A switched network consists of a series of intermediate nodes called switches. Switches are H/W and/or S/W devices capable of creating temporary connections between two or more devices linked to the switch not to each other.

Circuit switching creates direct physical connection between two devices.—is a device with n inputs and m outputs that creates temporary connection between an input link and an output link—no of inputs does not have to match no of outputs An n by n folded switch can connect n lines in full duplex mode.

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Synchronous optical network (SONET) is ANSI standard whereas Synchronous digital hierarchy (SDH) is ITU-T standard. SONET/SDH is a synchronous network. Single clock is used to handle the timing of transmission and equipment across the entire network so it adds a level of predictability to the system.—improved speed and reducing cost. SONET/SDH contains recommendations for the standardization of fiber optic transmission equipment (FOTS). SONET/SDH physical specification and frame design include mechanism that allow it to carry signals from incompatible tributary systems (e.g. asynchronous services –DS-0 and DS-1) SONET is a multiplexed transport mechanism and can be a carrier for broadband services e.g. ATM and B-ISDN. SONET defines a hierarchy of signaling levels called synchronous transport signals (STSs). The physical links defined o carry each level of STS is called Optical Carriers (OCs). OC describe the conceptual and physical specification of the links required to support each level of signaling.

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ISDN consists of following

Bearer Services: provide the means to transfer information between users without the network manipulating the contents of that
information. Belongs to first three layers.
Teleservices: the network may change or process the contents of the data. Correspond to 4-7 layers.
Supplementary services: provide additional functionality to the bearer services and teleservices.

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ATM: Asynchronous Transfer Mode

The most important technical innovation to come out of the standardization work on broadband ISDN is asynchrounous transfer mode (ATM), is a cell relay uses fixed size cell transmission technique. It has less overhead than frame relay and designed to operate at significantly  higher data rates than frame relay.

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Frame Relay

Frame Relay is a virtual circuit technology that provides low-level (physical and data link layers) services in response to following demands:
o Higher data rate at lower cost
o Bursty data—bandwidth on demand—bursty data requires different bandwidth allocation at different times.
o Less overhead due to improved transmission media
• The most technical innovation to come out of the standardization work on narrowband ISDN is the frame relay.
• Frame relay is a streamlined technique for packet switching that operates at the data link layer with much less overhead than packet switching X.25
• Standardization of the frame-mode bearer service, or frame relay, was done by both ITU-T and ANSI.

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Data Link Protocol

Data Link protocols
• Asynchronous Protocols
• Not complex and inexpensive to implement
• Data unit is transmitted with no timing co-ordination between sender and receiver.
• Receiver does not know when data unit is sent- only recognizes the beginning and    ending of frame-accomplished by start and stop bit.
• Used primarily in modem, features start and stop bits and variable length gaps between    characters.
• Synchronous Protocols

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Data Link Layer Functions

Line Discipline: co-ordinates the link system-determines which device can send and when it can send.
• Flow Control: co-ordinates the amount of data that can be sent before receiving ack.
• Error Control: error detection/correction-allows receiver to inform the sender of any frame lost or damaged and co-ordinate the retransmission

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Digital Communication Systems

Learning material by

Prof.H. S. Jamadagni, CEDT, IISc, Bangalore

Topics in Digital Communications

  1. Digital communication system advantages and disadvantages
  2. Digital communication system classification
  3. Digitization of analog signals
  4. Digital transmission systems
  5. Data communication systems
  6. Integrated Services Digital Network and other advanced digital communication systems

View / download : DCN-LearningMaterial (size 1.44MB)