PCI Express Terms and Acronyms (Nine)

Symbol Time The period of time required to place a Symbol on a Lane (10 times the Unit
Interval).
System Element A defined Device or collection of devices that operate according to distinct sets of
rules. The following System Elements are defined: Root Complex, Endpoint,
Switch, and Bridge.
Tag A number assigned to a given Non-posted Request to distinguish Completions
for that Request from other Requests.
TLP Prefix Additional information that may be optionally prepended to a TLP. TLP Prefixes
are either Local or End-End. A TLP can have multiple TLP Prefixes. See
Section 2.2.10.
Transaction Descriptor
An element of a Packet header that, in addition to Address, Length, and Type,
describes the properties of the Transaction.
Transaction ID A component of the Transaction Descriptor including Requester ID and Tag.
Transaction Layer The Layer that operates at the level of transactions (for example, read, write).
Transaction Layer
Packet, TLP A Packet generated in the Transaction Layer to convey a Request or Completion.
transaction sequence A single Request and zero or more Completions associated with carrying out a
single logical transfer by a Requester.
Transceiver The physical Transmitter and Receiver pair on a single chip.
Transmitter The component sending Packet information across a Link.
Transmitting Port In the context of a specific TLP or DLLP, the Port that transmits the Packet on a
given Link.
Unit Interval, UI Given a data stream of a repeating pattern of alternating 1 and 0 values, the Unit
Interval is the value measured by averaging the time interval between voltage
transitions, over a time interval long enough to make all intentional frequency
modulation of the source clock negligible.
Unsupported Request,
UR 1. A status that applies to a posted or non-posted Request that specifies some
action or access to some space that is not supported by the Completer. 2. A
status indication returned with a Completion for a non-posted Request that
suffered an Unsupported Request at the Completer.
Upstream 1. The relative position of an interconnect/System Element (Port/component)
that is closer to the Root Complex. The Port on a Switch that is closest
topologically to the Root Complex is the Upstream Port. The Port on a
component that contains only Endpoint or Bridge Functions is an Upstream Port.
The Upstream component on a Link is the component closer to the Root
Complex. 2. A direction of information flow where the information is flowing
towards the Root Complex.
variant A field of a TLP header that contains a value that is subject to possible
modification according to the rules of this specification as the TLP flows through
the PCI Express fabric.
wakeup An optional mechanism used by a component to request the reapplication of
main power when in the L2 Link state. Two such mechanisms are defined:
Beacon (using in-band signaling) and WAKE# (using sideband signaling).
warm reset A Fundamental Reset without cycling the supplied power.