EE 122 Lecture 4 Ion Stoica e mail istoica cs berkeley edu September 6 2001 Overview Encoding Framing istoica cs berkeley edu 2 Encoding Goal send bits from one node to other node on the same physical media This service is provided by the physical layer Problem specify an robust and efficient encoding scheme to achieve this goal Assumption we use two discrete signals high and low to encode 0 and 1 Signal Adaptor Adaptor Adaptor Adaptor Adaptor convert bits into physical signal and physical signal back into bits istoica cs berkeley edu 3 Non Return to Zero NRZ 1 high signal 0 low signal Disadvantages when there is a long sequence of 1 s or 0 s Sensitive to clock skew i e difficult to do clock recovery Difficult to interpret 0 s and 1 s baseline wander 0 0 1 0 1 0 1 1 0 NRZ non return to zero Clock istoica cs berkeley edu 4 Non Return to Zero Inverted NRZI 1 make transition 0 stay at the same level Solve previous problems for long sequences of 1 s but not for 0 s 0 0 1 0 1 0 1 1 0 NRZI non return to zero intverted Clock istoica cs berkeley edu 5 Manchester 1 high to low transition 0 low to high transition Addresses clock recovery and baseline wander problems Disadvantage needs a clock that is twice as fast as the transmission rate 0 0 1 0 1 0 1 1 0 Manchester Clock istoica cs berkeley edu 6 4 bit 5 bit Goal address inefficiency of Manchester encoding while avoiding long periods of low or high signals Solution Use 5 bits to encode every sequence of four bits such that no 5 bit code has more than one leading 0 and two trailing 0 s Use NRZI to encode the 5 bit codes 4 bit 0000 0001 0010 0011 0100 0101 0110 1111 11110 01001 10100 10101 01010 01011 01110 01111 5 bit 4 bit 1000 1001 1010 1011 1100 1101 1110 1111 5 bit 10010 10011 10110 10111 11010 11011 11100 11101 istoica cs berkeley edu 7 Overview Encoding Framing istoica cs berkeley edu 8 Framing Goal send a block of bits frames between nodes connected on the same physical media This service is provided by the data link layer Use a special byte bit sequence to mark the beginning and the end of the frame Problem what happens if this sequence appears in the data payload istoica cs berkeley edu 9 Byte Oriented Protocols Sentinel Approach 8 STX 8 Text Data ETX STX start of text ETX end of text Problem what if ETX appears in the data portion of the frame Solution If ETX appears in the data introduce a special character DLE Data Link Escape before it If DLE appears in the text introduce another DLE character before it Protocol examples BISYNC PPP DDCMP istoica cs berkeley edu 10 Byte Oriented Protocols Byte Counting Approach Sender insert the length of the data in bytes at the beginning of the frame i e in the frame header Receiver extract this length and decrement it every time a byte is read When this counter becomes zero we are done istoica cs berkeley edu 11 Bit Oriented Protocols 8 Start sequence 8 Text Data End sequence Both start and end sequence can be the same E g 01111110 in HDLC High level Data Link Protocol Sender inserts a 0 after five consecutive 1s Receiver when it sees five 1s makes decision on next two bits if next bit 0 this is a stuffed bit remove it if next bit 1 look at the next bit If 0 this is end of frame receiver has seen 01111110 If 1 this is an error discard the frame receiver has seen 01111111 istoica cs berkeley edu 12 Clock Based Framing SONET SONET Synchronous Optical NETwork Example SONET ST 1 51 84 Mbps istoica cs berkeley edu 13 Clock Based Framing SONET First two bytes of each frame contain a special bit pattern that allows to determine where the frame starts No bit stuffing is used Receiver looks for the special bit pattern every 810 bytes Size of frame 9x90 810 bytes overhead 9 rows Data payload SONET STS 1 Frame 90 columns istoica cs berkeley edu 14 Clock Based Framing SONET Details Overhead bytes are encoded using NRZ To avoid long sequences of 0 s or 1 s the payload is XOR ed with a special 127 bit patter with many transitions from 1 to 0 istoica cs berkeley edu 15 Summary Encoding specify how bits are transmitted on the physical media Challenge achieve Efficiency ideally bit rate clock rate Robust avoid de synchronization between sender and receiver when there is a large sequence of 1 s or 0 s Framing specify how blocks of data are transmitted Challenge Decide when a frame starts ends Differentiate between the true frame delimiters and delimiters appearing in the payload data istoica cs berkeley edu 16
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