Committed information rate (CIR) is the guaranteed rate at which a Frame Relay network will transfer information under normal line conditions.
In Frame Relay networks, CIR refers to the bandwidth associated with a logical connection in a permanent virtual circuit (PVC). Expressed in bits per second, CIR is the minimum data rate to be maintained between nodes in a PVC, i.e., the bandwidth a user is guaranteed all the time.
Different logical connections share the same physical path in a Frame Relay network, and some logical connections are given higher bandwidths than others. CIRs offer a way of assuring individual users minimum bandwidth speed, even when they share the same physical connection over Frame Relay.
In most Frame Relay services, the network provides a PVC so that the customer sees a continuous, dedicated connection without paying for a full-time leased line. Moreover, any necessary retransmission of data and error correction are left to the endpoints to speed up data transmission.
Frame Relay networks and virtual circuits
Frame Relay is a packet-switching telecommunication service that puts data in a variable-size unit -- the frame. It is usually used for data transmissions between geographically separated local area networks or across wide area networks. As a fast packet technology, it does not attempt to correct errors detected in a frame but simply drops them.
Frame Relay devices create either a switched virtual circuit (SVC) or a PVC. An SVC is a temporary connection that's only open during a data transfer. It is closed after the transfer is completed. In contrast, a PVC keeps the connection between two locations permanently open.
Both enable users to move data between remote sites without an expensive, full-time leased line. In a Frame Relay network, multiple users can use virtual circuits to share the same physical wires.
Permanent virtual circuit and committed information rate
A PVC provides enhanced connectivity, addressing, flexibility and cost effectiveness compared to private leased circuits. Through a single access port, PVCs in Frame Relay networks enable a single device to communicate with hundreds of others. The nodes are linked by PVCs, each of which provides a CIR. If network capacity is available, the maximum amount of bandwidth available can be higher than the CIR.
Depending on the network operator and the equipment supplier, the CIR can be set to a minimum value of 8 kilobits per second and can then be increased in blocks of 4 kbps. It is calculated over a small sliding window interval and can be implemented either on a per-interface basis or a per-PVC basis.
Under a CIR, a user is guaranteed a specific amount of bandwidth based on a service-level agreement (SLA). A customer sending streaming video over a content delivery network needs more bandwidth than a customer sending only data.
The CIR SLA determines which customer gets how much guaranteed bandwidth, regardless of how many other customers share the bandwidth pool. However, since total bandwidth is limited, customers must pay a premium price for the CIR.
The total CIR of all the PVCs through an interface can be greater than the physical speed of the interface, although it still cannot exceed the access speed.
Relationship between committed information fate and committed burst size
When average traffic rates are below the CIR, unused bandwidth capacity accumulates up to a maximum amount. This is essentially the maximum traffic burst size that the network commits to deliver during a specified time interval, even if it increases the bandwidth – temporarily -- beyond the allowed limit. The amount is defined by the committed burst size (CBS), specified in kilobytes (KB) or megabytes (MB).
Example: Allocated CIR = 3 megabits per second (Mbps) | Allocated CBS = 50 KB
The user is guaranteed a minimum of 3 Mbps and can burst up to 50 KB of traffic while staying within their SLA limits. If traffic bursts above 50 KB, it may be dropped or delayed.
Since CBS refers to the maximum data allowed to exceed the CIR, it indicates the flexibility of the provider to deliver additional bandwidth without discarding or shaping the traffic. This flexibility is limited since the network can only supply bandwidth up to the access rate of the network connection.
When traffic exceeds both the CIR and the CBS, it is considered nonconforming.
Color-based bandwidth profile policing
Bandwidth profile policing is done on the basis of a two-rate, three-color marker model. This model is implemented by a dual-rate, dual-token bucket algorithm.
The committed (C) bucket determines CIR-conformant frames.
The excess (E) bucket determines EIR-conformant, excess service frames.
Here, EIR refers to excess information rate -- the maximum, nonguaranteed traffic allowed during nonbusy, or noncongested, times.
The buckets are continuously replenished by adding tokens at the respective rate -- CIR/8 bytes for the C bucket, EIR/8 bytes for the E bucket. But ,when buckets are full, no further tokens are added.
The EIR has another parameter associated with it: excess burst size (EBS). Like CBS, EBS is specified in kilobytes or megabytes and refers to the size up to which the traffic is allowed to burst without being discarded.