Optical and Computer Network Technologies

Posted on Jan 12, 2025 in Optics and Optometry

Section C: Generations of Networks

First Generation Optical Networks

In these networks, optical fiber is used purely as a transmission medium, replacing copper cable. Switching and processing of the bits are handled by electronics. Examples include SONET, SDH, and FDDI.

Second Generation Optical Networks

In these networks, some of the switching and routing functions are done in the optical domain. Examples include OTDM and WDM.

Elements of SONET/SDH

  • TM: Terminal Multiplexers
  • UPSR: Unidirectional Path-Switched Rings
  • BLSR: Bidirectional Line-Switched Rings
  • DCS: Digital Crossconnect
  • ADM: Add/Drop Multiplexer

Multiplexing in SONET

  1. A lower-speed PDH stream is mapped into a Synchronous Payload Envelope (SPE).
  2. A set of overhead bytes, called path overhead, is added to this SPE. The overhead contains important information, such as the Bit Error Rate (BER).
  3. The SPE with its path overhead is called a Virtual Tributary (VT).
  4. A number of VTs may be multiplexed into a larger VT. Each VT consists of an overhead followed by a payload. The overhead includes a pointer to each smaller VT that is multiplexed into the larger VT. The small VT may, in turn, contain a number of even smaller VTs, with pointers to each of them. This structure makes it particularly easy to extract a low-speed stream from a higher-speed stream.

Computer Networks

  1. ESCON
  2. Fiber Channel
  3. HIPPI

Metropolitan Area Networks

  1. FDDI is a metropolitan-area standard, operating at 100Mb/s over multimode or single-mode fiber.
    1. It is commonly deployed in the form of two-fiber rings, similar to SONET BLSR/2s, and incorporates a similar protection mechanism as well.
    2. Standard specifications are met by LED transmitters operating in the 1.3mm band over multimode fiber using a PIN diode receiver. However, lasers and single-mode fiber can be used, and in this case, the distance between two stations can go up to 40km.
    3. FDDI uses a (4,5) line code to achieve some measure of DC balance. Thus, the actual rate of transmission over the optical fiber is 125Mbaud.
  2. ATM is a networking standard that was developed with many goals, one of which was the integration of voice and data networks.
    1. An ATM network uses packets or cells with a fixed size of 53 bytes. This packet size is a compromise between the conflicting requirements of voice and data applications.
    2. Of the 53 bytes in an ATM packet, at least 5 bytes constitute the header, which is the overhead required to carry information such as the destination of the packet.
    3. Advantages:
      • Ability to provide Quality of Service (QoS) guarantees, such as bandwidth and delay.
      • ATM employs switching even in a local-area environment, unlike other LAN technologies.
      • The fixed size of the packets used in an ATM network is particularly advantageous for the development of low-cost, high-speed switches.
  3. IP is a networking technology, or protocol, that is designed to work over a wide variety of lower layers, which are termed data link layers.
    1. IP operates over popular local area networks such as Ethernet, Token Ring, as well as FDDI.
    2. Compared to ATM, traditional IP provides only a simple datagram service to its higher layers—there is no notion of a connection.
    3. It also does not provide any QoS guarantees, unlike ATM.
    4. ATM standards define interfaces so that IP can operate using ATM as its immediately lower layer.
    5. IP uses variable-size packets, making it not useful for voice communications.

Network Layers (Not for Optical Transmission)

  • Physical Layer: Provides a “pipe” with a certain amount of bandwidth to the layer above it. In an optical network, the physical layer is an optical fiber with different types of physical parameters.
  • Data Link Layer: Is responsible for framing, multiplexing, and demultiplexing data sent over the physical layer.
  • Network Layer: Usually provides Virtual Circuits (VC) or datagram services to the higher layer.
  • 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 messages.

SONET/SDH (Optical Network) Layers

  • Path Layer: Is responsible for end-to-end (E2E) connections between nodes and is needed only at the ends of a SONET connection. If a SONET path fails, the traffic must be switched over to another path. It is responsible for monitoring and tracking the status of a connection. Each connection traverses a set of links and intermediate nodes in the network. The path layer may be thought of as being equivalent to the network layer in the classical protocol hierarchy.
  • Line Layer: Multiplexes a number of path-layer connections onto a single link between two nodes. It is also responsible for performing certain types of protection switching to restore service in the event of a line failure. Many SONET streams are to be multiplexed onto a higher-speed stream and transmitted over a SONET link.
  • Section Layer: Is present at each regenerator in the network. The line and section layers correspond to the data link layer in the classical protocol hierarchy. The error rate on a SONET link between regenerators is to be monitored.
  • Physical Layer: Responsible for the actual transmission of bits across the fiber.

Physical Components of SONET

  • PTE: Multiplexes/demultiplexes the STS payload. It can originate, access, modify, or terminate the path overhead, or it can perform any combination of these actions.
  • LTE: Originates/terminates the line signal. It can originate, access, modify, or terminate the line overhead, or it can perform any combination of these actions.
  • STE: A section is any two adjacent SONET network elements. An STE can be a terminating network element or a regenerator. It can originate, access, modify, or terminate the section overhead, or it can perform any combination of these actions.

Optical Layer (WDM Network)

Provides lightpaths to the higher layer. A lightpath is an end-to-end connection established across the optical network and uses a wavelength on each link in a path between the source and destination.

  • Lightpath Layer: Is at the top and takes care of the end-to-end routing of the light paths. Each lightpath traverses a number of links in the network, and each of these links carries multiple wavelengths.