Tuesday, 3 April 2007

Optical Transmission

Optical Fiber
(photo quoted from European Space Agency)
Why? Because it is immune to electrical interferences, it does not radiate signals, it uses less duct space than cooper or coax, goes longer distances and has now very low cost and huge information capacity.

An optical fiber consists of a very thin core (where light travels) and a large cladding which keeps the light in the core. A coating on the outside protects the fiber. The core not being perfectly circular create the optical pulse to get distorted (giving optical non linear effects).

Optical transmission

A laser generates optical pulses (pulses of light) controlled by the incoming electrical signal ‘1’ and ‘0’. A light sensitive component (photo diode) detects these pulses of light and reconstitutes the original electrical signal ‘1’ and ‘0’. Usually optical transmission is on a fiber pair – one for transmit and one for receive however transmit and receive can also travel on the same fiber. Optical transmission circuit characteristics (such as in a synchronous network): fixed size; pipe exists whether data flows or not; no concept of congestion in a transmission network as the total size of the pipes coming in, adds up exactly to the total size of the pipes going out.

Key differences between Metro and Long Haul networks
Metro Networks have a large range of services for 1.5M to 10G (DS1, DS3, Optical Ethernet, ESCON, FibreChannel, etc).

They are rapidly changing networks as new nodes are added for new customers, have short distances between nodes and lots of Network Elements hence need to keep NE cost to a minimum. Long Haul networks provide big transport pipes (moving to Terabits per fiber pair); a more stable network topology than Metro networks, less services than in the Metro (34M/45M, 2.5G, 10G, 40G future, GbE, 10GbE future) and greater distances between nodes (100’s of km). Long haul networks can be classified as backbone (many points, average circuit length less than 600km) and express (circuit length greater than 1000km).

Transmission requirements:
Optics performance adapted to the distance (cheap optics for the metro and optics to go thousands of km in the Long Haul); Flexibility (for instance in terms of traffic add/drop at a node or size of junction) and best use of fiber (which means 100’s of wavelengths in the Long Haul and 10’s in the Metro).

TDM and WDM

How to get the maximum capacity on a link: have the max number of bits per second for a signal (TDM) and have the max number of optical signals sharing a fiber (WDM).
Multiplexing: way to allow signals to share the same medium with each signal having the illusion to have their own line.

TDM gives a time slot to each signal. This means that the position in time determines which signal it is.

Multiplexing examples: 24 phones calls are multiplexed into a T1 in North America. 30 phones calls are multiplexed into an E1 outside North America.
WDM allows different optical signals (different bit rate and protocol) to share the same fiber by giving each signal a different frequency or color.

Types of circuit:

Fixed point to point: no bandwidth management/signal allocation flexibility – all signals are multiplexed at one end and demultiplexed at the other end. In synchronous networks (TDM) the mux is called Terminal mux or Line System. In optical networks it is an Optical Mux/Demux which multiplexes different optical signals.

Flexible networks can be a mesh of cross connect or switches or/and rings of ADM (Add Drop Muxes). A Cross connect is a piece of equipment with lots of ports: semi permanent connections between ports are under the control of the cross connect management system (not the end user of the network). Crossconnect with high capacity optical interfaces are called switches. They can be electrical inside (opaque switch) or purely optical (photonic switch).

An ADM allows traffic to get in and out of the main traffic flow.

Architecture for resilience:
A ring provides 2 ways to connect 2 points hence provides a fast protection mechanism; a Mesh provides various levels of protection (versus just protected or not) but this is more complex than a ring; Point to point system can be protected by sending the signal simultaneously on 2 transmitters and the receiver at the other end selects the signal.

Optical transmission:
Parameters affecting light transmission: attenuation causes the light pulse to loose intensity (the light pulse gets smaller); chromatic dispersion causes the light pulse to broaden.
Attenuation: Regenerators and amplifiers control attenuation. A regenerator terminates the optical signal, meaning that it converts the signal back to the original ‘1’ and ‘0’ and from that generates a brand new optical signal again. An amplifier gives energy to the optical signal and allows it to go further (a single amplifier amplifies an optical signal made of several wavelengths).

Amplifiers replace a regenerator ‘mountains’ since regenerator acts on a single channel and at a regenerator site, the signal has to be optically demultiplexed for each signal to be regenerated. Amplifier can be cascaded up to a certain number then regenerators need to be used.
Dispersion: Fiber types, Dispersion Compensation Module (DCM) and types of transmitter can be used to control dispersion;
Laying new fiber is expensive and some networks have already existing standard fiber;
DCM (length of special fiber) can be inserted in the network to compensate for dispersion;
Laser modulation in the transmitter controls dispersion too. Directly modulated transmitters are cheap however the laser going on and off creates heavy dispersion hence these transmitters are suitable only for short distances.

Externally modulated lasers (laser stays on and an external circuit masks the light to create the pulses) provide better pulse for long distances.

Nortel Networks

57 comments:

Melora said...

Good for people to know.

Mahesh said...

hm...nice...update perodically...

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