Optical Networks: A Practical Perspective - Part 5 potx

Optical fiber provided lower bit error rates and higher capacities than copper cables.. Examples of first-generation optical networks are SONET synchronous optical network and the essent

10 INTrODUCTiON TO OPTICAL NETWORI~S

that more sophisticated techniques can be used to improve the bandwidth efficiency but usually at the cost of slower restoration times This realization is stimulating the development of service offerings that trade off restoration time against bandwidth efficiency in the network

Thus carriers in the new world need to deploy networks that provide them with the flexibility to deliver bandwidth on demand when needed where needed, with the

appropriate service attributes The "where needed" is significant because carriers can rarely predict the location of future traffic demands As a result it is difficult for them

to plan and build networks optimized around specific assumptions on bandwidth demands

At the same time, the mix of services offered by carriers is expanding We talked about different circuit-switched and packet-switched services earlier What is not commonly realized is that today these services are delivered over separate overlay networks, rather than a single network Thus carriers need to operate and maintain multiple n e t w o r k s u a very expensive proposition over time For most networks, the costs associated with operating the network over time (such as maintenance, provi- sioning of new connections, upgrades) far outweigh the initial cost of putting in the equipment to build the network Carriers would thus like to migrate to maintaining a single network infrastructure that enables them to deliver multiple types of services

1.3 Optical Networks

Optical networks offer the promise to solve many of the problems discussed above

In addition to providing enormous capacities in the network, an optical network provides a common infrastructure over which a variety of services can be delivered These networks are also increasingly becoming capable of delivering bandwidth in

a flexible manner where and when needed

Optical fiber offers much higher bandwidth than copper cables and is less suscep- tible to various kinds of electromagnetic interferences and other undesirable effects

As a result, it is the preferred medium for transmission of data at anything more than

a few tens of megabits per second over any distance more than a kilometer It is also the preferred means of realizing short-distance (a few meters to hundreds of meters), high-speed (gigabits per second and above) interconnections inside large systems The latest statistics from the U S Federal Communications Commission [Kra99] indicate the ubiquity of fiber deployment Optical fibers are widely deployed today

in all kinds of telecommunications networks, except perhaps in residential access networks Although fiber is provided to many businesses today, particularly in large cities, it has yet to reach individual homes, due to the huge cost of wiring the infrastructure and the questionable rate of return on this investment seen by the

o

1,000,000

10,000

100

Long haul

Leased lines

/ x

0.01 t Residential access

Local-area networks

Year

Figure 1.3 Bandwidth growth over time in different types of networks

service providers Before providing some more data, we introduce some terminology first Each route in a network comprises many fiber cables Each cable contains many

fibers For example, a 10-mile-long route using 3 fiber cables is said to have 10 route miles and 30 sheath (cable) miles If each cable has 20 fibers, the same route is said to have 600 fiber miles As of the end of 1998, the local-exchange carriers in the United States had deployed more than 355,000 sheath miles of fiber, containing more than

16 million fiber miles More than 160,000 route miles of fiber had been deployed

by the interexchange carriers in the United States, containing more than 3.6 million miles of optical fiber [Kra99]

Fiber transmission technology has evolved over the past few decades to offer higher and higher bit rates on a fiber over longer and longer distances Figure 1.3 plots the growth in bandwidth over time of different types of networks, updated from a chart originally presented in [Fra93] This tremendous growth in bandwidth

is primarily due to the deployment of optical fiber communication systems

Note from Figure 1.3 that the bandwidths into our homes are still limited by the bandwidth available on our phone line, which is made of twisted-pair copper cable These lines are capable of carrying data at a few megabits per second using digital subscriber loop (DSL) technology, but voice-grade lines are limited at the central office to 4 kHz of bandwidth (Note that in some cases we measure bandwidth in hertz and sometimes loosely use bit rates when talking about bandwidth We will

12 INTRODUCTION TO OPTICAL NETWORKS

1.3.1

quantify the relationship between bit rate and bandwidth in Section 1.7.) The other alternative is the cable network, which again is capable of providing a few megabits per second to each subscriber on a shared basis using cable modem technology When we talk about optical networks, we are really talking about two gener- ations of optical networks In the first generation, optics was essentially used for transmission and simply to provide capacity Optical fiber provided lower bit error rates and higher capacities than copper cables All the switching and other intelligent network functions were handled by electronics Examples of first-generation optical networks are SONET (synchronous optical network) and the essentially similar SDH (synchronous digital hierarchy) networks, which form the core of the telecommuni- cations infrastructure in North America and in Europe and Asia, respectively, as well

as a variety of enterprise networks such as ESCON (enterprise serial connection)

We will study these first-generation networks in Chapter 6

Today we are seeing the deployment of second-generation optical networks, where some of the routing, switching, and intelligence is moving into the optical layer Before we discuss this new generation of networks, we will first look at the multiplexing techniques that provide the capacity needed to realize these networks

Multiplexing Techniques

The need for multiplexing is driven by the fact that it is much more economical

to transmit data at higher rates over a single fiber than it is to transmit at lower rates over multiple fibers, in most applications There are fundamentally two ways

of increasing the transmission capacity on a fiber, as shown in Figure 1.4 The first

is to increase the bit rate This requires higher-speed electronics Many lower-speed data streams are multiplexed into a higher-speed stream at the transmission bit rate

by means of electronic time division multiplexing (TDM) The multiplexer typically interleaves the lower-speed streams to obtain the higher-speed stream For example,

it could pick 1 byte of data from the first stream, the next byte from the second stream, and so on As an example, 64 155 Mb/s streams may be multiplexed into a single 10 Gb/s stream Today, the highest transmission rate in commercially available systems is around 10 Gb/s; 40 Gb/s TDM technology will be available soon To push TDM technology beyond these rates, researchers are working on methods to perform the multiplexing and demultiplexing functions optically This approach is called optical time division multiplexing (OTDM) Laboratory experiments have demonstrated the multiplexing/demultiplexing of several 10 Gb/s streams into/from

a 250 Gb/s stream, although commercial implementation of OTDM is still several years away We will study OTDM systems in Chapter 12 However, multiplexing and demultiplexing high-speed streams by itself is not sufficient to realize practical networks We need to contend with the various impairments that arise as these very

Figure 1.4 Different multiplexing techniques for increasing the transmission capacity

on an optical fiber (a) Electronic or optical time division multiplexing and (b) wavelength division multiplexing Both multiplexing techniques take in N data streams, each of B b/s, and multiplex them into a single fiber with a total aggregate rate of NB b/s

high-speed streams are transmitted over a fiber As we will see in Chapters 5 and 13, the higher the bit rate, the more difficult it is to engineer around these impairments However, similar bottlenecks have been encountered in the past, and people have always found ways to overcome them; so we can expect the transmission bit rates

to continue to increase, although perhaps not at the breakneck pace of the past two decades

Another way to increase the capacity is by a technique called wavelength division multiplexing (WDM) W D M is essentially the same as frequency division multiplex- ing (FDM), which has been used in radio systems for more than a century For some reason, the term FDM is used widely in radio communication, but W D M is used in

14 INTRODUCTION TO OPTICAL NETWORKS

1.3.2

the context of optical communication, perhaps because FDM was studied first by communications engineers and WDM by physicists The idea is to transmit data si- multaneously at multiple carrier wavelengths (or, equivalently, frequencies or colors) over a fiber To first order, these wavelengths do not interfere with each other pro- vided they are kept sufficiently far apart (There are some undesirable second-order effects where wavelengths do interfere with each other, and we will study these in Chapters 2 and 5.) Thus WDM provides virtual fibers, in that it makes a single fiber look like multiple "virtual" fibers, with each virtual fiber carrying a single data stream WDM systems are widely deployed today in long-haul and undersea networks and are being deployed in metro networks as well

WDM and TDM both provide ways to increase the transmission capacity and are complementary to each other Therefore networks today use a combination

of TDM and WDM The question of what combination of TDM and WDM to use in systems is an important one facing carriers today For example, suppose a carrier wants to install an 80 Gb/s link Should we deploy 32 WDM channels at 2.5 Gb/s each or should we deploy 10 WDM channels at 8 Gb/s each? The answer depends on a number of factors, including the type and parameters of the fiber used in the link and the services that the carrier wishes to provide using that link

We will discuss this issue in Chapter 13 Using a combination of WDM and TDM, systems with transmission capacities of around I Tb/s over a single fiber are becoming commercially available, and no doubt we will see systems with higher capacities operating over longer distances emerge in the future

Second-Generation Optical Networks

Optics is clearly the preferred means of transmission, and WDM transmission is now widely used in the network In recent years, people have realized that optical networks are capable of providing more functions than just point-to-point transmission Major advantages are to be gained by incorporating some of the switching and routing functions that were performed by electronics into the optical part of the network For example, as data rates get higher and higher, it becomes more difficult for electronics

to process data Suppose the electronics must process data in blocks of 53 bytes each (this happens to be the block size in asynchronous transfer mode networks) In

a 100 Mb/s data stream, we have 4.24 lZS to process a block, whereas at 10 Gb/s, the same block must be processed within 42.4 ns In first-generation networks, the electronics at a node must handle not only all the data intended for that node but also all the data that is being passed through that node on to other nodes in the network If the latter data could be routed through in the optical domain, the burden

on the underlying electronics at the node would be significantly reduced This is one

of the key drivers for second-generation optical networks

Optical networks based on this paradigm are now being deployed The ar- chitecture of such a network is shown in Figure 1.5 We call this network a

wavelength-routing network The network provides lightpaths to its users, such

as SONET terminals or IP routers Lightpaths are optical connections carried end to end from a source node to a destination node over a wavelength on each intermediate link At intermediate nodes in the network, the lightpaths are routed and switched from one link to another link In some cases, lightpaths may be converted from one wavelength to another wavelength as well along their route Different lightpaths in

a wavelength routing network can use the same wavelength as long as they do not share any common links This allows the same wavelength to be reused spatially in different parts of the network For example, Figure 1.5 shows six lightpaths The lightpath between B and C, the lightpath between D and E, and one of the lightpaths between E and F do not share any links in the network and can therefore be set

up using the same wavelength )~1 At the same time, the lightpath between A and F shares a link with the lightpath between B and C and must therefore use a different wavelength Likewise, the two lightpaths between E and F must be assigned different wavelengths Note that these lightpaths all use the same wavelength on every link in their path This is a constraint that we must deal with if we do not have wavelength conversion capabilities within the network Suppose we had only two wavelengths available in the network and wanted to set up a new lightpath between nodes E and

E Without wavelength conversion, we would not be able to set up this lightpath On the other hand, if the intermediate node X can perform wavelength conversion, then

we can set up this lightpath using wavelength )~2 on link EX and wavelength ~1 on link XE

The key network elements that enable optical networking are optical line ter- minals (OLTs), optical add~drop multiplexers (OADMs), and optical crossconnects

(OXCs), as shown in Figure 1.5 An OLT multiplexes multiple wavelengths into a single fiber and demultiplexes a set of wavelengths on a single fiber into separate fibers OLTs are used at the ends of a point-to-point W D M link An OADM takes

in signals at multiple wavelengths and selectively drops some of these wavelengths locally while letting others pass through It also selectively adds wavelengths to the composite outbound signal An OADM has two line ports where the composite

W D M signals are present, and a number of local ports where individual wavelengths are dropped and added An OXC essentially performs a similar function but at much larger sizes OXCs have a large number of ports (ranging from a few tens

to thousands) and are able to switch wavelengths from one input port to another Both OADMs and OXCs may incorporate wavelength conversion capabilities The detailed architecture of these networks will be discussed in Chapter 7

Optical networks based on the architecture described above are already being deployed OLTs have been widely deployed for point-to-point applications OADMs

16 INTRODUCTION TO OPTICAL NETWORKS

Figure 1.5 A WDM wavelength-routing network, showing optical line terminals

(OLTs), optical add/drop multiplexers (OADMs), and optical crossconnects (OXCs) The network provides lightpaths to its users, which are typically IP routers or SONET terminals

are now used in long-haul and metro networks OXCs are beginning to be deployed first in long-haul networks because of the higher capacities in those networks

Before delving into the details of the optical layer, we first introduce the notion of

a layered network architecture Networks are complicated entities with a variety of different functions being performed by different components of the network, with equipment from different vendors all interworking together In order to simplify our view of the network, it is desirable to break up the functions of the network into different layers, as shown in Figure 1.6 This type of layered model was proposed

by the International Standards Organization (ISO) in the early 1980s Imagine the layers as being vertically stacked up Each layer performs a certain set of functions

Figure 1.6

(NE)

Layered hierarchy of a network showing the layers at each network element

and provides a certain set of services to the next higher layer In turn, each layer expects the layer below it to deliver a certain set of services to it The service interface between two adjacent layers is called a service access p o i n t (SAP), and there can be multiple SAPs between layers corresponding to different types of services offered

In most cases, the network provides connections to the user A connection is established between a source and a destination node Setting up, taking down, and managing the state of a connection is the job of a separate network control and management entity (not shown in Figure 1.6), which may control each individual layer in the network There are also examples where the network provides con-

messages across a network, without having to pay the overhead of setting up and taking down a connection for this purpose We will confine the following discussion

to the connection-oriented model

Within a network element, data belonging to a connection flows between the layers Each layer multiplexes a number of higher-layer connections and may add some additional overhead to data coming from the higher layer Each intermediate network element along the path of a connection embodies a set of layers starting from the lowest layer up to a certain layer in the hierarchy

18 INTRODUCTION TO OPTICAL NETWORKS

Figure 1.7 The classical layered hierarchy

It is imi0ortant to define the functions of each layer and the interfaces between layers This is essential because it allows vendors to manufacture a variety of hard- ware and software products performing the functions of some, but not all, of the layers, and provide the appropriate interfaces to communicate with other products performing the functions of other layers

There are many possible implementations and standards for each layer A given layer may work together with a variety of lower or higher layers Each of the different types of optical networks that we will study constitutes a layer Each layer itself can

in turn be broken up into several sublayers As we study these networks, we will explore this layered hierarchy further

Figure 1.7 shows a classical breakdown of the different layers in a network that was proposed by the International Standards Organization The lowest layer in the hierarchy is the physical layer, which provides a "pipe" with a certain amount of bandwidth to the layer above it The physical layer may be optical, wireless, or coaxial

or twisted-pair cable The next layer above is the data link layer, which is responsible for framing, multiplexing, and demultiplexing data sent over the physical layer The framing protocol defines how data is transported over a physical link Typically data is broken up into frames before being transmitted ove~ a physical link This is necessary to ensure reliable delivery of data across the link The framing protocol provides clear delineation between frames, provides sufficient transitions in the signal

so that it can be recovered at the other end, and usually includes additional overhead that enables link errors to be detected Examples of data link protocols suitable for operation over point-to-point links include the point-to-point protocol (PPP) and the

high-level data link control (HDLC) protocol Included in the data link layer is the

media access control layer (MAC), which coordinates the transmissions of different nodes when they all share common bandwidth, as is the case in many local-area networks, such as Ethernets and token rings

Above the data link layer resides the network layer The network layer usually provides virtual circuits or datagram services to the higher layer A virtual circuit (VC) represents an end-to-end connection with a certain set of quality-of-service parameters associated with it, such as bandwidth and error rate Data transmitted

by the source over a VC is delivered in sequence at its destination Datagrams, on the other hand, are short messages transmitted end to end, with no notion of a connection The network layer performs the end-to-end routing function of taking a message at its source and delivering it to its destination The predominant network layer today is IP, and the main network element in an IP network is an IP router

IP provides a way to route packets (or datagrams) end to end in a packet-switched network IP includes statistical multiplexing of multiple packet streams and today also provides some simple and relatively slow and inefficient service restoration mechanisms The Internet Protocol has been adapted to operate over a variety of data link and physical media, such as Ethernet, serial telephone lines, coaxial cable lines, and optical fiber lines

The transport layer resides on top of the network layer and is responsible for ensuring the end-to-end, in-sequence, and error-free delivery of the transmitted mes- sages For example, the transmission control protocol (TCP) used in the Internet belongs to this layer Above the transport layer reside other layers such as the ses- sion, presentation, and application layers, but we will not be concerned with these layers in this book

Another important packet-switched layer is ATM ATM provides a connection- oriented service (virtual circuits) and is capable of providing a variety of quality-of-service guarantees Packets in ATM are called cells and are of fixed length (53 bytes) ATM is being used by many carriers as a vehicle to deliver re- liable packet-switched services More on this subject in Chapters 6 and 13

This classical layered view of networks needs some embellishment to handle the variety of networks and protocols that are proliferating today A more realistic lay- ered model for today's networks would employ multiple protocol stacks residing one on top of the other Each stack incorporates several sublayers, which may pro- vide functions resembling traditional physical, link, and network layers To provide

a concrete example of this, consider an IP over SONET network shown in Fig- ure 1.8 In this case, the IP network treats the SONET network as providing it with point-to-point links between IP routers The SONET layer itself, however, internally routes and switches connections, and in a sense, incorporates its own link, physical, and network layers