The Network Access Level:

1
SECTION 7

• The Network Access Level
• Datapac X.25
• OSI Level 3, no counterpart in Internet Stack

2
Text reading

• Chapter 10, especially
• Sec 10.3 X.25

3
Review

• Key point about Cyclades

End-to-end transport call entirely here
Subnet Datagram only
4
Bell Canadas Datapac

• Designed in 1973-4
• Cut over in 1975
• Worlds first public psn offered under tariff by
  a telco
• Pioneered the design implementation of X.25
• which is still in service - check the phone book

5
Datapac

• Heavily influenced by Cyclades 73 Arpanet
  71
• Initially 3 nodes
• Toronto
• Ottawa
• Montreal
• 56 Kb/s interswitch trunks !
• 1200 - 9600 b/s host access !!

6
Datapac

• Later
• Access from dumb terminals via host emulator
  called Packet Assembler/Disassembler at OSI L5
  session
• Terminal speeds 300 b/s, , 19,200 b/s
• Host access at 56 Kb/s
• Access by local dialup from all Cdn cities of
  population gt5 000
• Check your local phone book

7
Datapac

• Still in operation!
• Originally mostly for terminal-computer traffic,
• now used mostly for that plus client PC to
  server traffic
• Using the same protocols
• Most first-world countries built similar telco
  packet nets Transpac, DBN, . . .

8
Datapac access protcolsStandard Network Access
Protocol SNAP

• Became X.25
• Actually a protocol stack
• Level 1 physical
• ISO V.24 EIA RS-232 or
• ISO X.21 never implemented
• Level 2 managing the link
• HDLC or
• SDLC for IBM users System Network Architecture
  or SNA

9
Datapac

• Level 3 managing the host-host ? path - the
  virtual call
• SNAP Level 3, or SNAP for short
• SNAP syntax, semantics significance
• THE IMPORTANT PART OF THIS TALK!

10
SNAP Semantics What does it do?

• Builds, manages tears down virtual calls --
  initially no datagram service !
• I.e., multiplexes the single physical DTE-DCE
  link into
• a set of logical links , channels, or Virtual
  Circuits or Calls
• Always runs on top of HDLC - which also
  multiplexes the channel
• Why?

11
SNAP Assumptions

• Underlying channel is
• Slow 10 000 to 1 000 000 b/s
• Noisy BER of 1/100 to 1/1000
• Transit delay is small relative to line service
  time T gt
• Just a few bits in flight or stored in the
  channel gt
• ARQ will work well

12
SNAP Assumptions

• RIGHT for
• Wirepair, coax,
• DEAD WRONG for
• Optical fibre
• Some other wireless 802-11

13
SNAP level 3 Procedure
Call request
Call incoming
Build ckt ________ Data xfer ___________ Ckt
teardown
Call accepted
data
data
data
data
Clear Req
Clear Confirmed
Clear Confirmed
14
The Key IssueEnd-to-end significance

• For SNAP Version I Cyclades
• Control packets from the other peer, e.g.
• Call Accepted or
• Rotate Window 3 positions or . . .
• Come from the peer SNAP process in the other host
  gt
• End-to-End Significance
• Subnet knows nothing about calls - just moves
  datagrams

15
End-to-end significanceSNAP version 1 L3 was a
Transport Protocol ISO L4
Host subnet Host
hdlc
hdlc
link
link
SNAP
SNAP v1 TRANSPORT STATION IN HOST -just like
Cyclades
16
SNAP v1 L3 was a Transport Protocol ISO L4
SNAP L3 L2 L1
SNAP
SNAP
link
link
link
link
phys
phys
phys
phys
SNAP v1 TRANSPORT STATION IN HOST
17
SNAP Version 2

• A funny thing happened on the way to Geneva
• In SNAP Version 2 CCITT/ISO Standard X.25 Level
  3 . . .
• End-to-end significance was lost!
• Therefore . . .

18
SNAP Version 2

• Call Accepted, or
• Call Torn Down, or
• ACK packets lt3, or . . .
• Come from the local DTE, who may be throwing your
  packets on the floor!

19

End-to-end significanceGONE!
Host subnet Host
hdlc
hdlc
link
link
SNAP
SNAP
X.25 L3 IN DTE HOST AND in DCE switch
20

END-TO END SIGNIFICANCE -- .. GONE! SNAP V2
X.25 Level 3
DTE DCE DCE DTE
L3 L2 L1
SNAP
SNAP
SNAP
SNAP
link
link
link
link
phys
phys
phys
phys
NOTICE the redundancy of link SNAP layers! -
both just mux the DTE/DCE link!
21
Commentary

• X.25 is solely concerned with the DTE-DCE link
• X.25 L3 adds nothing but muxing of the link
• which could easily be done by extending Level 2
  -- HDLC

22

• X.25 is not about
• Host-host communications , but
• Host - network communications
• talking to the net, not through the net
• Hence ISO Layer 3 - Network Layer -had to be
  added to the stack to hold it

23

• X.25 L3 must be implemented in
• DTE AND DCE
• Of both ends
• 4 copies in total
• It gets worse . . .

24
It gets worse . . .

• Many carriers Bell Canada implemented E-E
  significance
• But refused to promise it to customers
• Later attempt to patch this blunder by
  introducing
• D-bit request E-E significance
• Meanwhile , still no transport protocol!!
• Nothing with End-to-End significance

25
Final remark

• Datagram service fast select packets finally
  added Nippon Tel Tel proposal to CCITT
• See Manning, Eric On Datagram Service in
  Public Packet Nets Computer Networks,
  North-Holland, 1976

26
X.25 Level 3 packet formats

27
X.25 L3 Data Packet
D 0 1
LOGICAL
Q
Q data qualifier bit M more data bit D confirm
delivery bit
CHANNEL NUMBER
PS
0
PR
M
PAYLOAD
28
X.25 Call Request/Incoming Call Packet
ID
LOGICAL
CHANNEL NUMBER


0 0 0 0 1 0 1 1

Caller adrlen
Called adrlen
DTE ADDRESS
0 0 0 0
FACILITIES LEN
0 0
Facilities . . .
PAYLOAD 0-16 octets
29
X.25 L3 Call Accepted/Connected
ID
LOGICAL
CHANNEL NUMBER


0 0 0 0 1 1 1 1

Caller adrlen
Called adrlen
DTE ADDRESS
0 0 0 0
FACILITIES LEN
0 0
Facilities . . .
30
Clear Request / Indication
ID
LOGICAL
CHANNEL NUMBER


0 0 0 1 0 0 1 1

Caller adrlen
Called adrlen
Clearing cause
Diagnostic code
31
X.25 State transition graph for call progress
Free Channel
DTE Clear Confirm
DCE Clear Confirm
DTE Call Req
DCE cleared
DTE cleared
DTE waits
DTE Clear req
DCE Clear indn
Call connected
DCE Clear indication
DTE Req Clear
Data transfer