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Knowledge Base

Forwarding Capability of the Port-4 10GBase WAN-XFP Interface Board on an NE5000E Is 10 Gbps, But the Traffic Rate Detected by the NMS Can Reach Only 9.1 Gbps

Publication Date:  2019-05-14  |   Views:  155  |   Downloads:  0  |   Document ID:  EKB1100015109


Issue Description

The forwarding capability of the port-4 10GBase WAN-XFP interface board on an NE5000E is 10 Gbps. However, the traffic rate detected by the NMS can only reach 9.1 Gbps.


For WAN interfaces, Ethernet packets are encapsulated in SDH frames of POS interfaces. The overhead of WAN interfaces includes the overhead of Ethernet interfaces and the overhead of POS interfaces. Therefore, the valid bandwidth is even lower.

When a packet is sent from a WAN interface, the 12-frame timeslot plus 8 preamble are added to ETH, and 64/66B conversion is implemented to convert the 64 bits composed by 8 bytes into 66 bits. Then, the 66-bit data is encapsulated into SDH frames. Therefore, the valid bandwidth of WAN interfaces is calculated as follows: (26/27) x (64/66 x [L/(L+20)] x interface bandwidth. The 10G WAN interface bandwidth and the 10G POS interface bandwidth are both 9.95328 Gbps. The valid bandwidth of WAN interfaces is relevant to the packet length.
For example, if the packet length is 980 bytes, the valid bandwidth is calculated as follows: (26/27) x (64/66) x [980/(980+20)] x 9.95328 Gbps = 9.11 Gbps.

NE5000E interfaces use the input byte and output byte to calculate the input rate and output rate. The input byte and output byte refer to the valid payload of packets, excluding the overhead. Therefore, the input rate and output rate can reach the valid interface bandwidth, instead of the interface's line bandwidth.

The NMS uses the input byte and output byte obtained through walk nodes ifHCInOctets and ifHCOutOctets to calculate the interface rate. Therefore, the interface rate on the NMS cannot exceed the valid interface bandwidth and the bandwidth utilization cannot reach 100%. However, the NE5000E counts overhead in during calculation of the inbound utility rate and outbound utility rate. Therefore, the bandwidth utilization is displayed as 100%.