MT90220
length cells; asynchronous in the sense that the
recurrence of cells containing information from an
individual user is not necessarily periodic.
Frame is Aligned on all links. Differential link delay
can cause the reception to be ‘mis-aligned‘ in time.
Alignment can be recovered using a link delay
synchronization mechanism. The ICP ‘Stuff‘
mechanism is a controlled violation of the IMA
consecutive frame definition.
Note: Although ATM cells are transmitted synchronously to main-
tain the clock between sender and receiver, the sender transmits
data cells on an as available basis and transmits empty cells when
idle. The sender is not limited to transmitting data every Nth cell.
Blocked - The Blocked State is a Group State
indicating that the Group has been inhibited.
IMA Group - The IMA Group is a ‘group‘ of links at
one end of a ’circuit’ that establish an IMA virtual link
to another end.
Blocking - Blocking is a transitional state that allows
graceful transition into the Unusable State without
loss of ATM layer cells.
IMA Sublayer - The IMA Sublayer is part of the
Physical Layer that is located between the interface
specific Transmission Convergence Sublayer and the
ATM Layer.
Cell - Fixed-size information package consisting of
53 bytes (octets) of data; of these, 5 bytes represent
the cell header and 48 bytes carry the user payload
and required overhead.
IMA Virtual Link - The IMA Virtual Link is a virtual
circuit established between two IMA ends over a
number of Physical Links (i.e., IMA Group).
Cell Delay Variation (CDV) - a QoS parameter that
measures the peak-to-peak cell delay through the
network; results from buffering and cell scheduling.
Inhibiting - Inhibiting is a voluntary action that
disables the capacity of a group or link to carry ATM
Layer cells for reasons other than reported problems.
Common Transmit Clock (CTC) Configuration -
This is a configuration where the transmit clocks of
all links within the IMA group are derived from the
same clock source.
Insufficient Links - This is a Group State indicating
that the group does not have sufficient links to be in
the Operational State.
Constant Bit Rate - An ATM service category
supporting a constant or guaranteed rate, with timing
control and strict performance parameters. Used for
services such as voice, video, or circuit emulation.
Independent Transmit Clock (ITC) Configuration -
This is a configuration where the transmit clock of at
least one link within the IMA Group is not derived
from a common clock source.
Filler Cell - A Filler Cell is used to fill in the IMA
frame when no cells are available at the ATM layer. It
is used for performing cell rate decoupling at the IMA
sublayer (e.g., similar to the Idle Cell used in single
link interfaces).
Isochronous - The temporal property of an event or
signal recurring at known periodic time intervals (e.g.
125 µs). Isochronous signals are dependent on
some uniform timing, or carry their own timing
information embedded as part of the signal.
Examples are DS-1/T1 and E1. From the root words,
“iso” meaning equal, and “chronous” meaning time.
Header Error Control (HEC) - ATM equipment
(usually the PHY) uses the fifth octet in the ATM cell
header to check for an error and correct the contents
of the header; CRC algorithm allows for single-error
correction and multiple-error detection.
ITU-T - International Telecommunications Union
Telecommunications Standards Sector.
Layer Management Functions
-
The Layer
I.363 - ITU-T Recommendation specifying the AALs
for B-ISDN.
Management Functions relate to processing of
actions such as configuration, fault monitoring and
performance monitoring within the group.
IMA Frame - The IMA Frame is used as the unit of
control in the IMA protocol. It is defined as M
consecutive cells, on each of N links, where 1 ≤ N ≤
32 (determined by the UM and IMA link start-up
procedure), in an IMA Group. One of the M cells on
each of the N links is an ICP cell that occurs within
the frame at the ICP cell offset position. This offset
position may be different between links. The IMA
Loss of Cell Delineation (LCD) - The LCD defect is
reported when the OCD anomaly persists for the
period of time specified in ITU-T Recommendation
I.432(30)¸. The LCD defect is cleared when the OCD
anomaly has not been detected for the period of time
specified in ITU-T Recommendation I.432.
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