Introduction to Networks – Part III
The focus of part 3 of this series is on the different types of wired networks. A key aspet of many networks is that QoS is just one concern, another key issue being how to meet guarantees for delivery of voice services. How is voice delivered? Integrated Services Digital Network (ISDN) is an international standard for voice, video, and data transmission over digital telephone or copper lines. It has two service levels. The first is Basic Rate Interface (BRI), which supports 2 bearer channels at 64kbps each and 1 D channel at 16 kbps. The second is the Primary Rate Interface (PRI), which supports 23 bearer channels (in the US) at 64 kbps each and 1 D channel at 16 kbps. The signaling/data (D) channel runs the ISDN signaling protocol based on
Q.931. This video is a good high level introduction of ISDN. T1 and ISDN are used in access networks, together with technologies like IP and MPLS.
Optical Networks
In newer generations of networks, the core is fiber (instead of copper) because it can deliver terabits per second. Installation and management of fiber networks is also much easier than copper networks. Fiber optic signals are analog – (in the infrared range).
Light sources used for fiber optic communication include light-emitting diodes (LEDs), laser diodes, vertical cavity surface emitting lasers (VCSELs), Fabry-Perot lasers, and distributed feedback lasers.
The packet transport network is another key piece to understand. Customers send traffic to metro access, aggregation, and core portions of the network where voice and data are converged. In the packet core, wavelengths are being added and dropped by add-drop multiplexers. There are several types of ADMs with links to explanations about them from various vendors:
- Fixed OADM (FOADM)
- Reconfigurable OADM (ROADM)
- Flexible ROADM
- Open ROADM, which works to address the fact that optical systems have been proprietary (e.g. because SD FEC algorithms) on transponders are not interoperable and there are proprietary control loops between transponders and other optical components).
The next video gave me a better understanding of customer concerns with ROADMs and FOADMs.
Other major types of network components include amplifiers, regenerators, and equalization nodes. Transponders map client side signals to wavelengths for high speed transport. They can be contained in a field-replaceable unit (FRU). Common types of pluggable optics include SFP+ (Small Form-factor Pluggable), CFP4, and QSFP28. Amplification is an analog process of boosting signal strength and is done in the optical domain (no conversion to electrical). Any impairments in the signal are boosted as well. A single pump laser is used for this. Regeneration can reshape and retime the optical signal but requires conversion to the electrical domain then back to the optical domain, making it more expensive to implement.
Major types of amplifiers in optical networks include EDFA (Erbium Doped Fiber Amplifer), Raman amplifier, and Hybrid Raman-EDFA amplifier. These are great explanations of these amplifiers:
Wavelength Selective Switch (WSS) was first implemented using MEMS but did’t work well because the hinges would fail. Liquid Crystal on Silicon (LCoS) is now commonly used to implement WSS since it has no moving parts. It can also support Flexgrid.
Optical patch panels are another component in fiber networks. They are used to join optical fibers where a connect/disconnect capability is required.
Handling Failure
There are 2 types of protection in networks:
- Network protection: ensures that customer SLAs are met by preventing failures. Optical protection examples include mesh restoration (GMPLS, SDN), SNCP (OTN), UPSR & BLSR for SONET, and 1+1 or 1:1 circuits (active vs inactive backup circuit). Packet protection examples include MPLS fast reroute, LAG, G.8031, G.8032.
- Equipment protection: focuses on protecting individual nodes.
I couldn’t emphasize this enough: this is such a broad field with so many technologies! What an introduction to networking!
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