HDLC Principle Frame content

1
HDLC Principle - Frame content

• The first character after the disappearance of
  the flags is an address byte
• Specifies the destination of the frame
• In X.25 we only have 2 addresses 01 for DCE, 03
  for DTE
• The second character is the control byte.
• Specifies the type of frame
• The last 2 characters before the reappearance of
  the flags are the FCS bytes.
• is a CRC code allowing error detection (not
  correction!)
• The remaining bytes in between are the info bytes
• The info field contains the data.

Control
Flags
Address
Info
FCS
Flags
Flags
Flags
FLAG Binary ' 0 1 1 1 1 1 1 0' Hex
' 7 E'
High level Data Link Communication protocol
2
Call set-up
Calling user address 5678
Called user address 1234
DCE
DCE

• Call set-up
• Call request
• Incoming call
• Call accepted
• Call connected
• Call clear-down
• clears a SVC
• LCN available again
• by user (DTE)
• by network

DTE
DTE
Call request
Call
Incoming Call
Call Accepted
Accept
Call Connected
Clear request
Clear Indication
Clear
Clear Confirm
Clear Confirm
ClearConfirmation
3
Call procedure bearer capability

• Purpose
• Select appropriate network resource
• Procedure
• B.C. Info Element is mandatory
• Checked by network
• Checked by called user (if no L.L.C.)
• Resources
• Speech
• Unrestricted digital
• 3,1 kHz audio (for network with analogue areas)
• Others ...

Network resources available
ISDN Network
Speech
TE
ET
ET
TE
Digital 64 kbps
analogue
Service Network resource used Speech Good
quality, compressed and with echo
suppression Digital data 64 kbps, guaranteed bit
integrity analogue Low quality, no
suppression, raw line
4
Call principle

• Calling side
• Tx Dial digits
• Rx Call proceeded
• Called side
• Rx Set-up
• TE can accept call
• Generate Ring
• Tx Alerting
• Phone is taken
• Accept call
• Tx Connect
• Rx Connect Ack
• Caller side
• Rx Connect
• Tx Connect Ack

D-Channel transfer
Setup
Dial
Setup
Call Proceeding
Ringing
Alerting
Alerting
Connect
Answer
Ring Back
Connect
Connect Ack
Ring Back
Connect Ack
DATA TRANSFER in B-channel
5
Frame Relay Committed Information Rate -
Principle

• CIR and EIR principle applied for one
  userAll data transferred within the
  Committed Information Rate (CIR) is guaranteed
• CIR is also called average throughput in bps
• all usage above the CIR EIR
• Non-committed rate is called Excess Information
  Rate
• is tagged with the discard eligible bit (DE)
• in the event the network runs into congestion,
  tagged frames will be discarded
• Available Information Rate is sum of CIR EIR

6
Connection types

• P/P
• unidirectional / bi-directional
• symmetric / asymmetric
• P/M
• unidirectional from Root to Leaves