Virtual output queueing (VOQ) is a technique used in certain
network switch
A network switch (also called switching hub, bridging hub, and, by the IEEE, MAC bridge) is networking hardware that connects devices on a computer network by using packet switching to receive and forward data to the destination device.
A netw ...
architectures where, rather than keeping all traffic in a single queue, separate queues are maintained for each possible output location. It addresses a common problem known as
head-of-line blocking
Head-of-line blocking (HOL blocking) in computer networking is a performance-limiting phenomenon that occurs when a line of packets is held up in a queue by a first packet. Examples include input buffered network switches, out-of-order delivery a ...
.
Description
In VOQ, the physical buffer of each input port maintains a separate virtual queue for each output port. Therefore congestion on an egress port will block only the virtual queue for this particular egress port. Other packets in the same physical buffer destined to different (non-congested) output ports are in separate virtual queues and can therefore still be processed. In a traditional setup, the blocked packet for the congested egress port would have blocked the whole physical buffer, resulting in head-of-line blocking.
It has been shown that VOQ can achieve 100% throughput performance with an effective
scheduling algorithm
In computing, scheduling is the action of assigning ''resources'' to perform ''tasks''. The ''resources'' may be processors, network links or expansion cards. The ''tasks'' may be threads, processes or data flows.
The scheduling activity is ca ...
. This scheduling algorithm should be able to provide a high speed mapping of packets from inputs to outputs on a cycle-to-cycle basis. The VOQ mechanism provides throughput at a much higher rate than the
crossbar switches without it.
There are many algorithms for design and implementation of fast VOQ. For example,
Nick McKeown Nicholas (Nick) William McKeown FREng, is the SVP/GM of the Network and Edge Group at Intel and a professor in the Electrical Engineering and Computer Science departments at Stanford University. He has also started technology companies in Silicon ...
and a group at
Stanford University published a design in 1997.
Quality of service
Quality of service (QoS) is the description or measurement of the overall performance of a service, such as a telephony or computer network, or a cloud computing service, particularly the performance seen by the users of the network. To quantitat ...
and
priority are extensions found in literature of the same time.
VOQ scheduling is often referred to as "
arbitration" (resolving the concurrent access wishes), whereas the ordering of packets ("packet
scheduling
A schedule or a timetable, as a basic time-management tool, consists of a list of times at which possible tasks, events, or actions are intended to take place, or of a sequence of events in the chronological order in which such things are ...
") is an additional task
following the VOQ arbitration.
References
{{Queueing theory
Switches
Queue management