Optical Networks: Capacity and Components
- Three Generations of Digital Transport Networks
- A Brief Introduction to WDM and TDM
- The Optical Marketplace
- Wireless Optical Systems
- Key Optical Nodes
- Other Key Terms
- Evolution of Optical Systems
- Key Attributes of Optical Fiber
- Summary
This chapter is from the book
This chapter is from the book
This chapter is from the book
Three Generations of Digital Transport Networks
The focus of this book is on third generation digital transport networks, usually shorted to 3G, or 3rd generation, transport networks. The main characteristics of three generations of digital transport networks are provided in Table 1–1. The information in this table will be helpful as you read the remaining chapters in this book. Most of the terms in the table are self-explanatory, or, if not, are explained in this chapter.
Table 1–1 Three Generations of Digital Transport (Carrier) Networks
|
Name |
Family |
Designed for |
Mux/SW Schemes at Inception |
Principal Media at Inception |
Capacity |
Typical Payload |
Protocol Inter-Working? |
|
T1/E1 |
First |
Voice, Non-BOD, Static |
TDM/E/E/E |
Copper: (Early1960s) |
Mbit/s |
Fixed Length |
No |
|
SONET/SDH |
Second |
Voice, Non-BOD, Static |
TDM/O/E/O |
Copper, Fiber: (Mid–1980s) |
Gbit/s |
Fixed Length |
Somewhat: PPP, IP, ATM |
|
OTN |
Third |
Voice, Video, Data, Tailored QOS, BOD, Dynamic |
WDM/O/O/O |
Fiber (Late1990s to Early 2000s) |
Tbit/s |
Fixed or Variable Lengths |
Yes: PPP, IP, ATM, MPLS |
The first column in the table is the name (or names) usually associated with the technology. The first generation systems are known as T1 or E1. The second generation systems are called SONET (for the Synchronous Optical Network) or SDH (for the Synchronous Digital Hierarchy). These terms are explained in more detail in later parts of this book. However, the industry has not yet settled on a handle for the third generation digital carrier network, but the term Optical Transport Network (OTN) is widely used. The second column identifies the generation family.
The third column shows what kinds of user payloads the networks are designed to support. Although the first and second generation networks are designed to support voice traffic, they can and do transport data and video images. But they are not "optimized" for data and video traffic. In contrast, the 3G transport network is designed to support voice, video, or data payloads. When used with multiprotocol label switching (MPLS), the resource reservation protocol (RSVP), and DiffServ, as well as some of the new specifications dealing with optical bandwidth on demand, they are also designed to provide tailored quality-of-service (QOS) features for individual customers. The point will be made repeatedly in this book that the 3G transport network no longer consists of fixed, static "pipes" of capacity; it can dynamically change to meet the changing requirements of its users.
The third column also contains the notations of Non-BOD or BOD. The first and second generation systems are not designed to provide bandwidth of demand (BOD). The bandwidth is configured with crafting operations at each node. 3G systems are more dynamic and allow bandwidth to be requested on demand.
The fourth column lists the predominant multiplexing schemes: TDM or WDM. The fourth column also lists the manner in which the networks switch traffic when they were first deployed (at their inception). First generation systems were solely E/E/E operations: (a) they accepted electrical signals (the first E), (b) processed them (the second E), and (c) sent them to another node (the third E). Second generation systems are O/E/O operations: (a) they accept optical signals (the first O), (b) convert them to electrical signals for processing (the E), and (c) convert the electrical signals back to optical signals for transmission (the second O). Third generation systems are intended to be all optical (O/O/O), in that they process optical payloads, and do not need to convert the bits to electrical images for processing. Today, all three generations are mainly O/E/O oriented.
The fifth column lists the principal media used by the technologies at their inception, as well as the time that these networks were first introduced into the industry. All three generations now use a combination of copper, fiber, and wireless media.
The sixth column lists the typical capacity of the generation. It is evident that each succeeding family has increased its transport capacity by orders of magnitude.
The seventh column goes hand-in-hand with the third column ("Designed For"). The first and second generation networks were designed for fixed-length voice traffic, based on the 64 kbit/s payload, with a 125-μsec clocking increment. The third generation network supports this signal, but also supports variable-length payloads, an important capability for carrying data traffic. As well, the first and second generation networks can carry variable-length traffic, but they are not very efficient in how they go about transporting variable-length data traffic.
The eighth column explains whether any of the generations were designed to interwork with and directly support other protocols. T1/E1 was not so designed; again, 1st generation transport systems were set up to support voice traffic. Any efforts to devise methods of carrying other payloads were an afterthought and in vendor-specific procedures. With the advent of 2nd generation systems with SONET/SDH, efforts were made by the standards groups to define procedures for carrying certain kinds of data traffic, and many manufacturers adapted these standards into their products.
3rd generation transport networks are geared toward supporting many kinds of payloads, and specifically the Internet, ATM, and MPLS protocol suites. As we shall see as we proceed though this book, extensive research has resulted in many specifications defining how MPLS contributes to the operations of the third generation digital (optical) transport network.
All Features Are Not Yet Available
Not all the features and attributes cited in Table 1–1 are available in 3G transport networks. In fact, third generation transport networks are just now appearing in the marketplace, and some capabilities that are touted for them are still in the lab. Nonetheless, many people think full-featured 3rd generation transport networks will be in the marketplace by around 2004. Certainly, pieces are emerging, such as bandwidth on demand, and of course, WDM and terabit networks. Other parts of 3G transport networks have yet to be implemented. For example, O/O/O operations are far from reaching commercial deployment on a mass scale.
Optical Fiber Capacity
To gain an appreciation of the transmission capacity of optical systems operating today, consider the facts in Table 1–2. Prior to the advent of optical fiber systems, a high-capacity network was capable of operating (sending and receiving traffic) at several million bits per second (Mbit/s). These electrical/electromagnetic transmissions take place over some form of metallic medium such as copper wire or coaxial cable, or over wireless systems such as microwave. In contrast, optical fiber systems transmit light signals through a glass or plastic medium. These systems are many orders of magnitude "faster" than their predecessors, with the capability of operating in the terabits-per-second (Tbit/s) range.
Table 1–2 Magnitudes and Meanings
|
Magnitude |
Term |
Initial |
Meaning |
|
1 000 000 000 000 000 000=1018 |
exa |
E |
Quintillion |
|
1 000 000 000 000 000=1015 |
peta |
P |
Quadrillion |
|
1 000 000 000 000=1012 |
tera |
T |
Trillion |
|
1 000 000 000=109 |
giga |
G |
Billion |
|
1 000 000=106 |
mega |
M |
Million |
|
1 000=103 |
kilo |
k |
Thousand |
|
100=102 |
hecto |
h |
Hundred |
|
10=101 |
deka |
da |
Ten |
As depicted in Figure 1–1, a terabit fiber carries 1012 bits per second. At this rate, the fiber can transport just over 35 million data connections at 28.8 kbit/s, or about 17 million digital voice channels, or just under 500,000 compressed TV channels (or combinations of these channels). Even the seasoned telecommunications professional pauses when thinking about the extraordinary capacity of optical fiber.
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Figure 1–1 Capacity of one fiber with a 1 Tbit/s rate.
A logical question for a newcomer to optical networks is, why are they of much greater capacity than, say, a network built on copper wire, or coaxial cable? The answer is that optical signals used in optical networks operate in a very high position and range of the frequency spectrum, many orders of magnitude higher than electromagnetic signals. Thus, the use of the higher frequencies permits the sending of many more user payloads (voice, video, and data) onto the fiber medium.
Figure 1–2 shows the progress made in the transmission capacity of optical fiber technology since 1982 [CHRA99]. The top line represents experimental systems, and the bottom line represents commercial systems. The commercial results have lagged behind the experimental results by about six years. The dramatic growth in the experimental capacity was due to improved laboratory techniques and the progress made in dispersion management, a subject discussed later in this book. As the figure shows, the transmission capacity of optical fiber has been growing at an extraordinary rate since the inception of the technology.
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Figure 1–2 Transmission capacity as a function of year [CHRA99].