Chapter 3.11 - LAPS

3.11 LAPS

The Link Access Procedure-SDH (LAPS) is a simple specification that includes data
link service and protocol designed to transport point-to-point IP or Ethernet traffic
over legacy SDH. ITU-T (X.86, pg 9) defines LAPS as “a physical coding sub-layer
which provides point-to-point transferring over SDH virtual containers and interface
rates.” In that respect, ITU-T has specified the Link Access Procedure-SDH (LAPS)
as a low-cost physical coding sublayer to transport point-to-point IP or Ethernet traffic
over SDH virtual containers and interface rates, and yet provide low latency variance,
flow control in bursty traffic, capability of remote-fault indications, ease of use,
and ease of maintenance. Encapsulation of IPv4, IPv6, PPP, and other layer protocols
is accomplished with the service access point identifier (SAPI).

Figure 3.17 A Resilient Packet Ring network architecture. Nodes on the ring are capable of passing all traffic destined to another node in transit.

Two ITU-T documents address the encapsulation and rate adaptation of IP and
Ethernet over LAPS: ITU-T X.85/Y.1321 defines IP over SDH using LAPS and
ITU-T X.86 defines Ethernet over LAPS.

ITU-T X.85 and X.86/Y.1321 Recommendations treat SONET/SDH transports
as octet-oriented, synchronous point-to-point links. Thus, frames are octet-oriented,
synchronous multiplex mapping structures that specify a series of standard rates,
formats, and mapping methods.

Table 3.1 shows the bandwidth value of the VCs. Control signals are not required,
and a self-synchronous scrambling/descrambling (x⁴³ + 1) function is applied
during insertion/extraction into/from the synchronous payload envelope.*
The frame structure of LAPS is illustrated in Figure 3.18.

Because LAPS is defined to encapsulate IP and Ethernet packets, an octet staffing
procedure known as transparency is defined by ITU-T X.85 and X.86. Since each
frame begins and ends with the same flag (0x7E), there is a probability that the octet
0x7E may also occur within the information field, thus emulating the flag sequence
of the frame. To avoid this, at the transmitter, such occurrence of the code 0x7E is

converted to the sequence 0x7D 0x5E. In addition, occurrences of 0x7D are also converted
to the sequence 0x7D 0x5D. The receiver recognizes these sequences (0x7D
0x6E, 0x7D 0x5D) and replaces them with the original octets. This full transparency
is also guaranteed (by ITU-T X.86) for Ethernet over LAPS, and LAPS over SDH.

Because LAPS is defined to map asynchronous data frames over synchronous
SONET/SDH frames, it is evident that a rate adaptation must take place. This
means that asynchronous frames are buffered and adapted to the SDH rate. However,
during this operation, “empty” payload occurs, which must be filled with special
codes that can be recognized and removed at the receiver. This code is a sequence
of {0x7D 0xDD}, as long as necessary.

Rate adaptation is performed right after transparency processing and before the
end flag is added. At the receiver, the rate adaptation sequence of {0x7D 0xDD}

Figure 3.18 The LAPS frame structure.

is detected and removed, again right after the flag is detected and before transparency.

_________________________________________
*Scrambling is a function encountered in most data transmission cases, such as SONET, ATM, and so
on. The scrambling is different in each case.

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