Data Communication X.25 ~ Most widely used today ~ an interface between DTE and DCE for terminal operation at the packet mode on public data network. l A packet switching protocol used in a wide area network l use virtual circuit and asynchronous TDM

Data Communication X.25 Layers l Error detection and correction in both the data link and the network layers l Network layer: establishing the connection transferring the data terminating the connection n Layers

Data Communication Encapsulation of PLP packet

Data Communication Packet Layer Protocol n PLP packets : Information packets and Control packets n Information Packets (I-packets) l transmit user data l consists of a header and a user data field l short and long packets: long packets to support facilities with long delays l The last bit in the header = 0 for I-packets

Data Communication Packet Layer Protocol n I-packet fields l General format identifier (GFI) field: 4 bits äQ bit : not defined in the standard. Users define two types of data. äD bit: for packet sequencing. äMod: the length of the header. (01: the header short, 10: long) l Virtual circuit identification fields äLogical channel group number (LCGN) : 4 bits äLogical channel number (LCN) : 8 bits ätotal 12 bits to identify the virtual circuit for a transmission l Control field äP(S) and P(R) : packet sequence numbers for flow and error control äP(S) for packet send, P(R) for packet receive: sliding window ARQ

Data Communication Packet Layer Protocol n Control Packets (C-packets) ~ flow and error control ~ connection, termination and management control l Category I (for flow and error control) äRR: Receive Ready - the station is ready to receive more packets äRNR: Receive Not Ready äREJ: Reject - an error in the packet (go-back-n ARQ)

Data Communication Packet Layer Protocol n Control Packet (C-packet) l Category II äfor connection, termination and management control äcontrol packet types: Table 16.1

Data Communication Control Packets n Category II l Figure 16.5 l Call Request/Incoming Call ärequest the establishment of a connection between two DTEs l Call Accepted/Call Connected äindicate the acceptance of the requested connection by the called system

Data Communication Control Packets n Category II (cont’d) l Clear Request/Clear Indication äto disconnect the connection at the end of an exchange l Clear Confirm äsent in response to the clear indication l and More...

Data Communication Packet Layer Protocol n Complete Packet Sequence l A message size may be too small or too large for a network and not compatible with a network. l X.25 mechanism to break up a long message among multiple packets but still keep those packets as a single contiguous transmission l A packets: at least one additional packets needed to carry the remainder of a message (M=1,D=0) B packets: either stand alone or final packet (M=0, D=1) l M=1 : final packet in the sequence D=1 : need ACK.

Data Communication Packet Layer Protocol n Complete Packet Sequence (cont’d)

Data Communication Packet Layer Protocol n Virtual Channel ID Numbers l up to 4096 multiplexed channels to be identified between each DTE and DCE l not permanent, allocated dynamically l the calling and called hosts use different numbers