Telecommunication Technologies Week 9 HDLC (ISO 33009, ISO 4335)

EIE325: Telecommunication Technologies

EIE325: Telecommunication Technologies

HDLC Syntax Stations: Link HDLC (High level Data Link Control) ISO 33009, ISO 4335 Stations: Primary, Secondary, Combined Link Balance or Unbalanced Transfer Modes NRM, ABM, ARM Frames I-frames, S-frames, U-frames EIE325: Telecommunication Technologies

HDLC Station Types Primary station Secondary station Combined station Controls operation of link Frames issued are called commands Maintains separate logical link to each secondary station Secondary station Under control of primary station Frames issued called responses Combined station May issue commands and responses EIE325: Telecommunication Technologies

HDLC Link Configurations Unbalanced One primary and one or more secondary stations Supports full duplex and half duplex Balanced Two combined stations EIE325: Telecommunication Technologies

HDLC Transfer Modes Normal Response Mode (NRM) Unbalanced configuration Primary initiates transfer to secondary Secondary may only transmit data in response to command from primary Used on multi-drop lines Host computer as primary Terminals as secondary EIE325: Telecommunication Technologies

HDLC Transfer Modes Asynchronous Balanced Mode (ABM) Balanced configuration Either station may initiate transmission without receiving permission Most widely used No polling overhead EIE325: Telecommunication Technologies

HDLC Transfer Modes Asynchronous Response Mode (ARM) Unbalanced configuration Secondary may initiate transmission without permission form primary Primary responsible for line Rarely used EIE325: Telecommunication Technologies

HDLC Frame Types Three types of frames I-frames : Information (data) S-frames : Supervisory (ARQ) U-frames : Unnumbered (other) Frame structure the same in each case EIE325: Telecommunication Technologies

Frame Structure Synchronous transmission All transmissions in frames Single frame format for all data and control exchanges EIE325: Telecommunication Technologies

Frame Structure Diagram EIE325: Telecommunication Technologies

Flag Fields Delimit frame at both ends 01111110 May close one frame and open another Receiver hunts for flag sequence to synchronise Bit stuffing used to avoid confusion with data containing 01111110 0 inserted after every sequence of five 1s If receiver detects five 1s it checks next bit If 0, it is deleted If 1 and seventh bit is 0, accept as flag If sixth and seventh bits 1, sender is indicating abort EIE325: Telecommunication Technologies

Example with possible errors Bit Stuffing

Address Field Identifies secondary station that sent or will receive frame Usually 8 bits long May be extended to multiples of 7 bits LSB of each octet indicates that it is the last octet (1) or not (0) All ones (11111111) is broadcast EIE325: Telecommunication Technologies

Control Field Different for different frame type Information - data to be transmitted to user (next layer up) Flow and error control piggybacked on information frames Supervisory - ARQ when piggyback not used Unnumbered - supplementary link control First one or two bits of control field identify frame type EIE325: Telecommunication Technologies

Control Field Diagram EIE325: Telecommunication Technologies

Poll/Final Bit Use depends on context Command frame Response frame P bit 1 to solicit (poll) response from peer Response frame F bit 1 indicates response to soliciting command EIE325: Telecommunication Technologies

Information Field Only in information and some unnumbered frames Must contain integral number of octets Variable length EIE325: Telecommunication Technologies

Frame Check Sequence Field FCS Error detection 16 bit CRC Optional 32 bit CRC EIE325: Telecommunication Technologies

HDLC Operation Exchange of information, supervisory and unnumbered frames Three phases Initialisation Data transfer Disconnect EIE325: Telecommunication Technologies

EIE325: Telecommunication Technologies

HDLC commands EIE325: Telecommunication Technologies

E.g.: Setup and disconnect SABME: Set ABM, 7-bit sequence numbers UA: Unnumbered ACK DISC: Disconnect EIE325: Telecommunication Technologies

E.g.: Exchange I,n,m: Information n & m are sender and receiver sequence numbers RR: Receive ready EIE325: Telecommunication Technologies

E.g.: Busy RNR: Receive not ready EIE325: Telecommunication Technologies

E.g.: Reject recovery REJ: Reject EIE325: Telecommunication Technologies

E.g.: Timeout recovery EIE325: Telecommunication Technologies

Telecommunication Technologies Week 9 Synchronisation

Asynchronous and Synchronous Transmission Timing problems require a mechanism to synchronise the transmitter and receiver data rate bit duration inter-frame spacing Two solutions Asynchronous (data not contiguous) Synchronous (data blocks contiguous) EIE325: Telecommunication Technologies

Presumptions Serial, not Parallel Transmission One bit per signal element Sender and receiver’s clocks are different Errors in timing (sampling) as well as amplitude (quantisation) EIE325: Telecommunication Technologies

Asynchronous Data transmitted one character (5-8 bits) at a time Timing only needs maintaining within each character Resynchronise for each character Idle (binary 1) between characters Start bit is a binary 0 EIE325: Telecommunication Technologies

Asynchronous (diagram) EIE325: Telecommunication Technologies

Asynchronous Timing Error 10kbps transmission … bit duration of 0.1ms. Over 8 bits a 5% timing error is acceptable A 6% timing error is not. EIE325: Telecommunication Technologies

Asynchronous - Behavior In a steady stream, interval between characters is uniform (length of stop element) In idle state, receiver looks for transition 1 to 0 Then samples next seven intervals (char length) Then looks for next 1 to 0 for next char Simple Cheap Overhead of 2 or 3 bits per char (~20%) Good for data with large gaps (keyboard/terminal) EIE325: Telecommunication Technologies

Asynchronous Errors Timing errors Framing errors extreme discrepancy between sender and receiver’s clocks. Framing errors Erroneous start bits EIE325: Telecommunication Technologies

Synchronous - Bit Level Block of data transmitted without start or stop bits Clocks must be synchronised Can use separate clock line Good over short distances Subject to impairments Embed clock signal in data Manchester encoding Carrier frequency (analog) EIE325: Telecommunication Technologies

Synchronous - Block Level Need to indicate start and end of block Use preamble and “postamble” e.g. series of SYN (hex 16) characters e.g. block of 11111111 patterns ending in 11111110 More efficient (lower overhead) than asynchronous E.g. HDLC EIE325: Telecommunication Technologies

Synchronous e.g. HDLC EIE325: Telecommunication Technologies

Echo Cancellation Transceiver: Transmitter/Receiver Two parties transmit on the same data path simultaneously Each is aware of their own transmission and can subtract that from the resultant to receive the other! EIE325: Telecommunication Technologies

Telecommunication Technologies Week 9 Interfacing

Interfacing Data processing devices (or data terminal equipment, DTE) do not (usually) include data transmission facilities Need an interface called data circuit terminating equipment (DCE) e.g. modem, NIC DCE transmits bits on medium DCE communicates data and control info with DTE Done over interchange circuits Clear interface standards required EIE325: Telecommunication Technologies

Interfacing EIE325: Telecommunication Technologies

Characteristics of Interface Mechanical Connection plugs Electrical Voltage, timing, encoding Functional Data, control, timing, grounding Procedural Sequence of events EIE325: Telecommunication Technologies

Two examples Modem ISDN EIE325: Telecommunication Technologies

E.g.: V.24/EIA-232-F ITU-T V.24 specifies functional and procedural EIA-232-F (USA) (originally RS-232): Mechanical ISO 2110 Electrical V.28 Functional V.24 Procedural V.24 EIE325: Telecommunication Technologies

Mechanical Specification

Data Pins EIE325: Telecommunication Technologies

Control Pins EIE325: Telecommunication Technologies

Timing Pins EIE325: Telecommunication Technologies

Electrical Specification Signaling specification Values interpreted as data or control, depending on circuit More than -3v is binary 1, more than +3v is binary 0 (NRZ-L) Signal rate < 20kbps Distance <15m For control, more than-3v is off, +3v is on EIE325: Telecommunication Technologies

Electrical Specification 1 +3V ?????? -3V EIE325: Telecommunication Technologies