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. A packet switching protocol used in a wide area network use virtual circuit and asynchronous TDM

Data Communication

X.25 Layers Error detection and correction in both the data link and

the network layers Network layer: establishing the connection

transferring the dataterminating the connection

Layers

Data Communication

Encapsulation of PLP packet

Data Communication

Packet Layer Protocol PLP packets : Information packets and Control packets Information Packets (I-packets)

transmit user data consists of a header and a user data field short and long packets: long packets to support facilities

with long delays The last bit in the header = 0 for I-packets

Data Communication

Packet Layer Protocol I-packet fields

General format identifier (GFI) field: 4 bitsQ bit : not defined in the standard. Users define two types of data.D bit: for packet sequencing.Mod: the length of the header. (01: the header short, 10: long)

Virtual circuit identification fieldsLogical channel group number (LCGN) : 4 bitsLogical channel number (LCN) : 8 bitstotal 12 bits to identify the virtual circuit for a transmission

Control fieldP(S) and P(R) : packet sequence numbers for flow and error control

P(S) for packet send, P(R) for packet receive: sliding window ARQ

Data Communication

Packet Layer Protocol Control Packets (C-packets)

~ flow and error control~ connection, termination and management control

Category I (for flow and error control)RR: Receive Ready - the station is ready to receive more packetsRNR: Receive Not ReadyREJ: Reject - an error in the packet (go-back-n ARQ)

Data Communication

Packet Layer Protocol Control Packet (C-packet)

Category II for connection, termination and management control

control packet types: Table 16.1

Data Communication

Control Packets Category II

Figure 16.5 Call Request/Incoming Call

request the establishment of a connection between two DTEs Call Accepted/Call Connected

indicate the acceptance of the requested connection by the called system

Data Communication

Control Packets Category II (cont’d)

Clear Request/Clear Indicationto disconnect the connection at the end of an exchange

Clear Confirmsent in response to the clear indication

and More...

Data Communication

Packet Layer Protocol Complete Packet Sequence

A message size may be too small or too large for a network and not compatible with a network.

X.25 mechanism to break up a long message among multiple packets but still keep those packets as a single contiguous transmission

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) M=1 : final packet in the sequence

D=1 : need ACK.

Data Communication

Packet Layer Protocol Complete Packet Sequence (cont’d)

Data Communication

Packet Layer Protocol Virtual Channel ID Numbers

up to 4096 multiplexed channels to be identified between each DTE and DCE

not permanent, allocated dynamically the calling and called hosts use different numbers