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Figure 5.22. PPP frame format.<br />

The PPP frame contains <strong>the</strong> following fields:<br />

• Start/End Flag—This flag indicates <strong>the</strong> start/end of a new frame. The binary<br />

sequence of 01111110 is always used <strong>to</strong> mark <strong>the</strong> start of a new frame.<br />

• Address—This field contains <strong>the</strong> binary sequence of eight ones or <strong>the</strong><br />

hexadecimal FF, which indicates all stations or a broadcast. PPP does not<br />

support station addressing.<br />

• Control—All PPP frames use <strong>the</strong> HDLC unsequenced frame type. This is<br />

indicated by a hexadecimal 03 or <strong>the</strong> binary equivalent 00000011.<br />

• Pro<strong>to</strong>col—The pro<strong>to</strong>col field is technically inside <strong>the</strong> HDLC information field.<br />

This 2-byte field identifies <strong>the</strong> pro<strong>to</strong>col associated with <strong>the</strong> data contained in<br />

<strong>the</strong> data field.<br />

• Data—This field can contain a maximum of 1,500 bytes. The <strong>to</strong>tal data field<br />

length, with <strong>the</strong> pro<strong>to</strong>col field, is 1502 bytes.<br />

• Frame CRC—This field contains <strong>the</strong> 16- or 32-bit CRC information for <strong>the</strong><br />

frame. The default is 16 bits, however, a 32-bit CRC can be negotiated as<br />

part of <strong>the</strong> link establishment process.<br />

In addition <strong>to</strong> data, <strong>the</strong> PPP frame is also used <strong>to</strong> transmit LCP messages. There are<br />

three types of LCP message frames: link-establishment, link-maintenance, and<br />

link-termination. All <strong>the</strong> PPP message frames use <strong>the</strong> control tag for <strong>the</strong> HDLC<br />

unsequenced frame type.<br />

Summary<br />

This chapter discussed <strong>the</strong> various technologies and pro<strong>to</strong>cols associated with WAN<br />

<strong>network</strong>ing. WAN <strong>network</strong>ing is typically seen as a distinct function from LAN<br />

<strong>network</strong>ing because WANs are almost exclusively centered around telephony, which<br />

is commonly disassociated from computing services in large information technology<br />

environments. Due <strong>to</strong> <strong>the</strong> ever-increasing demands <strong>to</strong> be "wired" all <strong>the</strong> time,<br />

however, this distinction is quickly disappearing. At <strong>the</strong> end of this chapter, you<br />

should have an understanding of <strong>the</strong> following concepts and terms:<br />

• The PSTN, T-carrier, SONET, and SDH digital transport systems<br />

• Packet-switched PSTN <strong>network</strong> technologies such as X.25 and Frame Relay<br />

• Asynchronous Transfer Mode and Broadband-ISDN

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