Transcript Lecture 2 - Egan Family

ECS5365 Lecture 2
ISDN Protocols and Standards
Philip Branch
Centre for Telecommunications and Information
Engineering (CTIE)
Monash University
http://www.anspag.monash.edu.au/~pbranch/lect02.ppt
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Outline
• ISDN protocol stack
• ISDN Physical layer
– Basic Rate Interface
– Primary Rate Interface
• ISDN Layer 2 Protocols
– LAPD and LAP-B
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Layered Protocols in ISDN
– Each layer provides a service to the above layer
– Layer 1 physical layer
• transmission issues
– Layer 2 data link layer
• connection from point to point in a network
– Layer 3 network layer
• connection between any two points in a network
– Layer 4 transport layer
• reliable connection between any points in a network
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ISDN Standards
• Physical layer (layer 1)
– I.430 (BRI) and I.432 (PRI)
• Data link layer (layer 2)
– LAPD (Q.291) and LAP-B
• Network layer (layer 3)
– Q.931 and X.25
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D Channel
• Main function is signalling
– Setting up, controling and releasing calls
• Low priority packet data
– using LAPD and X.25 packet layer
• Telemetry (undefined)
– eg meter reading
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B Channel
• Main function is user data
• Packet switching interface defined
– X.25 / LAP-B
• Non ISDN terminal interface defined
– I.465/V.120
• User is free to define protocols over B
channel
– eg. Point to Point protocol for IP
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Basic Rate Physical
• Connection between NT1 or NT2 and
customer equipment TE1 or TA
• Uses two pairs (four wires)
– one pair receive, one pair transmit
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2B + D channels are transmitted each frame
Each frame 48 bits long
bit rate 192 kbps
time length of frame 250 microseconds
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Basic Rate Coding
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•
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Pseudoternary coding
negative and positive voltage levels
binary 1 represented by no line signal
binary 0 alternates between positive and
negative voltages for synchronisation
• extra bits are added to frame to remove dc
bias
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Frame Format
• 16 bits per B channel and 4 bits per D
channel sent per frame
• Full duplex between TE and NT
• different frame format for TE to NT and NT
to TE
• The NT to TE frame echoes the values of
the D bits received from the TE
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Basic Rate Interface Frame
Structure
• Attachment
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Multidrop configuration
• Possible to use a passive bus to connect up
to 8 terminals
• Distance limit
– 200 metres if more than one terminal per BRI
– 1 km if one terminal per BRI
• D Channel shared by all terminals
• Contention resolution mechanism
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Contention resolution
– Each user terminal transmits 1s on D channel
when no information to send
– The NT reflects back the the D channel bits
– If a terminal wishes to send it waits for a string
of 1 bits greater than a threshold
– Terminal checks echo bits after sending D
channel bits
– If they don’t correspond, a collision has
occurred and the terminal must retransmit
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Contention resolution (continued)
• Threshold depends on terminal state
• String of ‘1’s corresponds to signal absence
• Terminal who writes a ‘0’ overrides
terminal who writes a ‘1’
– will transmit first
– other clients must resume waiting
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Example of contention resolution
• Attachment
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Threshold in contention
resolution
• Signalling
– normal 8, low 9
• Data
– normal 10, low 11
• After successful transmission threshold set
low
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Primary rate - Physical
• Uses two pairs (4 wires)
– one pair transmit, one pair receive
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30 B+D channels transmitted each frame
Each frame 256 bits long
Bit rate = 2048 kbps
Time length of frame = 125 microseconds
Based on G.703, G.704 transmission
standards
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Primary rate - Coding
• Pseudoternary coding - similar to basic rate
but opposite assignment of codes
• binary 0 - no line signal
• binary 1 - alternates between positive and
negative voltages for synchronisation
• strings of 4 zeros replaced by sequence that
violates the rules of alternate priorities
• High density bipolar - 3 zeros (HDB3) code
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Primary rate - Frame format
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Frame divided into 32 slots
1st slot used for frame alignment 0011011
30 slots are used for the 30 B channels
1 slot for the D channel
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Primary Rate Interface Frame
Structure
• Attachment
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Data link layer - D channel
• LAPD (Link Access Procedure) for the D
channel
• Based on LAP-B
– developed for X.25 and HDLC
– Link Access Protocol - Balanced
• Supports multiple user terminals across UNI
• Support multiple layer 3 entities
– signalling and X.25
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LAPD services
• Unacknowledged information transfer
– no flow control or error control
– point to point and broadcast
• Acknowledged information transfer
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similar to HDLC
guarantees all frames delivered in order
sliding window flow control
error control via retransmission
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LAPD Frame
• Messages sent in frames
• Flags identify location of frames in bit
stream
• Address field must cope with multiple user
devices per physical interface and multiple
layer 3 entities per device
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TEI
• TEI can be set manually by the user or
automatically by network
• LAPD supports multiple logical
connections via Data Link Connection
Identifier = combination of TEI and SAPI
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Protocol Nature
• peer-to-peer protocol
– terminal equipment and network termination
have equal status
• balanced operation
– once connection is established both sides can
send data either side can initiate disconnect
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Management functions
• TEI management
– request a TEI number from network
– check value of a TEI
– remove a TEI assignment
• Parameter negotiation
– each parameter has a default value
– XID command used to change parameter
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LAPD Frame Structure
• 01111110 flag
– bit stuffing
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Address field
Control field
Information field
Frame Check Sequence (FCS)
01111110 Flag
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Address Field
• TEI terminal endpoint identifier
• SAPI Service Access Point Identifier
• C/R command response bit
– user side commands 0, responses 1
– network side commands 1, responses 0
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Control Field
• Defines frame type
– Information (0)
– Supervisory (10)
– Unnumbered (11)
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Information Field
• Carries data for layer 3 entities
• Packet data if X.25
• Q.931 data if signalling
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FCS Field
• Frame check sequence field
• Cyclic redundancy check
• Error results in retransmission request
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Frame Types
• Information frames
– layer 3 call setup information
– flow and error control piggybacked
• Supervisory
– flow and error control
• Unnumbered
– link control functions
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Control field structure
• Attachment
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LAPD commands and responses
• Information
• Supervisory
– RR, RNR , REJ
• Unnumbered
– SABME, DM, UI, DISCUA FRMR, XID
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Examples of LAPD operation
• Attachment
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Summary
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•
•
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Physical layer of the BRI and PRI
Contention algorithm in BRI
LAPD format
LAPD messages and operation
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Preliminary Reading
• Signalling in ISDN
• Chapter 8 and 10 of Stallings
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Review Questions from last week
• Why don’t all TE1 devices need to connect to NT2
equipment?
• The BRI provides 2 B channels and 1 D channel,
total 144 kbps. However, a BRI interface is defined
at 192 kbps. Why?
• In what way might a carrier treat a 64 kbps 8kHz
structured speech bearer service differently to a
64kbps, unrestricted, 8kHz structured bearer
service?
• Which bearer services might be used for G4 fax?
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Review Questions
(not for assessment)
• The BRI D channel contention algorithm would fail
if any TE1 sent more than 8 consecutive ‘1’s as data
over the D channel. Why does this never happen?
• Why is the overhead for the BRI so much greater
than for the PRI?
• Why is there no contention mechanism for PRI?
• How would data consisting of the bit sequence
0111110 be coded within a LAPD frame?
• Why does LAPD define Supervisory frames for
flow and error control when Information frames can
piggyback the same information?
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