Telecommunication/Internetworking Devices (Continued)

Multiplexers

• What is multiplexing?• Identify an advantage associated with multiplexing,

and their two generic types of capacity constraints.• What is inverse multiplexing? Identify couple of

IMUX applications that are popular in the telecommunication industry.

• Identify some types of multiplexers.• Identify advantages/disadvantages associated with

different types of multiplexers.

Multiplexing

• Multiplexing means breaking up a higher speed circuit into several slower logical circuits (channels).

• The main advantage of multiplexing is cost savings.

• Example: NASA (Page 98-99 of textbook)

Multiplexed Circuit

Inverse Multiplexing• Inverse multiplexing is combining several lower speed

circuits to make them appear as one high-speed circuit.• Example 1: Room-to-room videoconferencing.

– Using IMUX, six 64 kbps lines can be combined to create an aggregate line of 384 kbps for transmitting video.

• Example 2: Dedicated Circuit Network Services– Lease a dedicated circuit such as a T-circuit or a OC

(Optical Carrier) circuit for a flat fee every month. • A T1 circuit inverse multiplexes 24 circuits of 64Kbps each,

giving a high-speed of 1.54 Mbps. An OC-3 cable combines 3 OC-1 cables of 51.84 Mbps each, giving 155.52 Mbps

– Synchronous Optical Network (SONET) is the ANSI standard for high-speed dedicated-circuit services.

• SONET speeds begin at the OC-1 level and are defined even as high as OC-192 level (10 Gbps). Few national ISPs are experimenting with OC-768 (80 Gbps), and planning for OC-3072 (160 Gbps).

Types of Multiplexing

• We consider four types of multiplexing:– Frequency division multiplexing (FDM)– Time division multiplexing (TDM) or

Synchronous TDM– Statistical time division multiplexing

(STDM) or Asynchronous TDM – Wavelength division multiplexing (WDM)

• Dense Wavelength division multiplexing (DWDM)

Frequency Division Multiplexing

• High-speed circuit is divided into a series of separate channels, each transmitting on a different frequency

• In order to prevent interference between channels, unused frequency bands called ‘Guardbands’ are used to separate the channels. Due to Guardbands, there is always some wasted capacity on an FDM circuit.

• CATV uses FDM. FDM was also commonly used to multiplex telephone signals before digital transmission became common and is still used on some older transmission lines.

Frequency Division Multiplexing

Time Division Multiplexing

• TDM shares the high-speed circuit among two or more end-user nodes by providing each node a dedicated capacity on the circuit for message transmission

• Capacity of high-speed circuit will be wasted only when there is no transmission from one or more end-user nodes

• More efficient than FDM, because there are no guardbands in TDM

Time Division Multiplexing

Statistical Time Division Multiplexing

• Capacity of the multiplexed circuit is less than the sum of the circuits it combines

• STDM is designed to make use of the idle time created when some end-user nodes are not using the multiplexed circuit.

• Selecting the transmission speed for the multiplexed circuit is based on a statistical/historical analysis of the usage requirements of the circuits to be multiplexed

• More efficient use of the circuit and saves money• Must have internal memory to store any message that it

cannot transmit immediately – causes delays• All messages must be identified by addresses

Wavelength Division Multiplexing (WDM)

• Optical fiber previously transmitted at only a single frequency, with a typical transmission rate being around 622 Mbps.

• Wavelength Division Multiplexing (WDM) is a version of FDM used in fiber optic cables. WDM works by transmitting different frequencies of light (i.e., colors) through the same fiber optic cable.

• The data transmission capacity of optical continues to increase dramatically. A new version of WDM, Dense WDM or DWDM promises data rates in the terabits, with over a hundred channels per fiber, each transmitting at a rate of 10 Gbps, making aggregate data rates in the low terabit range possible.

• Future versions of DWDM are expected to make petabit aggregate transmission rates possible as per channel data rates and the total number of channels both continue to rise.