Understanding the network.pdf - Back to Home

• Minimum Cell Rate (MCR)—Defines the minimum cell rate that must be
guaranteed for a VC
To enforce the QoS contracts and implement congestion control mechanisms, ATM
employs two traffic management functions: traffic policing and traffic shaping.
Traffic shaping occurs during the cell multiplexing phase, when various VC cell
streams are partitioned into buffers, which in turn adjust the cell flow into the PHY
layer according to the QoS contract. During periods when resources are limited,
cells associated with VCs that provide no service guarantee will be discarded.
To guard against cell loss, traffic policing is performed by the UNI interface, which
relays network congestion information to the ATM layer. Based on the traffic
shaping management information, if the network does not have the resources to
meet a VC's QoS requirements, the UNI will reject the establishment of the VC until
adequate network resources are available.
When the VC is established, a QoS contract is established. There are four QoS
categories supported by ATM for data delivery over VCs:
• Constant Bit Rate (CBR)—This provides a fixed rate bandwidth link and is
required for real-time video and audio applications. A CBR connection
establishes a PCR at call setup, and the resources to meet this requirement
are reserved at the same time. The MBS rate for a CBR connection is
unlimited, and the bandwidth is reserved for the duration of the VC.
• Variable Bit Rate (VBR)—VBR VCs utilize a PCR, SCR, and MBR to regulate
cell flow. There are two variations on the VBR contract. VBR-rt VCs can
tolerate some controlled network delay and generally operate at a consistent
bit rate (that might vary depending on traffic flow). VBR-nrt VCs have the
same operating rate requirements as VBR-rt, but are not affected by
network delays.
• Unspecified Bit Rate (UBR)—UBR VCs are used for network communications
that require no QoS contract. These applications can tolerate both data loss
and network delays and have no specific bandwidth availability requirements.
UBR is commonly used for ATM LAN applications where the ULPs can provide
recovery services for inconsistencies in the network transport.
• Available Bit Rate (ABR)—ABR VCs make use of the bandwidth available on
the network based on network congestion notification. Congestion
information is relayed by resource management cells.
ATM Adaptation Layer (AAL)
The adaptation layer has a control element and a user element. The control
adaptation layer element deals with transporting signaling information between two