Positron??™s Osiris product
(shown in Figure 11.3b) featured an early implementation of PPP/HDLC-based EoS.
Figure 11.2 SONET Add Drop Multiplexer
SONET
ADM
SONET
ADM
OC-N
fiber ring
OC-N
I/F
OC-N
I/F
OC-N
I/F
??¦
DS3
I/F
DS3
I/F
??¦
DS1
I/F
DS1
I/F
??¦
(optional)
STS
Mux
OC-N
I/F
VT
Mux
Add-Drop Multiplexer
SONET
ADM
SONET
ADM
304 Chapter 11
While these mappings worked and had some degree of standards??™ compliance, technical
shortcomings hampered both. Ethernet-over-ATM-over-SONET was saddled with
the infamous cell tax??”the large amount of protocol overhead required to segment
variable-length datagrams (such as Ethernet frames) into fixed-length, 53-byte ATM
cells. And, while ATM technology has found several successful (and rather large) niche
deployments, it has not enjoyed the ubiquitous deployment, and subsequent reduction
in costs, that many had hoped would happen.
HDLC-based implementations manifested a different technical problem??”bandwidth
expansion. HDLC uses a flag (a predefined pattern of eight bits) to delimit frames.
When that same bit pattern appears within the frame (i.e., as part of the actual user
data), an escape sequence (a different predefined pattern of eight bits) is added so the
receiving equipment does not confuse the bit pattern within the frame with a flag.
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