HDLC High-level Data Link Control

• Based on a number of ISO standards.
• Designed for point to point and multipoint operation.
• Is mostly used for point to point.
• Three Types of stations
  • Primary Station
    • Controls the operation of links
    • Issues commands, or polls other stations.
    • Employs the unbalanced configuration one primary and one or more secondary stations.
  • Secondary Station
    • Other half of the controller-controlled side
    • Responds to requests/commands from primary
  • Stallings and others say these two modes are infrequently used.
  • Combined Station
    • Employs both half and full duplex connections.
    • Point to point
    • Balanced Configuration
    • Both stations appear to be equals
• They discuss three modes of data transfer
  • Normal Response Mode (NRM), for Unbalanced configuration
    • Server transmits, clients respond.
    • Server is in complete control
  • Asynchronous response mode (ARM) , for Unbalanced configuration
    • Clients may initiate a transfer
    • Server is still responsible for all problems
  • Asynchronous balanced mode (ABM)
    • For peer to peer configuration.
• This works as either a connection-oriented or connectionless service
  • In a connection oriented service the participants have some notion that a connection has been formed.
  • Generally it knows when the exchange begins and ends.
  • TFTP built a connection oriented service, it maintained corresponding port numbers and such.
  • HTTP is connectionless.
  • A connection is probably required for flow control.
• The frame format
  • There is a single frame structure.
  • 
  • The flag field, at the beginning and end
    • 01111110
    • This marks the beginning and end of the frame.
    • the protocol uses bit stuffing to replace any sequence of 1s longer than 5 between the headers.
    • When the bit string 111111111110 is encountered
    • It is replaced with 111110111110110
    • And then transmitted.
    • The receiver knows to strip the next 0 when five 1s are encountered
    • If a sixth 1 is encountered, followed by a 0 it marks the end of the packet.
  • The address field
    • This is the address of the secondary station
    • Is 8 bits, but can be extended arbitrarily
      • If it begins with a 0, it is extended.
      • The next 8 bits are also part of the address.
      • And the MSB of these is used as above.
    • If it is all 1, it is a broadcast
    • This field is not used in point to point, but retained for uniformity.
  • Control Field.
    • This is either 8 or 16 bits.
    • There are three types of frames
      • Information - contains user data
        • Flow and error control is piggybacked
      • Supervisory
        • Acks/NACKS when piggybacking is not possible
      • Unnumbered
        • Link control information
    • The first bit indicates the type of frame.
    • A sequence number of 3 bits for the sender and receiver
    • A poll/final bit
    • In the Uframes, the m fields indicate the command.
      • These establish or terminate connections
      • Set up properties, like the number of bits in the control field.
      • Wikipedia has a nice chart
  • FCS
    • CRC-CRTT is the default
    • CRC-32 is optional
• operation
  • HDLC operates in three phases
    • Initialization
      • Either side sends one of the connection request commands
      • SNR, SAR, SAB, with an optional extend (7 bit sequence number)
      • Acknowledged with a UA, ok or DM , not ok.
    • Data Transfer
      • I frames with data are sent.
      • These are acked with either
        • A return I frame if data is exchanged both ways
        • A S frame if the data exchange is one way.
      • The ack contains the number of the frame that the entity expects to receive next.
    • Disconnect
• He works through an example, which is most informative.