B-49
Appendix B: Glossary
SNACP
SNA Control Protocol. An NCP in the PPP protocol suite that is used to establish
a point-to-point connection between hosts sending SNA packets. For more
information on SNACP, see RFC 2043 (at
http://www.ietf.org/rfc/rfc2043.txt
).
SNMP Simple Network Management Protocol. An Application Layer protocol that
supports the exchange of management information between network devices.
An SNMP network consists of agents, managed devices, and network-manage-
ment systems. Hierarchically organized information about network devices is
stored in and accessed from a management information base (MIB). For more
information on SNMP, see RFC 1157 (at http://www.ietf.org/rfc/rfc1157.txt).
SNR Signal-to-Noise Ratio. The ratio of the amplitude of a desired analog or digital
data signal to the amplitude of noise in a transmission channel. The SNR
measures the quality of a transmission channel or of an audio signal over a
network channel. ADSL devices periodically measure a line’s SNR to
determine whether the line needs to be taken down and retrained.
SONET Synchronous Optical NETwork. The ANSI standard for synchronous data
transmission on optical media. The equivalent international standard is SDH.
Complementary standards set by SDH and ANSI allow digital networks to
interconnect internationally. The standards also allow existing transmission
systems to take advantage of optical media through tributary attachments.
SONET is backward compatible with T-carrier lines. See also SDH.
Table G-4. SONET and SDH digital hierarchies
SPI Security Parameters Index. An arbitrary value that uniquely identifies an SA;
the SPI is used by VPN peers to match packets to keys contained in that SA.
Peers agree up the SPI when they negotiate the IPSec SA. When a peer secures
a packet to be sent over an IPSec SA, it adds the corresponding SPI to the
packet’s ESP or AH header. When a device receives a packet over a VPN tunnel,
it reads the packet’s SPI to determine which keys to use to authenticate and
decrypt the packet.
SONET STS
designator
SONET OCX
designator
SDH STM
designator
Line rate
(Mbps)
Overhead
rate (Mbps)
Payload rate
(Mbps)
STS-1 OC-1 — 51.840 1.728 50.112
STS-3 OC-3 STM-1 155.520 5.184 150.336
STS-12 OC-12 STM-4 622.080 20.736 601.344
STS-48 OC-48 STM-16 2488.320 82.944 2405.376
STS-192 OC-192 STM-64 9953.280 331.776 9621.504
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