Lightwave System Architectures
The preceding tutorials focused on the three main components of a fiber-optic communication system - optical fibers, optical transmitters, and optical receivers. In the following tutorials we consider the issues related to system design and performance when the three components are put together to form a practical lightwave system.
From an architectural standpoint, fiber-optic communication systems can be classified into three broad categories - point-to-point links, distribution networks, and local-area networks. This tutorial focuses on the main characteristics of these three system architectures.
1. Point-to-Point Links
Point-to-point links constitute the simplest kind of lightwave systems. Their role is to transport information, available in the form of a digital bit stream, from one place to another as accurately as possible. The link length can vary from less than a kilometer (short haul) to thousands of kilometers (long haul), depending on the specific application. For example, optical data links are used to connect computers and terminals within the same building or between two buildings with a relatively short transmission distance (<10 km). The low loss and the wide bandwidth of optical fibers are not of primary importance for such data links; fiber are used mainly because of their other advantages, such as immunity to electromagnetic interference. In contrast, undersea lightwave systems are used for high-speed transmission across continents with a link length of several thousands of kilometers. Low losses and a large bandwidth of optical fibers are important factors in the design of transoceanic systems from the standpoint of reducing the overall operating cost.
When the link length exceeds a certain value, in the range 20-100 km depending on the operating wavelength, it becomes necessary to compensate for fiber loses, as the signal would otherwise become too weak to be detected reliably. The figure below shows two schemes used commonly for loss compensation.
Until 1990, optoelectronic repeaters, called regenerators because they regenerate the optical signal, were used exclusively. As seen in figure (a) above, a regenerator is nothing but a receiver-transmitter pair that detects the incoming optical signal, recovers the electrical bit stream, and then converts it back into optical form by modulating an optical source. Fiber losses can also be compensated by using optical amplifiers, which amplify the optical bit stream directly without requiring conversion of the signal to the electric domain. The advent of optical amplifiers around 1990 revolutionized the development of fiber-optic communication systems. Such amplifiers are especially valuable for WDM systems as they can amplify a large number of channels simultaneously.
Optical amplifiers solve the loss problem but they add noise and worsen the impact of fiber dispersion and nonlinearity because signal degradation keeps on accumulating over multiple amplification stages. Indeed, periodically amplified lightwave systems are often limited by fiber dispersion unless dispersion-compensation techniques are used. Optoelectronic repeaters do not suffer from this problem as they regenerate the optical bit stream and thus effectively compensate for all sources of signal degradation automatically. However, their repeated use in WDM systems (every 80 km or so) is not cost effective. Although considerable research effort is being directed toward developing all-optical regenerators, most terrestrial systems employ a combination of the two techniques shown in the figure above and place an optoelectronic regenerator after a certain number of optical amplifiers. Submarine systems are often designed to operate over a distance of more than 5000 km using only cascaded optical amplifiers.
The spacing L between regenerators or optical amplifiers, often called the repeater spacing, is a major design parameter simply because the system cost reduces as L increases. However, the distance L depends on the bit rate B because of fiber dispersion. The bit rate-distance product, BL, is generally used as a measure of the system performance for point-to-point links. The BL product depends on the operating wavelength, since both fiber losses and fiber dispersion are wavelength dependent. The first three generations of lightwave systems correspond to three different operating wavelengths near 0.85, 1.3, and 1.55 μm. Whereas the BL product was ~ 1 (Gb/s)-km for the first-generation systems operating near 0.85 μm, it becomes ~ 1 (Tb/s)-km for the third-generation systems operating near 1.55 μm and can exceed 1000 (Tb/s)-km for the fourth-generation systems.
2. Distribution Networks
Many applications of optical communication systems require that information is not only transmitted but also distributed to a group of subscribers. Examples include local-loop distribution of telephone services and broadcast of multiple video channels over cable television (CATV, short for common-antenna television). Considerable effort is directed toward the integration of audio and video services through a broadband digital network. The resulting bit stream can be transmitted using a variety of standards developed for this purpose. Transmission distances are relatively short (L < 50 km), but the bit rate can be as high as 100 Gb/s.
The figure below shows two topologies for distribution networks.
In the case of hub topology, channel distribution takes place at central locations (or hubs), where an automated cross-connect facility switches channels in the electrical domain. Such networks are called metropolitan-area networks (MANs), or simply metro networks, as hubs are typically located in major cities. The role of fiber is similar to the case of point-to-point links. Since the fiber bandwidth if generally much larger than that required by a single hub office, several offices can share a single fiber headed for the main hub. Telephone networks employ hub topology for distribution of audio channels within a city. A concern for the hub topology is related to is reliability - outage of a single fiber cable can affect the service to a large portion of the network. Additional point-to-point links can be used to guard against such a possibility by connecting important hub locations directly.
In the case of bus topology, a single fiber cable carries the multichannel optical signal throughout the area of service. Distribution is done by using optical taps, which divert a small fraction of the optical power to each subscriber. A simple CATV application of bus topology consists of distributing multiple video channels within a city. The use of optical fiber permits distribution of a large number channels (100 or more) because of its large bandwidth compared with coaxial cables. The advent of high-definition television (HDTV) also requires lightwave transmission because of a large bandwidth associated with each video channel.
A problem with the bus topology is that the signal loss increases exponentially with the number of taps and limits the number of subscribers served by a single optical bus. Even when fiber losses are neglected, the power available at the Nth tap is given by
PN = PTC[(1 - δ)(1 - C)]N-1
where PT is the transmitted power, C is the fraction of power coupled out at each tap, and δ accounts for insertion losses, assumed to be the same at each tap. If we use δ = 0.05, C = 0.05, PT = 1 mW, and PN = 0.1 μW as illustrative values, N should not exceed 60. A solution to this problem is offered by optical amplifiers which can boost the optical power of the bus periodically and thus permit distribution to a large number of subscribers as long as the effects of fiber dispersion remain negligible.
3. Local-Area Networks
Many applications of fiber-optic communication technology require networks in which a large number of users within a local are (e.g., a university campus) are interconnected in such a way that any user can access the network randomly to transmit data to any other user. Such networks are called local-area networks (LANs). Optical-access networks used in a local subscriber loop also fall in this category. Since the transmission distances are relatively short (< 10 km), fiber losses are not of much concern for LAN applications. The major motivation behind the use of optical fibers is the large bandwidth offered by fiber-optic communication systems.
The main difference between MANs and LANs is related to the random access offered to multiple users of a LAN. The system architecture plays an important role for LANs, since the establishment of predefined protocol rules is a necessity in such an environment. Three commonly used topologies are known as bus, ring, and star configurations.
The bus topology is similar to that shown in the previous figure. A well-known example of bus topology is provided by the Ethernet, a network protocol used to connect multiple computers and used by the Internet. The Ethernet operates at speeds of up to 10 Gb/s (10 GbE) by using a protocol based on carrier-sense multiple access (CSMA) with collision detection. A new standard known as 100 Gb Ethernet (officially IEEE 802.3ba) became operational in 2010. Its advent boosts the traffic speed on the Internet to a bit rate of 100 Gb/s. The figure below shows the ring and star topologies for LAN applications.
In the ring topology, consecutive nodes are connected by point-to-point links to form a closed ring. Each node can transmit and receive the data by using a transmitter-receiver pair, which also acts as a repeater. A token (a predefined bit sequence) is passed around the ring. Each node monitors the bit stream to listen for its own address and to receive the data. It can also transmit by appending the data to an empty token. The use of ring topology for fiber-optic LANs has been commercialized with the standardized interface know as the fiber distributed data interface, FDDI for short.
In the star topology all nodes are connected through point-to-point links to a central node called a hub, or simply a star. Such LANs are further subclassified as active-star or passive-star networks, depending on whether the central node is an active or passive device. In the active-star configuration, all incoming optical signals are converted to the electrical domain through optical receivers. The electrical signal is then distributed to drive individual node transmitters. Switching operations can also be performed at the central node since distribution takes place in the electrical domain. In the passive-star configuration, distribution takes place in the optical domain through devices such as directional couplers. Since the input from each node is distributed to many output nodes, the power transmitted to each node depends on the number of users. Similar to the case of bus topology, the number of users supported by passive-star LANs is limited by distribution losses. For an ideal N x N star coupler, the power reaching each node is simply PT/N (if we neglect transmission losses) since the transmitted power PT is divided equally among N users. For a passive star composed of directional couplers, the power is further reduced because of insertion losses and can be written as
PN = (PT/N)(1 - δ)log2N
where δ is the insertion loss of each directional coupler. It we use δ = 0.05, PT = 1 mW, and PN = 0.1 μW as illustrative values, N can be as large as 500. This value of N should be compared with N = 60 obtained for the case of bus topology. A relatively large value of N makes star topology attractive for LAN applications.