GFP also provides relatively little protocol overhead;
in fact, GFP is more efficient than IEEE 802.3 Ethernet at mapping Ethernet frames
into the physical layer.3
3 IEEE 802.3 Ethernet requires a minimum of 20 bytes of protocol overhead (12 bytes for the interframe
gap and 8 bytes for the preamble/start of frame delimiter) between successive MAC frames. Typical GFP
implementations require only 12 bytes of protocol overhead (4 bytes for the core header, 4 bytes for the payload
header, and 4 bytes for the payload FCS) to carry an Ethernet MAC frame (see Figure 11-4). Table 11-2
illustrates the reduced overhead of GFP.
Figure 11.3 Early EoS implementations
(a) Fujitsu FLM 150 ADM (b) Positron Osiris XTS
SONET/MSPP 305
ITU-T also standardized another EoS mapping, Link Access Protocol for SONET
(LAPS) [9]. Some early router and CPE implementations still use X.86 for their EoS
interfaces. However, this mapping uses the fundamentals of HDLC and, therefore, carries
its technical disadvantages. Because of its technical superiority and broad basis in
North American and international standards, GFP appears to be gaining momentum
as the preferred EoS mapping.
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