8 INTRODUCTION
link from router A to OXC 1, OXC 1 to OXC 2, OXC 2 to OXC 3, and OXC 3 to router B is assumed to be a single fiber with W wavelengths, referred to as λ
1
, λ
2
, . . . , λ
W
. Data is transmitted only in one direction: from router A to router B. Another set of fibers not
shown in Figure 1.5a has to be used in order to transmit data in the opposite direction i.e. from router B to router A.
Assume that IP router A wants to transmit data to IP router B. Using a signaling protocol, A requests the establishment of a connection to B. The connection between
routers A and B is established by allocating the same wavelength, say wavelength λ
1
, on all of the links along the path from A to B i.e., links A to OXC 1, OXC 1 to OXC 2,
OXC 2 to OXC 3, and OXC 3 to B. Also, each OXC is instructed to switch λ
1
through its switch fabric transparently. As a result, an optical path is formed from router A to B, over
which data is transmitted optically. This optical path is called a lightpath, and it connects routers A and B in a unidirectional way from A to B. In order for B to communicate with
A, a separate lightpath has to be established in the opposite way over a different set of fibers which are set up to transmit in the opposite direction.
1.3 ORGANIZATION OF THE BOOK
In this book, we explore two connection-oriented packet-switching networks: ATM net- works and MPLS-enabled networks. ATM is a legacy network that was developed in the
late 1980s and early 1990s. It is used in the backbone to transport IP traffic, in access networks such as ADSL-based networks and ATM passive optical networks APON, and
in cellular telephony. The MPLS architecture can be seen as an extension of ATM, and it can be used to introduce QoS in IP networks.
Two circuit-switching networks – SONETSDH and optical wavelength routing net- works
– are also presented in this book. SONETSDH has been around for along time, whereas optical wavelength routing networks are relatively new. SONETSDH is the
underlying transport network of the telephone system. It is also used in all modern packet-switching networks, such as IP and ATM. Wavelength routing networks are also
circuit-switching networks since the transmission of data is done using optical circuit- switching connections, known as lightpaths. We also present a new optical networking
scheme, which has not yet been standardized, known as optical burst switching OBS. This type of optical network can be seen as lying between packet switching and cir-
cuit switching.
Finally, the book contains a chapter on access networks, such as ADSL-based networks, cable modems, and passive optical networks, and a chapter on voice over ATM and voice
over MPLS. The book consists of twelve chapters, which cover the following topics:
• Chapter 1: Introduction
• Chapter 2: SONETSDH
• Chapters 3, 4, and 5: ATM networks
• Chapters 6 and 7: MPLS
• Chapters 8, 9, and 10: Optical networks
• Chapter 11: Access networks
• Chapter 12: Voice over ATM and MPLS
ORGANIZATION OF THE BOOK 9
Below, we briefly examine the content of each chapter. Chapter 2: SONETSDH and the Generic Frame Procedure GFP
In this chapter, we focus on the SONETSDH transport technology. We first start with a description of T1 and E1, and then we present in detail the SONETSDH hierar-
chy, the SONET STS-1 frame structure, overheads, payload, and the SONET STS-3 frame structure.
Subsequently, we describe the SONETSDH devices and SONETSDH rings. One of the main features of a SONETSDH rings is that they are self-healing. That is, a
SONETSDH ring can automatically recover when a fiber link fails. Link failure can result from a fiber being accidentally cut, or the optical components that are used to transmit
on a fiber fail, or the SONETSDH switch fails. We describe various architectures for self-healing rings, such as two-fiber and four-fiber protection schemes.
We conclude this chapter with a description of the generic framing procedure GFP and data over SONETSDH DoS. GFP is a lightweight adaptation scheme that permits
the transmission of different types of traffic over SONETSDH and, in the future, over G.709. DoS is a network architecture that uses GFP together with two other mechanisms
to provide an efficient transport of integrated data services over SONETSDH.
Chapter 3: ATM networks The asynchronous transfer mode ATM architecture was standardized in 1987 by ITU-T as
the preferred architecture for the broadband integrated services data network B-ISDN.
ATM is a mature technology that is primarily used in the backbone. For instance, it is widely used in the backbone of internet service providers ISPs and it
has been deployed to provide point-to-point and point-to-multipoint video connections. It is also used in cellular telephony to carry multiple voice connections using the ATM
adaptation layer 2 AAL 2. It is also used for circuit emulation, a service that emulates a
point-to-point T1E1 circuit over an ATM network. ATM is also used in access networks such as ADSL-based residential access networks and ATM passive optical networks. ATM
is not visible to the networking users, as is the IPTCP protocol, and because of this, it is often mistaken as a network that it is no longer in use
The ATM architecture was a novel departure from previous networking architectures; it has built-in mechanisms that permit the transport of different types of traffic with
different QoS. Until the advent of multi-protocol label switching MPLS architecture in the late 1990s, ATM was the only networking technology that provided QoS. From the
educational point of view, it is a good idea to develop a working knowledge of ATM and its congestion control schemes before proceeding to MPLS in Chapter 6.
This chapter is organized as follows. We first present the main features of the ATM architecture, such as the structure of the header of the ATM cell, the ATM protocol
stack, and the physical layer. Then we briefly describe the ATM shared memory switch architecture, which is the dominant switch architecture, and various scheduling algorithms
used to determine the order in which ATM cells are transmitted out. Subsequently, we describe the three ATM adaptation layers AAL: AAL 1, AAL 2, and AAL 5. We conclude
the chapter with a description of classical IP and ARP over ATM, a technique standardized by IETF to transport IP over ATM.
