Question

A) Explain why the stuffing is needed for PPP protocol.

B)Explain why parity checking for error detection.

Answer 1

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Stuffing is a method to maintain transparency in a communications channel when certain specific bit or byte sequences have special meaning to lower layer protocols.

For example, HDLC delimits physical frames with the 8-bit flag sequence 01111110 (hex 7e). Without stuffing, if an application sent this value as data the HDLC receiver would misinterpret it as the end of a frame. So bit-synchronous HDLC specifies that whenever 5 consecutive 1's appears in the user data stream, a 0 is inserted (stuffed) at the transmitter before the framing flags are added so that if the next two user data bits happen to be '10' it cannot form a "false flag". (A run of more than 7 1's on the channel aborts the current frame, so bit stuffing prevents this as well).

After flag detection at the receiver, the sequence ...111110 is converted to ...11111, i.e., the stuffed 0-bit is removed to restore the original sequence of five user 1-bits.

An alternate byte-oriented form of HDLC is used in the Point-to-Point (PPP) protocol, as the dialup modems for which it was designed usually carry 8-bit bytes, not bits. This required a different kind of stuffing rule, known as byte stuffing. When the byte 7e hex appears in the user data stream, it is translated to the two-byte sequence 7d 5e: the "control escape" code 7d and the original data value 7e exclusive-ORed with 20 hex. At the receiver, the value 7d is removed and the following byte exclusive-ORed with 20 hex to restore the original data byte 7e hex. If the user sends 7d as actual data, this becomes the two-byte sequence 7d 5d, and the receiver similarly turns this back to the original data byte of 7d.

Parity checking uses parity bits to check that data has been transmitted accurately. The parity bit is added to every data unit (typically seven or eight bits) that are transmitted. The parity bit for each unit is set so that all bytes have either an odd number or an even number of set bits.

Parity checking is the most basic form of error detection in communications. Although it detects many errors, it is not foolproof, because it cannot detect situations in which an even number of bits in the same data unit are changed due to electrical noise. There are many other more sophisticated protocols for ensuring transmission accuracy, such as MNP and CCITT V.42.

Parity checking is used not only in communications but also to test memory storage devices. Many PCs, for example, perform a parity check on memory every time a byte of data is read.

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