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Asynchronous and Synchronous Transmission

Two techniques are used by the DLC to provide timing and proper reception of the traffic at the receiver:

  1. Asynchronous transmission : each data byte has start and stop bits (i.e. synchronizing signals) placed around it. The puprpose is to a) alert the receiver that data are arriving b) to give the receiver sufficient time to perform certain timing functions before the next character arrives. Widely used because equipment is relatively inexpensive. Most PCs use asynchronous interfaces. Loose timing is allowed because the inaccurace can be corrected with the net arriving character.
  2. Synchronous transmission - eliminates the intermittent start/stop signals around each character and provide signals that precede and sometimes follow the user data stream. The principal function is to alert the receiver to an incoming frame and to provide a means to determine when all bits in the frame have been transmitted. This process is called framing.
  3. Async transmission is cost effective for low-speed and low-volume transmissions such as those from keyboard entry terminals. For larger volumes, sync transmission is more efficient.

          Assume a 128-byte user block to be transmitted:

Asynchronous Synchronous
2 (start/stop)/8 = 0.25 6(bytes of control overhead)*8 bits = 48 bits    128 (bytes of data) * 8 bits = 1024 bits          48/1024 = 0.46

 

25% Overhead                     4.6% Overhead

       

Synchronous Link Protocols

  1. Character Oriented Protocols - developed in 1960s, remain in use today, although limited.Protocols rely on a specific code set (ASCII, EBCDIC) to interpret the control fields; thus they are code dependent. Communicating systems must recognize the same code set in order to communicate. Need special "transpacrency" characters to deal with control characters in the data stream (give examples DLE)
  2. Count Oriented Protocols - developed in 1970s to address code dependency problem. These protocols are more effective in handling of user data transparency: they insert a count field at the transmitting station. This field specifies the length of the user data field, and as a consequence the receiver does not have examine the user data field. Count-oriented protocols may encounter problems when the signals are transmitted across a digitial link. A digitial system may delete a  frame  to recover clocks and resynchronize, it may also insert timing/control data into the transmission.
  3. Bit Oriented Protocols - developed in the 1970s and are now prevalent throughout the industry. Does not rely on specific codes (ASCII or EBCDIC) for line control.Individual bits within an octet are set to effeect control functions. An eight-bit flag pattern of 011111110 is the usual flag value. It is generated at the beginning and end of a transmission.. Bit stuffing is used to deal with similar pattern occuring in the data stream.

Function of Timers

Many link control protocols uyse timers in conjunction with logic states to verify that an event occurs within a prescribed time.

When a transmitting station sends a frame onto the channe, it starts a timer and enters a wait state. The value of the timer, T1, is set to expiure if the receiving station does not respond to the transmitted frame within the set period. The value of T1 depends on :

The receiving station may use a parameter T2 in conjunction with T1. Its value is set to ensure an acknowledgement frame is sent to the transmitting station before the T1 at the transitter expires. This action precludes the transmitter from resending frames unncessarily.

T1T >= T2R + PD + FPTr +TTcur +TTack + FPTT

T2r <= T1t -PD - FPTr -TTcur -TTack -FPTt

Where: T is the transmitter; R is the receiver; PD is the round-trip propagation delay; FTP is the frame processing time; TTack is the transmission time of the acknowledgement frame; TTcur is the time to complete thte transmission of the ongoing frames that already are in the transmit queue and cannot be pushed down into the queue.

Other timer include:

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