While technologies such as GFP solve the most fundamental technical issue with EoS
(i.e., how does SONET actually carry Ethernet?), they do not address an issue that is
nearly as critical: How does SONET carry Ethernet efficiently?
SONET was designed to carry DS1 and DS3 signals. Its rate structure (see Table 11.1)
is optimized for this. Beyond the STS-3 rate, SONET rates grow by factors of four.
The fundamental Ethernet rates look nothing like DS1 or DS3 rates and grow in multiples
of ten. This means that, while GFP is a very efficient protocol, the rate mismatch
of SONET and Ethernet can still result in tremendous bandwidth inefficiencies, as
Table 11.2 illustrates.
Virtual concatenation (VCAT) helps address these inefficiencies by allowing SONET
payloads to combine into a single, virtual payload. VCAT provides a byte-wise inverse
Figure 11.4 GFP frame format
Core header
Payload area
Payload length
indicator
cHEC (CRC-16)
Payload Header
Optional payload FCS
(CRC-32)
Client payload
information field
(Carries an Ethernet
MAC frame)
2 octets
2 octets
4 - 64 octets
4 octets
4 - 65,
535 octets
cHEC: Core HEC
CRC: Cyclic Redundancy Check
HEC: Header Error Check
FCS: Frame Check Sequence
306 Chapter 11
multiplexing of the overall payload (e.
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