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Chapter Objectives
After completing this chapter you will be able to:
– Understand the ATM header information and how it is utilised
– Outline the UNI and NNI cell headers
– Describe the functions of the ATM layer
ATM Layer
Adaptation
Layer
CS
SAR
Layer two
ATM Layer
TC
Physical Layer
Layer one
PMD
UNI Cell Header
Bits
8
1
GFC
VPI
1st Octet
VPI
VCI
2nd Octet
3rd Octet
VCI
PTI
VCI
HEC
48-octet data field
CLP
4th Octet
5th Octet
NNI Cell Header
Bits
8
1
VPI
1st Octet
VPI
VCI
2nd Octet
3rd Octet
VCI
PTI
VCI
HEC
48-octet data field
CLP
4th Octet
5th Octet
Generic Flow Control
Locally significant only (at UNI)
– Any value will be overwritten by the switch
Two modes of operation:
– Controlled mode
– Uncontrolled mode
Currently only uncontrolled mode is defined
– Uncontrolled GFC = 0000
GFC
VPI
VPI
VCI
VCI
VCI
PTI
HEC
48-octet data field
CLP
Virtual Path Identifier
Identifies this cell’s path
8 bits available at the UNI
12 bits available at the NNI
– 256/4096 possible simultaneous paths
– Maximum number of usable bits is negotiable between user and
network
VPI 57
VPI 68
GFC
VPI
VPI
VCI
‘Real’ physical link
VCI
VCI
PTI
HEC
48-octet data field
CLP
Virtual Channel Identifier
Identifies this cell’s channel
16 bits available at the UNI & NNI
– 65,536 possible simultaneous channels per path
– Maximum number of useable bits is negotiable on a per-path
basis
VCI 39
VCI 40
VCI 38
VCI 39
VPI 68
VPI 57
VPI 68
VPI 68
VCI 39
VCI44
VCI 40
VCI 41
GFC
VPI
VPI
VCI
VCI
VCI
PTI
HEC
Physical Interfaces
4- octet data field
CLP
Virtual Paths
Multiple channels destined
for a common location can
be quickly and simply
switched by the network if
they share a common VPI
channels
131
145
117
channels
131
145
117
Reserved Virtual Connections
The following VPI/VCI combinations have been reserved:
– VPI = 0
– VPI = 0
– VPI = ALL
VCI = 0 to 15
VCI = 16 to 31
VCI = 1 to 5
ITU-T
ATM Forum
In practice, carriers regard VCIs 0 to 31 as reserved for
all VPIs
Payload Type Identifier
PTI Coding
(MSB first)
Interpretation
000
User data cell, congestion not experienced, SDU type = 0
001
User data cell, congestion not experienced, SDU type = 1
010
User data cell, congestion experienced, SDU type = 0
011
User data cell, congestion experienced, SDU type = 1
100
Segment OAM F5 flow-related cell
101
End-to-end OAM F5 flow-related cell
110
Resource management cell
111
Reserved for future functions
GFC
VPI
VPI
VCI
VCI
VCI
PTI
HEC
48-octet data field
CLP
Congestion Control
Bit 2 of the PTI may be used to indicate to the
destination that congestion has taken place in the
network
The bit is called Explicit Forward Congestion Indicator
(EFCI)
This will occur when switches are discarding cells with
CLP =1
EFCI
GFC
VPI
VPI
VCI
VCI
VCI
PTI
HEC
48-byte data field
CLP
Cell Loss Priority
CLP operates independently on each active VPI/VCI
A switch may flip CLP from 0 to 1, for example, if traffic
on a VPI/VCI exceeds the maximum agreed sustainable
cell rate
CLP = 0
CLP = 1
CLP = 1
CLP = 0
GFC
VPI
VPI
VCI
VCI
Private UNI
Private NNI
Public UNI
Public NNI
VCI
PTI
HEC
48-octet data field
CLP
Header Error Check
The HEC is performed on the header only
– Supports forward correction of single-bit errors
– Supports detection of multiple-bit errors
 Faulty cells are discarded
– At the UNI:
 Error detection is mandatory
 Error correction is optional
The HEC is generated/verified at the TC part of the
physical layer
GFC
VPI
VPI
VCI
VCI
VCI
PTI
HEC
48-octet data field
CLP
Virtual Paths and Channels
ATM Switch
Virtual Channel Switch
VCI1
VCI2
VPI1
VCI3
VCI4
VPI2
VPI5
VCI4
VPI5
VCI3
VCI1
VCI2
VPI1
VPI2
VPI3
VPI4
VCIa
VCIb
VCIa
Virtual Path Switch
VCIb
Virtual Paths and Channels
CPE A
Switching Node
Virtual Channel Link
CPE B
Virtual Channel Link
Virtual Channel Connection
The Switch Map
ATM Cell
1
2
VPI/VCI = A/B
ATM Cells
VPI/VCI = X/Y
Switch Map (1)
VPI
A
-
VCI
B
-
Interface
2
-
VPI
X
-
VCI
Y
-
VPI/VCI is of
LOCAL Significance
Only