Cisco 4XIB SuperFlex Cable DDR Ready, 3m CAB-04XS-03= Manuel D’Utilisation
Codes de produits
CAB-04XS-03=
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White Paper
InfiniBand SDR, DDR, and QDR Technology Guide
The InfiniBand standard supports single, double, and quadruple data rate that enables an InfiniBand link to transmit
more data. This paper discusses the characteristics of single data rate (SDR), double data rate (DDR), and quad data
rate (QDR) InfiniBand transmission with respect to performance, transmission distances, and cabling
considerations.
INFINIBAND DDR
InfiniBand supports DDR and QDR transmission to increase link bandwidth. In the context of InfiniBand, DDR and QDR differ with
respect to computer DDR and QDR transmission as the InfiniBand 2.5-Gbps lane is clocked two times (DDR) or four times (QDR) faster,
instead of transferring two bits (DDR) or four bits (QDR) per clock cycle. By increasing the clock-rate by a factor of two or four times,
DDR and QDR transmission provide a simple method of increasing bandwidth capacity and reducing serialization delay.
Note:
Commercial InfiniBand QDR products are not available at this time.
Because the link data rate is twice as fast (5 Gbps) or four times faster (10 Gbps) than InfiniBand’s base 2.5 Gbps, bandwidth is increased
by a factor of two or four and latency is reduced because packets are serialized faster. This benefits applications that are either bandwidth-
intensive (also referred to I/O-intensive) or are particularly sensitive to interprocess latency. See Table 1.
Table 1.
InfiniBand SDR and DDR Link Characteristics
InfiniBand Link
Signal Pairs
Signaling Rate
Data Rate (Full Duplex)
1X-SDR
2
2.5 Gbps
2.0 Gbps
4X-SDR
8
10 Gbps (4 x 2.5 Gbps)
8 Gbps (4 x 2 Gbps)
12X-SDR
24
30 Gbps (12 x 2.5 Gbps)
24 Gbps (12 x 2 Gbps)
1X-DDR
2
5 Gbps
4.0 Gbps
4X-DDR
8
20 Gbps (4 x 5 Gbps)
16 Gbps (4 x 4 Gbps)
12X-DDR
24
60 Gbps (12 x 5 Gbps)
48 Gbps (12 x 4 Gbps)
1X-QDR
2
10 Gbps
8.0 Gbps
4X-QDR
8
40 Gbps (4 x 5 Gbps)
32 Gbps (4 x 8 Gbps)
12XQDDR
24
1200 Gbps (12 x 5 Gbps)
96 Gbps (12 x 8 Gbps)
Note:
Although the signaling rate is 2.5 Gbps, the effective data rate is limited to 2 Gbps because of the 8B/10B encoding scheme:
(2.5 x 8) ÷ 10 = 2 Gbps
For applications that move large data files, such as distributed databases and data-mining applications, InfiniBand 4X DDR provides
significant performance benefits. If an application transmits many small messages, there may be limited performance improvement
realized by deploying InfiniBand DDR depending upon the size of the high-performance computing (HPC) cluster and application.
However, because DDR serializes packets to line twice as fast as SDR, end-to-end latency can be significantly reduced in large, multi-stage
clusters. Although the difference in latency is quite small—in the order of 120 to 600 nanoseconds (ns) for back-to-back configurations—