09.12.2012 Views

Understanding the network.pdf - Back to Home

Understanding the network.pdf - Back to Home

Understanding the network.pdf - Back to Home

SHOW MORE
SHOW LESS

You also want an ePaper? Increase the reach of your titles

YUMPU automatically turns print PDFs into web optimized ePapers that Google loves.

Along with <strong>the</strong> SAP, <strong>the</strong>re is a 5-byte Sub<strong>network</strong> Access Pro<strong>to</strong>col<br />

(SNAP) discrimina<strong>to</strong>r (see <strong>the</strong> SNAP header portion of Figure 3.6).<br />

This is used <strong>to</strong> identify <strong>the</strong> pro<strong>to</strong>col family in which <strong>the</strong> packet belongs.<br />

Two SNAP pro<strong>to</strong>col discrimina<strong>to</strong>rs used <strong>to</strong> define AppleTalk packets<br />

exist:<br />

Figure 3.6. E<strong>the</strong>rTalk message frame formats.<br />

• $080007809B defines that <strong>the</strong> frame contains an AppleTalk data<br />

packet.<br />

• $00000080F3 defines that <strong>the</strong> frame is an AARP packet.<br />

For all pro<strong>to</strong>cols (E<strong>the</strong>rnet, TokenTalk, and so on), <strong>the</strong> 80F3 always<br />

identifies <strong>the</strong> AARP packet. Data packets vary.<br />

TokenTalk<br />

TokenTalk provides non-collision-based media access control (<strong>to</strong>ken<br />

passing) over 4, and 16Mbps IEEE 802.5 Token Ring (see Figure 3.7).<br />

TokenTalk is only supported in AppleTalk Phase 2 <strong>network</strong>s. Like<br />

E<strong>the</strong>rTalk, TokenTalk has a transmission-media- specific access<br />

control pro<strong>to</strong>col called TokenTalk Link Access Pro<strong>to</strong>col (TLAP) that<br />

manages all <strong>the</strong> UPL interaction with <strong>the</strong> transport and uses AARP for<br />

hardware-<strong>to</strong>-AppleTalk address translation. TLAP constructs packets

Hooray! Your file is uploaded and ready to be published.

Saved successfully!

Ooh no, something went wrong!