This document discusses how IP data is carried over an ATM network. It explains that IP datagrams are encapsulated into ATM cells using AAL5. The cells are then routed individually through the ATM network based on their addresses to the exiting point router. ATMARP is used to map IP addresses to physical ATM addresses to allow cells to be properly delivered from entering to exiting point routers across the ATM network. Logical IP subnets can also be defined to facilitate protocols like ATMARP that require simulating broadcast domains.

1. CChhaapptteerr 2233
IIPP OOvveerr AATTMM
Objectives
Upon completion you will be able to:
• Review the features of an ATM WAN
• Understand how an a datagram can pass through an ATM WAN
• Understand how an IP packet is encapsulated in cells
• Understand how cells are routed in an ATM network
• Understand the function of ATMARP
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2. 23.1 ATM WANS
We review some features of the ATM WAN nneeeeddeedd ttoo uunnddeerrssttaanndd IIPP
oovveerr AATTMM.. TThhee oonnllyy AAAALL uusseedd bbyy tthhee IInntteerrnneett iiss AAAALL55,, ssoommeettiimmeess ccaalllleedd
tthhee ssiimmppllee aanndd eeffffiicciieenntt aaddaappttaattiioonn llaayyeerr ((SSEEAALL))..
TThhee ttooppiiccss ddiissccuusssseedd iinn tthhiiss sseeccttiioonn iinncclluuddee::
LLaayyeerrss
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3. Figure 23.1 An ATM WAN in the Internet
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4. Figure 23.2 ATM layers in routers and switches
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5. NNoottee::
End devices such as routers use all
three layers, while switches use only
the bottom two layers.
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6. Figure 23.3 AAL5
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7. NNoottee::
The AAL layer used by the IP protocol
is AAL5.
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8. Figure 23.4 ATM layer
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9. Figure 23.5 ATM headers
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10. 23.2 CARRYING A DATAGRAM
IN CELLS
We show how an example of a datagram encapsulated iinn ffoouurr cceellllss aanndd
ttrraannssmmiitttteedd tthhrroouugghh aann AATTMM nneettwwoorrkk..
TThhee ttooppiiccss ddiissccuusssseedd iinn tthhiiss sseeccttiioonn iinncclluuddee::
WWhhyy UUssee AAAALL55??
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11. Figure 23.6 Fragmentation
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12. NNoottee::
Only the last cell carries the 8-byte
trailer added to the IP datagram.
Padding can be added only to the last
cell or the last two cells.
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13. NNoottee::
The value of the PT field is 000 in all
cells carrying an IP datagram
fragment except for the last cell;
the value is 001 in the last cell.
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14. Figure 23.7 ATM cells
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15. 23.3 ROUTING THE CELLS
The ATM network creates a route between two rroouutteerrss.. WWee ccaallll tthheessee
rroouutteerrss eenntteerriinngg--ppooiinntt aanndd eexxiittiinngg--ppooiinntt rroouutteerrss..
TThhee ttooppiiccss ddiissccuusssseedd iinn tthhiiss sseeccttiioonn iinncclluuddee::
AAddddrreesssseess
AAddddrreessss BBiinnddiinngg
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16. Figure 23.8 Entering-point and exiting-point routers
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17. 23.4 ATMARP
ATMARP finds (maps) the physical address of the eexxiittiinngg--ppooiinntt rroouutteerr
ggiivveenn tthhee IIPP aaddddrreessss ooff tthhee eexxiittiinngg--ppooiinntt rroouutteerr.. NNoo bbrrooaaddccaassttiinngg iiss
iinnvvoollvveedd..
TThhee ttooppiiccss ddiissccuusssseedd iinn tthhiiss sseeccttiioonn iinncclluuddee::
PPaacckkeett FFoorrmmaatt
AATTMMAARRPP OOppeerraattiioonn
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18. Figure 23.9 ATMARP packet
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19. TTaabbllee 2233..11 OOPPEERR ffiieelldd
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20. NNoottee::
The inverse request and inverse reply
messages can bind the physical
address to an IP address in a PVC
situation.
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21. Figure 23.10 Binding with PVC
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22. Figure 23.11 Binding with ATMARP
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23. NNoottee::
The request and reply message can be
used to bind a physical address to an
IP address in an SVC situation.
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24. NNoottee::
The inverse request and inverse reply
can also be used to build the server’s
mapping table.
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25. Figure 23.12 Building a table
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26. 23.5 LOGICAL IP SUBNET (LIS)
An ATM network can be divided into logical (not pphhyyssiiccaall)) ssuubbnneettwwoorrkkss..
TThhiiss ffaacciilliittaatteess tthhee ooppeerraattiioonn ooff AATTMMAARRPP aanndd ootthheerr pprroottooccoollss ((ssuucchh aass
IIGGMMPP)) tthhaatt nneeeedd ttoo ssiimmuullaattee bbrrooaaddccaassttiinngg oonn aann AATTMM nneettwwoorrkk..
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27. Figure 23.13 LIS
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28. NNoottee::
LIS allows an ATM network to be
divided into several logical subnets. To
use ATMARP, we need a separate
server for each subnet.
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