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 Physical Layer Adaptation Layer PMD TC SAR CS Layer two Layer one

UNI Cell Header 48-octet data field GFC VPI VCI PTI HEC 8 1 1st Octet 2nd Octet 3rd Octet 4th Octet 5th Octet Bits CLP

NNI Cell Header 48-octet data field VPI VCI PTI CLP HEC 8 1 1st Octet 2nd Octet 3rd Octet 4th Octet 5th Octet Bits

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 = octet data field VPI VCI PTI CLP HEC GFC

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 48-octet data field VCI PTI CLP HEC VPI GFC ‘ Real’ physical link VPI 57 VPI 68

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 VPI 57 VPI 68 VCI 39 VCI 40 VCI 38 VCI 39 VPI 68 VCI 39 VCI44 VCI 40 VCI 41 Physical Interfaces 4- octet data field PTI CLP HEC VPI GFC VCI

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 channels

Reserved Virtual Connections The following VPI/VCI combinations have been reserved: –VPI = 0VCI = 0 to 15ITU-T –VPI = 0VCI = 16 to 31ATM Forum –VPI = ALLVCI = 1 to 5 In practice, carriers regard VCIs 0 to 31 as reserved for all VPIs

Payload Type Identifier PTI Coding (MSB first) Interpretation User data cell, congestion not experienced, SDU type = 0 User data cell, congestion not experienced, SDU type = 1 User data cell, congestion experienced, SDU type = 0 User data cell, congestion experienced, SDU type = 1 Segment OAM F5 flow-related cell End-to-end OAM F5 flow-related cell Resource management cell Reserved for future functions octet data field VPI VCI CLP HEC GFC PTI

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 48-byte data field VPI VCI CLP HEC GFC PTI EFCI

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 Private UNIPrivate NNIPublic UNIPublic NNI 48-octet data field VPI VCI HEC GFC PTI 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 48-octet data field VPI VCI CLP GFC PTI HEC

Virtual Paths and Channels ATM Switch Virtual Channel Switch Virtual Path Switch VCI1VCI2VCI3VCI4 VCI1 VCI2 VCI3 VCI4 VCIa VCIb VCIa VCIb VPI1 VPI2 VPI3 VPI1 VPI2 VPI4 VPI5

Virtual Paths and Channels

The Switch Map ATM Cell ATM Cells VPI/VCI = A/B VPI/VCI = X/Y 1 2 Switch Map (1) VPIVCIInterface VPI VCI A B 2 X Y VPI/VCI is of LOCAL Significance Only