Where we are We can send strings of bits We can keep the transmitter and receiver clock synchronized What next? Datalink layer Logical link control Media access contro Framing Error detection/ correction Flow control Access ecture 6 15441@2008 16
Where we are •We can send strings of bits •We can keep the transmitter and receiver clock synchronized •What next? – Datalink layer • Logical link control • Media access control – Framing – Error detection/correction – Flow control – Access Lecture 6 15-441 © 2008 16
Framing 0010101011100101010101011101110000001111010101110 How do we differentiate the stream of bits into frames? ecture 6 15441@2008 17
Framing •How do we differentiate the stream of bits into frames? Lecture 6 15-441 © 2008 0100010101011100101010101011101110000001111010101110101010101101011010111001 17
Framing A link layer function defining which bits have which function Minimal functionality mark the beginning and end of frames Some techniques Out-of-band: delimiters(e.g 4B/5B control symbols) In-band: frame delimiter characters with char stuffing frame delimiter codes with bit stuffing Clock based:(e. g SONET) ecture 6 15441@2008 18
Framing •A link layer function, defining which bits have which function. •Minimal functionality: mark the beginning and end of frames. •Some techniques: – Out-of-band: delimiters (e.g. 4B/5B control symbols) – In-band: • frame delimiter characters with char stuffing • frame delimiter codes with bit stuffing – Clock based: (e.g. SONET) Lecture 6 15-441 © 2008 18
Out-of-band: E. g, 802.5 Start Access Frame Dest Src End Frame delim ctrl ctrl aar adr Body checksum delim status 802.5/token ring uses 4b/5b start delim end delim are "illegal"codes ecture 6 15441@2008
Out-of-band: E.g., 802.5 •802.5/token ring uses 4b/5b •Start delim & end delim are “illegal” codes Lecture 6 15-441 © 2008 Start delim Access ctrl Body checksum Frame ctrl Dest adr Src adr End delim Frame status 19
Sentinal based: E. g, BISyNC SYN SYN SOH Header STX Body ETX CRC SyN: sync character SOH: start of header STX: start of tex↑ ·ETx: end of tex十 What happens when eTX is in body ecture 6 15441@2008
Sentinal based: E.g., BISYNC •SYN: sync character •SOH: start of header •STX: start of text •ETX: end of text •What happens when ETX is in Body? Lecture 6 15-441 © 2008 SYN SYN SOH Header STX Body ETX CRC 20