Spirent SmartBits System, Reference Manual

Page: 263 / 278

Glossary of Terms
Features and Terminology
SmartBits System Reference 255
cut through and store & forward devices
For the SmartMetrics Ethernet latency tests, latency is defined as the difference between
the time that a tagged byte leaves the transmitting port and the time that it arrives at the
receiving port. This is typically used to measure the performance of "cut through" devices.
Cut Through devices begin retransmission after receiving packet headers. This increases
performance over "store and forward devices."
Measurement of latency through one or more "store and forward" switches can be
calculated by adding the time to transfer any remaining bytes out of the transmitting port.
Store and Forward devices receive a complete packet on the input port before beginning
transmission on the output port. For these devices, the latency is defined as the difference
between the time the last byte leaves the transmitting device and the time that the first byte
arrives at the destination device.
In SmartWindow, the results are presented to cover both possibilities. A result of <NO> is
shown in the Store and Forward column if the system is able to detect the type of device
under test is a cut-through switch.
data integrity
Applicable to gigabit Ethernet traffic and cards, Data Integrity controls payload integrity.
When enabled for a card, it informs you that the payload portion–and only the payload
portion of the frame–has an error. The SmartMetrics
Sequence and Latency Test and
Capture function can detect and record a data integrity error event.
encapsulation types
The following types of encapsulation are available:
•
VC Mux Routed (null encapsulation)
VC Multiplexing creates a binding between an ATM VC and the type of the network
protocol carried on that VC. Thus, there is no need for protocol identification informa-
tion to be carried in the payload of each AAL5 CPCS-PDU. This reduces payload
overhead and can reduce per-packet processing. VC multiplexing can improve effi-
ciency by reducing the number of cells needed to carry PDUs of certain lengths.
(RFC2684).
•
SNAP
The SNAP header consists of a three octet Organizationally Unique Identifier (OUI)
and a two octet Protocol Identifier (PID). The OUI is administered by IEEE and iden-
tifies an organization that administers the values that might be assigned to the PID.
The SNAP header thus uniquely identifies a routed or bridged protocol. The OUI
value 0x00-00-00 indicates that the PID is an EtherType. (RFC2684).
•
LANE 802.3 (LAN Emulation)
The ATM Forum LAN Emulation specification provides an environment where the
ATM network is enhanced by LAN Emulation Server(s) to behave as a bridged LAN.
Stations obtain configuration information from, and register with, a LAN Emulation