1 CS4550: Computer Networks II Review Data Link Layer

data link layer phys. layer subject to errors; not reliable; and only moves information as bits, which alone are not meaningful. DL layer adds these, and combines bits into frames, or messages. purpose of DL: transform unreliable physical bit stream into reliable data communications link... PHY + DL = DATA COMMUNICATIONS MAC layer (media access control) - takes place of DL layer in LANs (together with LLC)

data link layer : functions framing and frame synchronization –frames marked by sync/async technique error control flow control addressing control access, data on same link (unlike EIA232) link management (3 phases)

HDLC: high level data link control ISO standard for a data link protocol other DL standards exist, but are very similar; e.g., PPP HDLC combines various functions of the DL layer - flow control, error control, sequencing, framing, etc. - into a single protocol standard

HDLC frame types and formats f a c data fcs f u I-frame(information/data) u S-frame (supervisory) u U-frame (Ip datagrams with a = ) 0 seq p/f next 1 0 type p/f next 1 1 type p/f modifier

data link layer : error control 3 basic techniques in this course (more complex techniques exist) –parity checking very simple and easy error detection –CRC - cyclic redundancy check more complex, but very effective and efficient –Hamming code limited error correction; based on complex combinations of parity checks

data link layer : flow and error control purpose : regulate the flow of data from sender S to receiver R, so that R is neither overwhelmed nor kept idle unnecessarily. secondary purpose may also be used to avoid swamping the network or link with traffic. technique : send control information between S and R, synchronizing on buffer space, transmission rates, etc. protocols: –stop-and-wait, alternating bit –sliding window (go-back-N, selective repeat/reject)

CSMA / CD : basic protocol transmissions are stopped when collisions are detected. first version used bus -broadcast topology when the MAC receives a packet to transmit: (1) sense the carrier; {LISTEN} if no signal is detected then begin Xmitting message & continue sensing; if collision detected then Xmit jam, stop Xmitting, wait(BACKOFF), goto (1); when end-of-packet Xmitted, END. else {carrier is busy} go to (1)

CSMA / CD : frame format length: 64 to 1518 bytes preamble : 7 bytes; SOF : 1 byte; DA,SA : 2/6 bytes; length : 2 bytes; data : ; pad : 0-46; FCS : 4

CSMA / CD : standards IEEE : several physical configurations: 10BASE5 : baseband coaxial cable; original 10BASE2 : thin coaxial cable; cheaper alternative 10BASE-T : twisted pair, hub configuration 10BROAD36 : uses broadband coax (TV cable) 10BASE-F : fiber 100BASE-X : fiber OR twisted pair NOTE: all use the same frame format and basic MAC protocol

Token Ring access controlled by token

Token Ring Network - general each station receives a bit, checks it, and passes it on; this introduces delay of about bits per station note that a station does not receive entire message before passing it on only node with token transmits; others repeat. frames removed by station that generated them THT (token holding timer) limits time a station holds the token token passed when (a) no more data or (b) THT expires

Token Ring : frame formats token SD: JK0JK000 ED: JK1JK1IE frame AC: access control (8) FS: frame status (8) SD AC ED FC DA SAINFOFCS ED FSSDAC P P PR R R T M A C r r A C r r