Key Optical Nodes
We now leave the subject of the optical network marketplace, and focus our attention on the nodes (machines) that comprise the network. Figure 110 shows the key nodes in an optical network. The topology is a ring, but the topology can be set up either as a ring, a point-to-point, multipoint, or meshed system. In most large networks, the ring is a dual ring, operating with two or more optical fibers. The structure of the dual ring topology permits the network to recover automatically from failures on the optical links and in the link/node interfaces. This is known as a self-healing ring and is explained in later chapters.
Figure 110 Key nodes in the optical network.
End-user devices operating on LANs and digital transport systems (such as DS1, E1, etc.) are attached to the network through a service adapter. This service adapter is also called an access node, a terminal, or a terminal multiplexer. This node is responsible for supporting the end-user interface by sending and receiving traffic from LANs, DS1, DS3, E1, ATM nodes, etc. It is really a concentrator at the sending site because it consolidates multiple user traffic into a payload envelope for transport onto the optical network. It performs a complementary, yet opposite, service at the receiving site.
The user signals, such as T1, E1, ATM cells, etc., are called tributaries. The tributaries are converted (mapped) into a standard format called the synchronous transport signal (STS), which is the basic building block of the optical multiplexing hierarchy. The STS signal is an electrical signal. The notation STS-n means that the service adapter can multiplex the STS signal into higher integer multiples of the base rate, The STS signals are converted into optical signals by the terminal adapter and are then called OC (optical carrier) signals.
The terminal/service adapter can be implemented as the end-user interface machine, or as an add-drop multiplexer (ADM). The ADM implementation multiplexes various STS input streams onto optical fiber channels. OC-n streams are demultiplexed as well as multiplexed with the ADM.
The term add-drop means that the machine can add or drop payload onto one of the fiber links. Remaining traffic that is not dropped passes straight through the multiplexer without additional processing.
The cross-connect (CS) machine usually acts as a hub in the optical network. It can not only add and drop payload, but it can also operate with different carrier rates, such as DS1, OC-n, E1, etc. The cross-connect can make two-way cross-connections between the payload and can consolidate and separate different types of payloads. For example, the cross-connect can consolidate multiple low bit-rate tributaries into higher bit-rate tributaries, and vice versa. This operation is known as grooming.
Key Terms for the Cross-connect
The convention in this book is to use three terms to describe the optical cross-connect. There is a spate of terms to describe a cross-connect. I counted six terms in one paper alone. To make sure there is no ambiguity about the optical cross-connect in this book, the following terms are used:
Optical/Electrical cross-connect (OXC): Receives optical signals, converts them to electrical signals, makes routing/switching and/or ADM decisions, then converts the electrical signals back to optical signals for transmission. These operations are also noted as O/E/O. This technique is also called an opaque operation.
Photonic cross-connect (PXC): Performs the functions of the OXC, but performs all operations on optical signals. These operations are also noted as O/O/O, and are also called transparent operations.
Cross-connect (XC): A more generic term, used when it is not necessary to distinguish between the OXC or the PXC.
Switch: Some recent literature distinguishes between a cross-connect and a switch. This literature states that cross-connect is an outdated term! Well, the term switch has also been around for quite a while. Anyway, the book uses the terms cross-connect and switch synonymously.
Other terms and different definitions of optical nodes are used by various vendors, network operators, and standards groups. In some cases, they are the same as those just cited; in other cases, they are different. Where appropriate, I will distinguish and explain these other terms.