TCP/IP Tutorial and Technical Overview - IBM Redbooks

in two different ways: NLPID value, indicating IP, or NLPID value, indicating
SNAP.
Although both of these encapsulations are supported under the given definitions,
it is advantageous to select only one method as the appropriate mechanism for
encapsulating IP data. Therefore, encapsulate IP data using the NLPID value of
0xcc, indicating an IP packet. This option is more efficient, because it transmits
48 fewer bits without the SNAP header and is consistent with the encapsulation
of IP in an X.25 network.
2.8 PPP over SONET and SDH circuits
This discussion describes the use of the PPP encapsulation over Synchronous
Optical Network (SONET) and Synchronous Digital Hierarchy (SDH) links, which
is documented by RFC 2615. Because SONET and SDH are, by definition,
point-to-point circuits, PPP is well suited for use over these links. SONET is an
octet-synchronous multiplex scheme that defines a family of standard rates and
formats. Despite the name, it is not limited to optical links. Electrical
specifications have been defined for single-mode fiber, multimode fiber, and
CATV 75 ohm coaxial cable. The transmission rates are integral multiples of
51.840 Mbps, which can be used to carry T3/E3 bit-synchronous signals. The
allowed multiples are currently specified as shown in Table 2-1. Additionally, the
CCITT Synchronous Digital Hierarchy defines a subset of SONET
transmission rates beginning at 155.52 Mbps.
Table 2-1 SONET speed hierarchy
Kind Length Meaning
0 - End of option list
1 - No-Operation
2 4 Maximum segment size
3 3 Window scale
4 2 Sack-Permitted
5 X Sack
8 10 Time stamps
Chapter 2. Network interfaces 45