required with specific optical characteristics, such as WDM transmitter wavelength. Beyond these technological difficulties, there are principal limitations, such as nonlinear effects and optical dispersion. Nonlinear effects in the optical link, particularly four-wave mixing, cause channel crosstalk and lead to significant performance degradation for the overall system. The effect of four-wave mixing is intensified as the number of equally spaced (in frequency) channels increases and (or) as the power per channel increases. Non-zero fiber dispersion is vital for minimization of nonlinear effects. In some types of the optical fiber, such as Dispersion Shifted Fiber (DSF5) having low dispersion in the range of 1550 nm, multichannel WDM transmission is not feasible.
Notes:
1EDFA – erbium-doped fiber amplifier – усилитель на оптическом волокне, легированном ионами эрбия.
2DCM – dispersion compensation modules – модуль компенсации дисперсии.
3WDM – wavelength division multiplexed ‑ мультиплексирование (мультиплексированный) с разделением по длине волны.
4CDM – coherence division multiplexed – мультиплексирование (мультиплексированный) с разделением по когеренции.
5DSF – dispersion shifted fiber – волокно со смещенной дисперсией.
6. Match the letter of the correct answer to the following questions.
1. What do conventional optic fiber communication systems employ to transport information in optical telecommunication networks?
a) a metallic waveguide;
b) atmosphere;
c) optical fibers.
2. What is used to modulate the light emitted by an optical source?
a) magnetic field;
b) an electrical signal carrying information;
c) radioactive isotopes.
3. What does the light emerging from the optical fiber link illuminate?
a) an oscillator;
b) radioactive material;
c) an optical detector.
4. What was the only way to increase the bit rate in early development stages of optical communication systems?
a) to increase the modulation speed of the laser;
b) wavelength division multiplexing (WDM);
c) coherence division multiplexing (CDM).
5. What communication systems are the only multichannel systems deployed commercially nowadays?
a) coherence division multiplexing (CDM);
b) dispersion compensation modules (DCM);
c) wavelength division multiplexing (WDM).
7. Read the following statements and say whether they are true or false. Correct the false ones.
1. Nowadays WDM communication systems are not the only multichannel optical systems used commercially.
2. To reduce the optical fiber capacity, WDM communication systems use a lot of lasers and wavelength-selective passive components to multiplex and demultiplex a plurality of distinct optical channels onto a single fiber.
3. A conventional WDM communication system consists of a plurality of WDM transmitters, a wavelength division multiplexer and a wavelength division demultiplexer interconnected by an optical link, and a plurality of WDM optical receivers.
4. Each WDM transmitter operating at a specified distinct wavelength can accept an electrical input carrying an information channel.
5. If the information channel is coded in an optical domain, then the optical signals do not have to be turned into an electrical domain by plurality of transponders to drive WDM transmitters.
6) The number of individual information channels in modern WDM communication systems varies from 8 to 64.
8. Match the parts to complete the sentences.
| 1. A conventional optical link comprises 2. WDM communication systems though significantly enhancing capacity of communication networks have 3. Each span customarily comprises at least 4. For a conventional long haul link, each span has a length of 5. The number of spans depends on 6. A number of WDM receivers corresponds to 7. The maximum length of a link, which is determined by the requirement to regenerate the optical signal, is typically | a) one optical amplifier (EDFA), a segment of optical fiber, and, optionally, a dispersion compensation module (DCM). b) about 600 km. c) certain technical limitations. d) the WDM system design and length of the transmission line. e) the number of WDM transmitters. f) between 80 and 120 km. g) one ore more spans. |
9. Using information of paragraphs 1 ‑ 4, name advantages of a WDM communication system.
10. Make an outline of the text.
11. Make a short summary of the text in written form using your outline.
Part B
12. Scan the text and choose the best title for it.
a) The advantages of WDM systems.
b) The advantages of the present invention.
c) The advantages of optical devices.
d) The advantages of CDM channels.
13. Read the text and find out the topical sentences of the paragraphs.
Text B
In the present invention, the system and method employing both wavelength and coherence division multiplexing address the limitations of the existing WDM technology. The invention utilizes the benefits of existing WDM systems and enhances their capacity to a theoretical limit. It is a principal advantage of the present invention that impairments of WDM and disadvantages of CDM communication systems are substantially diminished or eliminated by loading one or more WDM transmission channels with a plurality of CDM transmission channels, and using an optical spectral range of one WDM channel for transmitting multiple CDM channels.
It is yet another significant advantage of the method and system of the present invention that the overall count of transmission channels is substantially increased, and the number of CDM channels added or dropped can be varied without changing the WDM physical infrastructure.
It is still another important advantage of the present invention that it utilizes a specially designed broadband optical source characterized by substantial noise reduction compared to the conventional broadband source used in the conventional CDM communication systems.
The invention provides a multichannel optical communication system for transmitting optical signals via an optical fiber. The system comprises a plurality of individual WDM transmission channels. A desired number of individual WDM transmission channels of this plurality is selected for transmission of WDM optical signals. Each WDM optical signal is transmitted via respective WDM transmission channel on a unique wavelength within a designated bandwidth. At least one WDM channel is assigned to transmit CDM optical signals. For CDM transmission, a CDM transmission unit is disposed within this at least one WDM transmission channel. The CDM transmission unit comprises one or more CDM transmission channels for transmitting CDM optical signals within the designated bandwidth of assigned at least one individual WDM transmission channel.
A broadband optical source significantly different from conventional broadband sources is used for transmitting CDM optical signals within one WDM transmission channel. According to one embodiment of the present invention a broadband optical source having continuous spectrum within one WDM channel comprises a semiconductor optical amplifier (SOA) for reducing relative intensity noise (RIN) originated from beating between different frequency components of this spectrum. According to another embodiment of the present invention a broadband optical source with reduced RIN has a discrete spectrum with equally spaced individual spectral lines wherein spacing between the spectral lines exceeds an electrical detection bandwidth of transmitted CDM optical signals.
The invention provides a method of multichannel optical transmission via optical fiber. According to the method, a plurality of individual WDM transmission channels is provided. A requested number of individual WDM transmission channels are selected for transmitting WDM optical signals. At least one WDM transmission channel is selected for transmitting CDM optical signal within a designated range of wavelengths assigned to this channel. A light beam is generated by a broadband source within the spectral range of this WDM transmission channel. This beam is split into a plurality of optical paths, one path chosen as a reference path, and other paths assigned to CDM transmission channels. In each CDM channel, the light beam is phase modulated and delayed by several coherence times relative to the reference path and other CDM channels. At the output of the optical link WDM and CDM channels are demultiplexed, and information channels detected.
14. Name the main problems of the text.
15. Make questions to the text.
16. Express your attitude to the facts given in the text. You may use the following phrases:
1. It is full of interesting information… .
2. I find the text rather / very cognitive… .
3. I’ve learnt a lot … .
4. I don’t agree with it… .
17. Say which facts presented in the text you’ve already been familiar with.
18. Give your point of view on the possibility of using presented in the text information in your future profession.
Part C
19. Scan the following text and say what problem is described in it. Entitle the text.
20. Read the text and arrange the following items of an outline:
a) Advantages of OFDM;
b) Frequency division multiplexing;
c) FDM Applications in Industry;
d) Orthogonal Frequency Divison Multiplexing (OFDM).
Text C
Digital communications systems require each channel to operate at a specific frequency and with a specific bandwidth. In fact, communication systems have evolved so that the largest amount of data can be communicated through a finite frequency range. Frequency division multiplexing (FDM1) and orthogonal frequency division multiplexing (OFDM2) are able to effectively utilize the frequency spectrum. OFDM systems are currently being implemented in some of the newest and most advanced communications systems.
Frequency division multiplexing (FDM) involves the allocation of each channel to a unique frequency range. This frequency range prescribes both the center frequency and channel width (bandwidth). Because these channels are non-overlapping, multiple users can operate concurrently simply by using different channels of the frequency domain. Note that each channel operates a different carrier frequency and that these channels are bandlimited to operate within a defined bandwidth.
It is important to note that the implementation of a pulse-shaping filter allows each channel to be bandlimited to a specific frequency range.
FDM is commonly used in a variety of communications protocols including Bluetooth and cellular protocols such as GSM3, TDMA4, and CDMA5. Bluetooth, a digital communications protocol that is utilized by cell phones, laptops, and PDA’s, is one example. It operates in the 2.4 GHz unlicensed band and implements FDM by defining 79 channels from 2.402 GHz to 2.480 GHz which are spaced at 1 MHz apart. Each channel is bandlimited through the implementation of a Gaussian filter.
As second common implementation of FDM is in the Global System for Mobile Communications protocol (GSM) which is a 3G cellular communication standard. With GSM, the frequency range is divided into downlink channels from 890 – 915 MHz and the uplink channels at 935 – 960 MHz. Moreover, these frequency bands are further divided so that there are 124 channels which are spaced at 200 kHz intervals. Again, the bandwidth of each channel can be limited through the implantation of a root raised cosine filter.
OFDM is a subset of frequency division multiplexing in which a single channel utilizes multiple sub-carriers on adjacent frequencies. In addition the sub-carriers in an OFDM system are overlapping to maximize spectral efficiency. Ordinarily, overlapping adjacent channels can interfere with one another. However, sub-carriers in an OFDM system are precisely orthogonal to one another. Thus, they are able to overlap without interfering. As a result, OFDM systems are able to maximize spectral efficiency without causing adjacent channel interference.
Orthogonal frequency division multiplexing is commonly implemented in many emerging communications protocols because it provides several advantages over the traditional FDM approach to communications channels. More specifically, OFDM systems allow for greater spectral efficiency reduced intersymbol interference (ISI6), and resilience to multi-path distortion.
Notes:
1FDM – Frequency Division Multiplexing – частотное уплотнение; метод частотного уплотнения каналов.
2OFDM – Orthogonal Frequency Division Multiplexing – мультиплексирование с ортогональным частотным разделением каналов.
3GSM – Global System for Mobile – глобальная система мобильной связи.
4TDMA – Time Division Multiple Access – многостанционный (множественный) доступ с временным разделением каналов.
5CDMA – Code Division Multiple Access – многостанционный доступ с кодовым разделением каналов.
6ISI – intersymbol interference – межсимвольная интерференция.
21. Find the following information in the text:
a) what digital communication systems require;
b) where the frequency spectrum can be effectively utilized;
c) what the common implementation of FDM is;
d) what the advantages of OFDM are.
22. Say where the information presented in the text can be used.
Appendix
SUPPLEMENTARY READING
Text 1
1. Read the text.
2. Divide the text into paragraphs.
3. Express the idea of each paragraph in one sentence.
4. Write a summary of the text in English.
DIGITAL TELEVISION
Digital television (DTV) is a telecommunication system for broadcasting and receiving moving pictures and sound by means of digital signals, in contrast to analog signals used by analog (traditional) TV. DTV uses digital modulation data, which is digitally compressed and requires decoding by a specially designed television set, or a standard receiver with a set-top box, or a PC fitted with a television card. Introduced in the late 1990s, this technology appealed to the television broadcasting business and consumer electronics industries as offering new financial opportunities. There are a number of different ways to receive digital television. One of the oldest means of receiving DTV (and TV in general) is using an antenna (known as an aerial in some countries). This way is known as Digital Terrestrial Television (DTT). With DTT, viewers are limited to whatever channels the antenna picks up. Signal quality will also vary. Other ways have been devised to receive digital television. Among the most familiar to people are digital cable and digital satellite. In some countries where transmissions of TV signals are normally achieved by microwaves, digital MMDS1 is used. Other standards, such as DMB2 and DVB-H3, have been devised to allow handheld devices such as mobile phones to receive TV signals. Another way is IPTV4, that is receiving TV via Internet Protocol with guaranteed quality of service (QoS). Finally, an alternative way is to receive TV signals via the open Internet infra-structure, usually referred to as Internet TV. Today, regardless of how viewers receive DTV, most will pick up digital television via a set-top box, which decodes the digital signals into signals that analog televisions can understand ‑ thus using the television purely as a monitor. However, a growing number of TV sets with integrated receivers are available – these are known as iDTVs. Many countries around the world currently operate a simulcast service where a broadcast is made available to viewers in both analog and digital at the same time. As digital becomes more popular it is likely that the existing analog services will be removed. In some cases this has already happened where a broadcaster has offered incentives to viewers to encourage them to switch to digital or simply switched their service regardless of whether they want to switch. In other cases government policies have been introduced to encourage the switch-over process, especially with regard to terrestrial broadcasts. Government intervention usually involves providing some funding for broadcasters to enable a switch-over to happen by a given deadline. Luxembourg was the first country to complete the move to digital broadcasting, on September 1, 2006.
· The Netherlands moved to digital broadcasting on December 11, 2006. The switch-off was helped greatly by the fact that about 90 percent of the households have cable that continues to use analogue broadcasts.
· In Finland, terrestrial analogue transmissions were terminated nationwide at 4am, September 1, 2007 (switch-off was previously planned for the midnight after August 31 but a few extra hours were added for technical reasons). Cable-TV viewers will continue to receive analogue broadcasts till the end of February 2008.
· Andorra completed its switch-off on September 25, 2007.
· Germany started the switch-off at different times in different regions. The first was the Berlin area, where the switch-off began on November 1, 2002 and was completed on August 4, 2003. Most other regions have followed, and in most populous areas the switch-off is completed, but a number of regions have not yet started. The switch-off is planned to be completed by the end of 2008.
· In the United Kingdom, the first switch off of analogue television was on 30 March 2005, in the villages of Llansteffan and Ferryside in Wales.The last regions will be switched off in 2012.
· In Ukraine, analogue transmissions will be terminated on July 17, 2015.
DTV has several advantages over traditional, analog TV, the most significant being that digital channels take up less bandwidth. This means that digital broadcasters can provide more digital channels in the same space, provide high-definition television service, or provide other non-television services such as multimedia or interactivity. DTV also permits special services such as multiplexing (more than one program on the same channel), electronic program guides and additional languages, spoken or subtitled. In many cases, viewers perceive DTV to have superior picture quality, improved audio quality, and easier reception than analog. However, DTV picture technology is still in its early stages. DTV images have some picture defects that are not present on analog television or motion picture cinema, due to present-day limitations of bandwidth and compression algorithms such as MPEG5-2.When a compressed digital image is compared with the original program source, some hard-to-compress image sequences may have digital distortion or degradation. For example: quantization noise, incorrect color, blockiness, a blurred shimmering haze. Broadcasters attempt to balance their needs to show high quality pictures and to generate revenue by using a fixed bandwidth allocation for more services.
Notes:
1MMDS – Multichannel Multipoint Distribution Service – многоканальная многоточечная распределенная служба (связи).
2DMB – Digital Multimedia Broadcasting – цифровое мультимедийное вещание.
3DVB-H – Digital Video Broadcasting – Handheld – цифровое телевизионное вещание для мобильных устройств.
4IPTV – Internet Protocol Television – интернет-телевидение.
5MPEG – Motion Pictures Experts Group – экспертная группа по кинематографии, группа, MPEG образована в 1988 г., занимается алгоритмами сжатия видеоизображений.
Text 2
1. Read the text and entitle it.
2. Express the idea of each paragraph in one sentence.
3. Write a summary of the text in English.
A person with a Wi-Fi enabled device such as a computer, cell phone or PDA can connect to the Internet when in proximity of an access point. The region covered by one or several access points is called a hotspot. Hotspots can range from a single room to many square miles of overlapping hotspots. Wi-Fi can also be used to create a mesh network. Both architectures are used in community networks, municipal wireless networks like Wireless Philadelphia, and metro-scale networks like
M-Taipei.
Wi-Fi also allows connectivity in peer-to-peer mode, which enables devices to connect directly with each other. This connectivity mode is useful in consumer electronics and gaming applications.
When the technology was first commercialized there were many problems because consumers could not be sure that products from different vendors would work together. The Wi-Fi Alliance began as a community to solve this issue so as to address the needs of the end user and allow the technology to mature. The Alliance created the branding Wi-Fi CERTIFIED to show consumers that products are interoperable with other products displaying the same branding.
A typical Wi-Fi setup contains one or more Access Points (APs) and one or more clients. An AP1 broadcasts its SSID (Service Set Identifier, “Network name”) via packets that are called beacons, which are usually broadcast every 100 ms. The beacons are transmitted at 1 Mbit/s, and are of relatively short duration and therefore do not have a significant effect on performance. Since 1 Mbit/s is the lowest rate of Wi-Fi it assures that the client who receives the beacon can communicate at at least 1 Mbit/s. Based on the settings (e.g. the SSID), the client may decide whether to connect to an AP. If two APs of the same SSID are in range of the client, the client firmware might use signal strength to decide which of the two APs to make a connection to. The Wi-Fi standard leaves connection criteria and roaming totally open to the client. This is a strength of Wi-Fi, but also means that one wireless adapter may perform substantially better than another. Since Wi-Fi transmits in the air, it has the same properties as a non-switched ethernet network. Even collisions can therefore appear as in non-switched ethernet LAN's. Unlike a wired Ethernet, and like most packet radios, Wi-Fi cannot do collision detection, and instead uses a packet exchange to try to avoid collisions.
Notes:
1AP ‑ access point – узел (точка, пункт) доступа; приемопередатчик беспроводной сети.
Text 3
1. Read the text.
2. Divide the text into paragraphs.
3. Express the idea of each paragraph in one sentence.
4. Write a summary of the text in English.
Standard Wi-Fi Devices
A wireless access point (AP) connects a group of wireless stations to an adjacent wired local area network (LAN). An access point is similar to an ethernet hub, but instead of relaying LAN data only to other LAN stations, an access point can relay wireless data to all other compatible wireless devices as well as to a single (usually) connected LAN device, in most cases an ethernet hub or switch, allowing wireless devices to communicate with any other device on the LAN. A wireless router integrates a wireless access point with an IP router and an ethernet switch. The integrated switch connects the integrated access point and the integrated ethernet router internally, and allows for external wired ethernet LAN devices to be connected as well as a (usually) single WAN device such as cable modem or DSL modem. A wireless router advantageously allows all three devices (mainly the access point and router) to be configured through one central configuration utility, usually through an integrated web server. A wireless Ethernet bridge connects a wired network to a wireless network. This is different from an access point in the sense that an access point connects wireless devices to a wired network at the data-link layer. Two wireless bridges may be used to connect two wired networks over a wireless link, useful in situations where a wired connection may be unavailable, such as between two separate homes. A wireless range extender (or wireless repeater) can increase the range of an existing wireless network by being strategically placed in locations where a wireless signal is sufficiently strong and nearby locations that have poor to no signal strength. An example location would be at the corner of an L-shaped corridor, where the access point is at the end of one leg and a strong signal is desired at the end of the other leg. Another example would be 75 % of the way between the access point and the edge of its useable signal. This would effectively increase the range by 75 %. There are very few stand-alone 802.11 wireless repeaters on the market, but some access points routers have a built-in repeater mode. Nearly all WLAN1 repeaters currently available today are actually built-in functions of access points. For example, the Cisco 350 and 1200 allow you to configure the access point to behave as a repeater (and not as an access point). Buffalo Technology, however, does offer a stand alone repeater in their AirStation Pro Series WLA-AWCG. The advantage of the stand alone repeaters is that they are generally less expensive. One downside of wireless repeaters, though, is that they reduce throughput on the WLAN. A repeater must receive and retransmit each frame on the same RF channel, which effectively doubles the number of frames that are sent. This problem compounds when using multiple repeaters because each repeater will duplicate the number of frames sent.
Notes:
1WLAN – wireless LAN – беспроводная локальная сеть.
Text 4
1. Read the text.
2. Express the idea of each paragraph in one sentence.
3. Write a summary of the text in English.