Compact SFP (CSFP) – What it is and when it is used?

SFP type and BiDi SFP type optical modules are very popular and widely used in optical communication while CSFP (Compact Small Form-Factor Pluggable) optical transceivers are much more rare. In this article we would like to explain main difference between SFP, BiDi SFP and Compact SFP, mention in which scenarios Compact SFPs are mostly used, which vendors in which platforms support them and how it relates back to Fiberland Optical Solutions CSFP portfolio.
Compact SFP (CSFP) – What it is and when it is used?
SFP, BiDi SFP and Compact SFP (CSFP) differences.

As an old Chinese proverb goes: “One Picture is Worth Ten Thousand Words” let’s start with a Picture where we can see detailed differences between SFP, BiDi SFP and Compact SFP (CSFP) optical modules link construction principles:

SFP principle:

We know that for one of most widely used product types in our industry SFP we have 2 ports one is TX which is used to transmit the signal and other port is RX which is used to receive the signal. This communication require 2 optical fiber connection. To make connection we simply use 2pcs of this product type put in Site A and in Site B, correctly plug optical cables and we will get the link up and communication can start.

BiDi SFP principle:

If we go further BiDi SFP modules are as well very popular, key advantage for BiDi SFP comparing to regular SFP is that we only need to use 1 optical fiber to make same 1xGE transfer. In situations where fiber resources are scarce or operator needs to rent out fiber, BiDi SFP brings significant benefit to the network design and can reduce the cost a lot. BiDi SFPs work in pairs, there are A side and B side modules. A side modules on standard distances 3-40km have Tx: 1310nm and Rx:1490nm or Tx:1310nm and Rx:1550nm, while B Side modules on standard distances will have Tx:1490nm Rx:1310nm or Tx:1550nm and Rx:1310nm. We should connect them accordingly in pairs Tx:1310nm/Rx:1490nm A side together with Tx:1490nm/Rx:1310nm B side OR Tx:1310nm/Rx:1550nm A side together with Tx:1550nm/Rx:1310nm B and we will get the link running.

Compact SFP (CSFP) principle:

Compact SFP (CSFP) goes even further, by using same SFP port we can have 2 BiDi bidirectional streams of traffic, thus we can get 2xGE out from one SFP port. Only thing which is required that equipment line card support CSFP and actually for this not so many vendor equipment has been designed to accept CSFP modules.

When Compact SFP (CSFP) is used?

Most commonly CSFP is used in Central Office/Aggregation site operating at Tx1&Tx2:1490nm Rx1&Rx2:1310nm and it get’s connected to 2 CPE sites which have simple BiDi SFP operating in Tx:1310nm Rx:1490 at each site. Compact SFP (CSFP) major application is to use it in FTTx scenario where we have (point-to-point) connections from Central Office/Aggregation Site to (CPE) Customer Premises Equipment. With help of CSFP we can double the Central Office/Aggregation Site port density (we can achieve 2xGE from one aggregation site port) and as well reduce power consumption in Central Office as we use 2 times less ports.

Which Vendor platforms support Compact SFP (CSFP)?

Most popular vendors which support Compact SFP are Cisco, Huawei and Nokia (ex. Alcatel) and here as well we need to be careful as not all Cisco, Huawei and Nokia (ex. Alcatel) platforms will support Compact SFPs.

Cisco Catalyst 4500 and GLC-2BX-D:

One of most popular Cisco platform which support CSFP optical modules is Cisco Catalyst 4500 Series which when used with X4640-CSFP-E line card accepts CSFP optical modules. Normally in this line card we can have 40xGE as 40 SFP ports are available, but if we use CSFP optical modules in same 40 SFP ports we can get 80xGE.
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