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Friday, November 12, 2010

Permanent Virtual Circuit (PVC)

Virtual circuit
In telecommunications and computer networks, a virtual circuit (VC), synonymous with virtual connection and virtual channel, is a connection oriented communication service that is delivered by means of packet mode communication. After a connection or virtual circuit is established between two nodes or application processes, a bit stream or byte stream may be delivered between the nodes; a virtual circuit protocol allows higher level protocols to avoid dealing with the division of data into segments, packets, or frames.
Virtual circuit communication resembles circuit switching, since both are connection oriented, meaning that in both cases data is delivered in correct order, and signalling overhead is required during a connection establishment phase. However, circuit switching provides constant bit rate and latency, while these may vary in a virtual circuit service because of reasons such as:
  • 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.
Many virtual circuit protocols, but not all, provide reliable communication service, by means of data retransmissions because of error detection and automatic repeat request (ARQ).
Examples of protocols that provide virtual circuits
Examples of transport layer protocols that provide a virtual circuit:
  • 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.
Examples of network layer and datalink layer virtual circuit protocols, where data always is delivered over the same path:
Permanent and switched virtual circuits in ATM, frame relay, and X.25
Switched virtual circuits (SVCs) are generally set up on a per-call basis and are disconnected when the call is terminated; however, a permanent virtual circuit (PVC) can be established as an option to provide a dedicated circuit link between two facilities. PVC configuration is usually preconfigured by the service provider. Unlike SVCs, PVC are usually very seldom broken/disconnected.
A switched virtual circuit (SVC) is a virtual circuit that is dynamically established on demand and is torn down when transmission is complete, for example after a phone call or a file download. SVCs are used in situations where data transmission is sporadic and/or not always between the same data terminal equipment (DTE) endpoints.
A permanent virtual circuit (PVC) is a virtual circuit established for repeated/continuous use between the same DTE.
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.
Frame relay is typically used to provide PVCs. ATM provides both switched virtual connections and permanent virtual connections, as they are called in ATM terminology. X.25 provides both SVCs and PVCs, although not all X.25 service providers or DTE implementations support PVCs as their use was much less common than SVCs. 
The unspecified bit rate (UBR) service category is one of five ATM service categories defined in the ATM Forum's Traffic Management Specification 4.0
The five service classes are:
UBR is intended for non-real-time applications that do not require any maximum bound on the transfer delay or on the cell loss ratio.
The purpose of this document is to clarify the differences between a UBR permanent virtual circuit (PVC) and a variable bit rate, non-real time (VBR-nrt) PVC by illustrating that two such virtual circuits (VCs) with the same peak cell rate (PCR) experience very different bandwidth guarantees and scheduling priorities. These differences may affect the level of performance that users are provided on the connection.
This document is not restricted to specific software and hardware versions.
Advantages and Disadvantages of 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.
Despite these disadvantages, a well-designed ATM network implementing congestion control, traffic shaping at the end systems, and intelligent cell discard mechanisms such as early packet discard (EPD) or tail packet discard can provide reasonable support for UBR. In other words, any quality of service (QoS) provided to the UBR PVC results from the network design guidelines and the end system applications as opposed to anything operating within ATM.
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)
Sustained Cell Rate (SCR)
Available bit rate (ABR)
Non-zero Minimum Cell Rate (MCR) if specified
Non-zero MCR if signaled by the router; applies to switched virtual circuit (SVCs) only on the PA-A3

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.

Traffic contract
If a service (or application) wishes to use a broadband network (an ATM network in particular) to transport a particular kind of traffic, it must first inform the network about what kind of traffic is to be transported, and the performance requirements of that traffic[1]. The application presents this information to the network in the form a traffic contract.
Contents 1 The Traffic descriptor

(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****

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