- varying packet queue lengths in the network nodes,
- varying bit rate generated by the application,
- varying load from other users sharing the same network resources by means of statistical multiplexing, etc.
- Transmission Control Protocol (TCP), where a reliable virtual circuit is established on top of the underlying unreliable and connectionless IP protocol. The virtual circuit is identified by the source and destination network socket address pair, i.e. the sender and receiver IP address and port number. Guaranteed QoS is not provided.
- SCTP, where a virtual circuit is established on top of either the IP protocol or the UDP protocol.
- X.25, where the VC is identified by a virtual channel identifier (VCI). X.25 provides reliable node-to-node communication and guaranteed QoS.
- Frame relay, where the VC is identified by a VCI. Frame relay is unreliable, but may provide guaranteed QoS.
- Asynchronous Transfer Mode (ATM), where the circuit is identified by a virtual path identifier (VPI) and virtual channel identifier (VCI) pair. ATM is unreliable, but may provide guaranteed QoS.
- General Packet Radio Service (GPRS)
- Multiprotocol label switching (MPLS), which can be used for IP over virtual circuits. Each circuit is identified by a label. MPLS is unreliable, but provides eight different QoS classes.
In a PVC, the long-term association is identical to the data transfer phase of a virtual call. Permanent virtual circuits eliminate the need for repeated call set-up and clearing.
The five service classes are:
- Constant bit rate (CBR)
- Variable bit rate non-real-time (VBR-nrt)
- Variable bit rate real-time (VBR-rt)
- Available bit rate (ABR)
- Unspecified bit rate (UBR) and UBR+
Following is a summary of the advantages and disadvantages of UBR VCs. This ATM service category has some important disadvantages related to bandwidth guarantees and scheduling priorities. These disadvantages are further illustrated in the next sections.
- Allows for a high degree of statistical multiplexing by not reserving any minimum bandwidth per VC. The VCs use the bandwidth up to the configured PCR when available.
- Models the best-effort service normally provided by the Internet. Suitable for applications tolerant to delay and not requiring real-time response. Examples include e-mail, fax transmission, file transfers, Telnet, LAN and remote office interconnections. Such applications are not sensitive to delay, but they are sensitive to cell loss. ATM switches, such as the Cisco Catalyst 8500 series, allocate larger maximum per-VC queue limits for UBR PVCs.
Note: Queuing minimizes loss at the expense of greater delay.
- The only attributes specified as part of UBR are the PCR and the cell delay variation tolerance (CDVT). The PCR only provides an indication of a physical bandwidth limitation within a VC.
Note: A relatively new variant of UBR, called UBR+, allows an ATM end-system to signal a minimum cell rate to an ATM switch in a connection request, and the ATM network attempts to maintain this minimum as an end-to-end guarantee. Refer to the document Understanding the UBR+ Service Category for ATM VCs.
- VCs of other ATM service categories have a higher priority as viewed by the ATM interface segmentation and reassembly (SAR) scheduler. When competition for a cell timeslot arises, the scheduler gives the timeslot to a VC of service classes with a higher priority.
- It does not place any bounds with respect to the cell loss ratio (CLR) or to the cell transfer delay (CTD). The end-system is expected to handle and adjust for any cell loss or delay.
- It does not guarantee cell delivery. Retransmission occurs at higher layers.
Understanding Bandwidth GuaranteesThis section illustrates how a router ensures that bandwidth guarantees are met by reserving or not reserving bandwidth for a particular VC depending on its ATM service class. In scheduling the next cell to be transmitted from a port, a process called the scheduler selects a cell from a PVC with guaranteed cell rates.
This table lists the cell rates that are guaranteed by the rate scheduler for each service category:
| Service Category || Cell Rate Guaranteed |
| Constant bit rate (CBR) || PCR |
| VBR-rt || Sustained Cell Rate (SCR) |
| VBR-nrt || SCR |
| Available bit rate (ABR) || Non-zero Minimum Cell Rate (MCR) if specified |
| UBR+ || Non-zero MCR if signaled by the router; applies to switched virtual circuit (SVCs) only on the PA-A3 |
| UBR || None |
Both ATM-attached routers and ATM switches take steps to meet bandwidth guarantees. The example below shows how a router accomplishes this.
In this example, PVCs are configured with service classes on a PA-A3 ATM port adapter.
- 2 Type of Service
- 3 Traffic Parameters
- 4 Quality of Service (QoS) Parameters
(Virtual Channel Identifier, Identificador de Canal Virtual) hace referencia a un campo de 16 bits en el encabezado de una celda ATM.
El VCI, junto con el VPI, se utilizan para identificar el próximo destino de una celda a medida que pasa a través de una serie de switches ATM en su recorrido hasta el destino. Los switches ATM utilizan los campos VPI/VCI para identificar el próximo VCL de red que una celda necesita para recorrer su camino hasta llegar al destino final. La función del VCI es similar a la del DLCI en Frame Relay.
Una gama de PVC se define por dos pares VPI-VCI.
La ruta de acceso virtual dos identificadores (VPIs) define un rango de VPI, y los dos identificadores de canal virtual (VCIs) definen un rango de VCI. El número de PVC en la gama de PVC es igual al número de VPIs en la gama VPI multiplicada por el número de VCIs en el rango de VCI.
****Entelnet vpi 0 vci 33 UBR without PĈR****