Every occurrence of the flag pattern within the frame results in an escape sequence??”
and the frame growing by one octet. When Ethernet is mapped into SONET using
HDLC, the mapping overhead is nondeterministic and a function of the contents of the
Ethernet frame. This subtle issue can adversely affect the performance of networks and
can prove difficult to identify as the culprit when performance problems do arise.
The industry clearly needed a standard EoS mapping that addressed the shortcomings
of both the ATM and HDLC-based approaches. In the late 1990s, several companies
(led by Lucent Technologies) began working in ANSI T1X1 toward this end. These
efforts brought the generic framing procedure (GFP), which was standardized first in
ANSI and then in ITU-T [8].
GFP works much like a variable-length version of ATM. Each GFP frame (see
Figure 11.4) carries an Ethernet medium access control (MAC) frame. GFP frames are
transmitted continuously within the SONET SPE; idle GFP frames are transmitted
when there is no Ethernet frame to carry. GFP delimits frames using the Header Error
Check field, much like ATM, and therefore obviates the need for flag sequences (and the
resulting bandwidth expansion).
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