Class of Service “COS”

The customer specifies a Category of Service, also called Class of Service (COS), for each virtual circuit in their network.  The Category of Service defines the performance parameters for that connection.  Each service category meets the unique networking requirements of different end-user applications.  

The provider’s ATM service supports the following service categories: 

Constant Bit Rate (CBR) ­­­ - CBR service is designed for timing sensitive traffic such as voice and interactive videoconferencing.  CBR provides a constant data rate and a consistent delay through the The provider ATM network.  CBR PVCs are given the highest priority throughout the ATM network and are provisioned to provide minimal Cell Delay Variation or “jitter.”  CBR service is ordered at the peak cell rate (PCR), the highest rate of transmission the logical connection will allow.  CBR is typically used to enable PBX tie-line or Video Codec services over ATM.

In order for a customer to utilize the ATM network for CBR applications at DS3 levels, the customer will be required to provide their own Stratum 1 traceable clock source at their premises.  The provider will not guarantee timing from the The provider network.  CBR traffic requires Stratum 1 traceability to the CPE ATM switches. 

This current situation, affecting all carriers, is related to the way the DS3 circuits are mapped and handed off between The provider and the LEC.   In order to send time sensitive traffic at a constant rate and consistent delay over the network via DS3 CBR service, PLCP (Physical Layer Convergence Protocol) is utilized.  PLCP is a DS3 framing format, which allows synchronization to be transferred on DS3 signals at Stratum 1 levels.  While the The provider SONET network is Stratum 1 clocked, and this clock can be transferred to the ATM network and mapped into a DS3 circuit, once it is handed off to the LEC it is vulnerable to being altered by the LEC prior to it reaching the customer’s site.  If any network element within the LEC reframes the DS3 signal, the PLCP framing could be destroyed, and the Stratum 1 clock traceability is destroyed as well.  The ultimate solution for reliable CBR service is the LEC guaranteeing the customer “transparent” transport of the PLCP framed circuit.  Since this is an unrealistic solution, customers will be better suited in generating their own clock source.

The provider’s Network Transport Planning group has identified several options for the customer in generating Stratum timing to the ATM CPE devices, these include the following:

·        Utilize existing on-site Stratum 1 traceable equipment (a PBX, or access to a GPS antenna, or install a low cost PRS (Primary Reference Source).

·        Obtain Stratum traceability from the LEC via DS1.

·        OCn from the LEC

·        Cesium beam

·        LEC-supplied DS1 tariffed sync timing

Variable Bit Rate (VBR-nrt) - VBR-nrt (non-real-time) service is designed for bursty data applications, and provides a pre-assigned variable data rate.  VBR-nrt service is ordered with two parameters:  peak cell rate (PCR), which defines the maximum rate of transmission, and sustainable cell rate (SCR), which provides an average throughput rate expected on the connection (Note:  SCR must be specified for VBR connections.  If there is no SCR indicated on an ATM DOC order, the default is set to 20% of the PCR.  SCR must fall within 20-70% of PCR).

Unspecified Bit Rate (UBR) - UBR service takes advantage of excess network bandwidth, and is best suited for communications applications where timing of delivered data is not critical, such as file transfer, e-mail, LAN-to-LAN communications, and voicemail.  UBR is well suited for TCP/IP LAN traffic, which has inherent reliability, and can tolerate occasional cell discarding.  The provider does not currently assign specific PCR or SCR values to UBR circuits.  A benefit of UBR is that ATM cells may be transmitted up to the line rate if bandwidth is available (rather than limited to pre-defined maximum PCR as with CBR and VBR-nrt services).

For customers served by the Vector Edge platform,  UBR VCCs will be provisioned with packet level discard, which supports the reliable transport of IP packets in large, busy networks.  This reliable transport results in intelligent discarding of cells to maximize throughput during periods of instantaneous link congestion to maximize “goodput”—the effective throughput, determined by cells which are successfully recombined at the receiver.— This “smart” discarding dramatically reduces the number of packets that must be retransmitted during congestion. Two complementary techniques are used:  Early Packet Discard (EPD) and Partial Packet Discard (PPD).

Early packet discard (EPD) takes advantage of a threshold level set in the switch buffers.  If the buffer ever reaches this threshold level, no new packets are permitted to enter the switch fabric.  This ensures that packets that have already started through the switch buffer can be completed.  Once the cells in the buffer have receded below the threshold, EPD stops discarding cells.

Partial packet discard (PPD) is the back-up capability that works hand-in-hand with early packet discard.  In the event that buffers overflow and cells must be discarded, partial packet discard intelligently discards all remaining cells in the packets which are currently incomplete.  This reduces the number of packets that must be retransmitted.  PPD is activated only when the output buffer on the network interface is completely full.

Performance tests have shown that UBR service with packet level discard performs significantly better under congestion conditions than the proprietary ABR (Available Bit Rate) schemes currently offered by some carriers. These current proprietary implementations of ABR do not operate in a multi-vendor environment and are not easily adapted to evolving standards.

The benefits of packet level discard will be provided to customers with no requirement for CPE upgrading, allowing for a simpler customer implementation.  But in order for customers to realize these benefits, they must use AAL 5 for their ATM adaptation.  The overhead of a TCP/IP packet that has been converted to ATM cells—via AAL 5--is located in the last cell of the sequence.  A field in the ATM cell header (the payload type indicator (PTI) field) indicates when that last cell is being transported.  It is important that this last cell is transported through the network because it provides the network with an indication of the start of the next packet.

Available Bit Rate (ABR) - The provider Availability TBD - ABR service is designed for data applications that need a high level of performance (low cell loss) but can tolerate variation in transmission speed and network delay. See section 4.4 ABR COS for more information.

Variable Bit Rate (VBR-rt) intended for compressed video and audio in teleconferencing and multi-media applications which need the same timing considerations as CBR but that produce a variable, or bursty, bit rate. See section 4.5 VBR-rt COS for more information.