Data communication in a computer network involves the exchange of data between two or more entities interconnected by communication links and subnetworks. Computer networking systems are packet switched networks often require transmission of data to a single end station or to multiple end stations within the network. The data originates from a user program, and is segmented into multiple data frames and subsequently transmitted in order to simplify processing and minimize the retransmission time required for error recovery. Network traffic travels over buses in the form of packets, each including a variable data payload that a source computer sends to a destination computer. A data packet is typically formed in a relatively high level of a communications protocol and before it is transmitted from an originating device it has attached to it a header which includes address data. The address data normally includes a protocol or network address, defining a `subnet` on which the destination station is located and usually also an identification of that destination station. Each packet includes a header conveying data the network devices need to properly route and process the packet. In a computer network, network devices such as routers, hubs, switches, bridges, repeaters and others may be used to divide network traffic and boost network signals. A network operating according to a given communications protocol may be expanded by using one or more repeaters, bridges or switches. A repeater is a hardware device that functions at the physical layer and re-transmits each received packet to every other port. A network hub is simply a repeater for receiving a data transmission at any of its I/O ports and for rebroadcasting the data transmission outward on all of its other I/O ports. Thus all network stations connected to a network hub receive all data transmissions sent to the hub. A bridge operates at the data link layer of OSI Reference Model and increases efficiency by filtering packets to reduce the amount of unnecessary packet propagation on each network segment. Packet switched computer networks typically employ a network switch that receives and forwards data frames to individual and/or multiple end stations. The switch makes forwarding decisions upon receipt of the data frames based on information contained in a header of each data frame. Typically, a network switch receives a packet from the source network component via an input port and sends a packet to the destination network component via an output port. A network switch creates a network among a plurality of end nodes, such as workstations, and other network switches connected thereto. Each end node is connected to one port of the network. The ports also serve to connect network switches together. Each end node sends packets of data to the network switch which the switch then routes either to another of the end nodes connected thereto or to a network switch to which the destination end node is connected. Each network switch has to temporarily store the packets of data which it receives from the units (end node or network switch) connected to it while the switch determines how, when and through which port to retransmit the packets. The network switch passes data frames received from a transmitting station to a destination station based on the header information and the received data frame. Packet transmission events typically are tracked to provide a basis for statistical analysis of network operation with respect to each data network switch port.

A network switch provides a switching function for transferring information, such as data frames, among entities of the network. Typically, the switch is a computer comprising a collection of components interconnected by a backplane of wires. Each card may include a limited number of ports that couple the switch to the other network entities over various types of media, such Ethernet, FDDI or token ring connections. A network switch is responsible for providing the communication link between the two sub-networks. A network switch may also act to provide a communication link between two individual computers, or between an individual computer and a sub-network. The switching function provided by the switch typically comprises receiving data at a source port from a network entity, transferring the data over the backplane to a destination port and, thereafter, transmitting that data over a medium to another entity of the network. A network switch typically includes a set of input ports for receiving packets arriving on the buses, a set of output ports for forwarding packets outward on the buses, and a switch fabric such as a crosspoint switch for routing packets from each input switch port to the output switch ports that are to forward them. Each input port includes a memory for storing each incoming packet until the input port can forward it via the switch to an output port. A network switch input port may include a protocol processor for converting each incoming packet to a sequence of cells of uniform size, and a traffic manager within the input port for storing the cells in a cell memory until it can forward them through the switch fabric to one of the output ports. The traffic manager in a network switch forwards packets of the same flow in the order that it receives them, but may forward packets of high priority flows before forwarding earlier received packets of low priority flows. Each output port also includes a memory for each packet arriving via the switch fabric until it can forward the packet outward on a network bus. An output switch port may include a traffic manager for storing the cells received via the switch fabric in its buffer memory and for later forwarding them to another protocol processor. The output port's protocol processor reassembles each cell into a packet and forwards the packet outward on a network bus. A network switch output port may be able to forward a packet outward to another network switch or to a network station on a selected channel of any of several different network buses, and the traffic manager of a network switch output port decodes the packet's FIN to determine which output bus or bus channel is to convey the packet away from the port. The output port's traffic manager may also decode a packet's a flow identification number (FIN) to determine a packet's minimum and maximum forwarding rate and priority. The network switch also includes an address translation system which relates a network destination address included in each incoming packet to an output port that can forward the packet to that network address. When an input port receives an incoming packet it stores the packet, reads its network destination address, consults the address translation system to determine which output port is to forward the packet, and then sends a routing request to the switch's arbitration system.

A network switch includes switching logic for receiving and forwarding frames to the appropriate destinations. One arrangement for generating a frame forwarding decision uses a direct addressing scheme, where the network switch includes a fixed address table storing switching logic for the destination addresses. Switches employing both media access control addresses (such as in bridges) or network addresses (such as in routers) provide temporal buffering both when the packets are received, in what are known as `receive queues`, and when they are assigned to transmit ports, in what are known as transmit queues. The transmission of packets from a transmit queue may depend on a variety of considerations, including possible congestion in a device to which the respective port is connected. A multiport network switch may be provided in a data communication network to enable data communication between multiple network nodes connected to various ports of the switch. A logical connection may be created between receive ports and transmit ports of the switch to forward received frames to appropriate destinations. Data sent by one station on a network to one or more other stations on the network are sent through the network switch. The data is provided to the network switch over a shared access medium according to an Ethernet protocol. The network switch, which receives the data at one of its multiple ports, determines the destination of the data frame from the data frame header. The network switch then transmits the data from the appropriate port to which the destination network station is connected. Multiple queues of different priorities may be arranged at each port to service receive and transmit data according to data priority. Network switch input and output ports often include buffer memories for storing packets until they can be forwarded thorough the switch fabric or outward on a network bus. An output port's buffer allows it to receive data faster than it can forward it, at least until the buffer fills up. When the buffer is full, incoming data is lost. A network switch port often uses one or more synchronous dynamic random access memories (SDRAMs) to implement its buffer memory because large SDRAMs capable of storing many cells are inexpensive. Some network switch input switch ports include protocol processors for converting each incoming packet to a sequence of cells of uniform size. The input port stores the cells in its buffer memory until it can forward them through the switch fabric to one of the output ports. Each output switch port in turn stores cells received via the switch fabric in its buffer memory and later forwards them to another protocol processor which reassembles them into a packet to be forwarded outward on a network bus. A performance of a digital switch is often assessed based on metrics such as the number of physical ports that are present, and the total bandwidth or number of bits per second that can be switched without blocking or slowing the data traffic.