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TCP/IP Tutorial and Technical Overview - IBM Redbooks

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RFC 2225 also describes extensions to the ARP protocol (RFC 826) in order to<br />

work over ATM. We discuss this separately in 2.10.1, “Address resolution<br />

(ATMARP <strong>and</strong> InATMARP)” on page 47.<br />

First, some ATM basics:<br />

Cells All information (voice, image, video, data, <strong>and</strong> so on) is<br />

transported through the network in very short (48 data<br />

bytes plus a 5-byte header) blocks called cells.<br />

Routing Information flow is along paths (called virtual channels)<br />

set up as a series of pointers through the network. The<br />

cell header contains an identifier that links the cell to the<br />

correct path that it will take toward its destination.<br />

Cells on a particular virtual channel always follow the<br />

same path through the network <strong>and</strong> are delivered to the<br />

destination in the same order in which they were received.<br />

Hardware-based switching<br />

ATM is designed such that simple hardware-based logic<br />

elements can be employed at each node to perform the<br />

switching. On a link of 1 Gbps, a new cell arrives <strong>and</strong> a<br />

cell is transmitted every .43 microseconds. There is not a<br />

lot of time to decide what to do with an arriving packet.<br />

Virtual Connection (VC)<br />

ATM provides a virtual connection switched environment.<br />

VC setup can be done on either a permanent virtual<br />

connection (PVC) or a dynamic switched virtual<br />

connection (SVC) basis. SVC call management is<br />

performed by implementations of the Q.93B protocol.<br />

End-user interface The only way for a higher layer protocol to communicate<br />

across an ATM network is over the ATM Adaptation Layer<br />

(AAL). The function of this layer is to perform the mapping<br />

of protocol data units (PDUs) into the information field of<br />

the ATM cell <strong>and</strong> vice versa. There are four different AAL<br />

types defined: AAL1, AAL2, AAL3/4, <strong>and</strong> AAL5. These<br />

AALs offer different services for higher layer protocols.<br />

Here are the characteristics of AAL5, which is used for<br />

<strong>TCP</strong>/<strong>IP</strong>:<br />

Message mode <strong>and</strong> streaming mode<br />

Assured delivery<br />

Non-assured delivery (used by <strong>TCP</strong>/<strong>IP</strong>)<br />

Blocking <strong>and</strong> segmentation of data<br />

Multipoint operation<br />

Chapter 2. Network interfaces 51

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