Optical Fibers and Transceivers

What is physical difference between 10Mbps and 100Mbps cable? Will higher value of this lead to faster data transfer?

Ans- For two cables that are made of identical materials and same length,

• If you think of the Internet as a series of tubes, latency is the length of the tube between two points. Bandwidth is how wide the tube is. Indeed, it’s named bandwidth because it describes the width of the communications band. The wider the tube the more data you can send in parallel.
• The key point here that gets missed is that, regardless of how much data you are sending, you still have to move it the distance from point A to point B. That takes time and that is the latency.
• Having 10 Mbps and 100 Mbps connections does not somehow allow a single bit data of data to travel that a distance any faster.
• A large bandwidth connection simply allows you to send or receive more data in parallel. The data still needs to travel to and from your computer.
• For example,

10 Mbps line:

100 Mbps line:

From above example, we see that the high bandwidth connection downloads the file faster than the low bandwidth connection because more data can travel in parallel. So faster transmission, latency still there.

MULTIMODE (MM) FIBER:

• MMF cables use a larger internal core diameter and can utilize lower cost LEDs for transmission
• Has larger core diameter- allows multiple modes (.ie. allows light to come in at different angles) and multiple wavelengths
• But, since this might cause loss and interference between the different modes over longer distance, it is used only for short distances (upto 2 km)- common in LAN deployments
• MMF cables are usually marked with “aqua blue” or “orange” color jackets

SINGLEMODE FIBER:

• The difference between SMF and MMF is in their physical characteristics
• SMF has a much smaller core diameter (typically 8-10 µm) and accepts signals coming in from a specific angle and on a specific mode only.
• SMF can be used for cable runs of very long distances (typically up to ~40 miles without repeaters depending on wavelength).
• SM cables are usually marked with ‘Yellow’ jackets

Single Mode and Multimode Optical Fibres?

SM	MM
Small diameter (9/125 um)	Large diameter (50/125 um)
Jacket is “yellow” color	Jacket is “aqua” or “orange” color
Long distance	Short distance
Avg distance is 30 km	Avg distance is 550 m
SIngle wavelength	Multiple rays (of same wavelength)
Very costly	Cheaper

• 

QUALITY RATING OF OPTICAL FIBERS:

 Just like we have category of cables in copper that tells us the distance for which we can use those cables, even for optical cables, there are categories

SR stands for short rage. So, shorter the wavelength supported, shorter the range. So, SR supports short wavelenths and have short range. We also see that Singlemode cables have longer range than multimode ones

interconnect	AKA	Wavelenth	Fiber Mode	Max range	Transceiver	Medium
SR	Short Reach	850nm	Multimode	300m	SFP+	Fiber
LR	Long Reach	1310nm	Multimode	10km	SFP+	Fiber
LRM	Long Reach Multimode	1310nm	Multimode	220m	SFP+	Fiber
ER	Extended Reach	1550nm	Singlemode	40km	SFP+	Fiber
ZR	Extended Reach +	1550nm	Singlemode	80km	SFP+	Fiber

How does Optical cable work?

• Fiber Optics is sending signals down hair-thin strands of glass or plastic fiber.
• The light is “guided” down the center of the fiber called the “core”.
• The core is surrounded by a optical material called the “cladding” that traps the light in the core using an optical technique called “total internal reflection.”

• The refractive index of core is greater than cladding
• It can be step-index or graded-index
• It can be OM3 or OM4

TYPES OF CONNECTORS:

Straight Tip (ST):

• The Straight Tip (ST) connector is often seen on the end of a multi-mode cable;
• It is being slowly replaced by multi-fiber connectors (LC and MTP).

SC (Subscriber Connector):

• 
• The Subscriber Connector (SC) can be seen commonly on MMF or SMF
• As with SC connectors, the ST connector is slowly being replaced by multi-fiber connectors. 

What is MTP- 12 and LC cables?

MTP:

• MPO is a 12 fiber connector for ribbon cable. It's main use is for pre-terminated cable assemblies and cabling systems. Here is a 12 fiber MT broken out into 12 STs (ST is the most popular connector for multimode networks). MPO means multi-position optical. This connector is sometimes called a MTP which is a commercial name.  (http://www.thefoa.org/tech/connID.htm)
• MTP or MPO 12 Strand Fiber Assemblies are a high density cables designed to carry one signal which increases bandwidth and reduces space. This plug and play fiber optic assembly is made with Corning fibre.
• The standard specifies MPO12 (or MTP12) as connector to the SR4 QSFP, which employs traditionally 12 fibres, but 40G only need 8 (4 pairs) to carry the 4 parallels bidirectional paths. You might know that QSFPs can be programmed to operate as 4 x 10G.
• When facing an SR4 QSFP or an MTP cable with their key up, those will always receive on their left (your right if you face them).
• They will always send on their right (your left if you face them).   
• It is larger than the other connectors but for good reason - it can support up to 24 fibers in a single ferrule.
• In high density patch environments such as datacenters they are used extensively, both at single mode and multi-mode wavelengths.
• Because the sequence of the fibers cannot physically be changed after termination, the connector is often supplied with a fan-out assembly at the opposing end (such as LC, SC FC etc.). This allows the operator to change channels simply by re-patching the fanned-out side of the cable. The consequence of this is that the small form high density design of the MTP/MPO will only benefit one side of the assembly.

LC:

• LC (Lucent Connector) fiber optic patch cables are used to send high-speed data transmissions throughout your network.
• LC fiber optic cables connect two components with fiber optic connectors. A light signal is transmitted so there is no outside electrical interference.
• Good performance, highly favored for singlemode.
• Its small size gives it huge popularity in datacoms and other high-density patch applications, as its combination of small size and latch feature make it ideal for densely populated racks/panels.
• 

HENCE,

In practice, one end is a MTP end and other end, we have many LC cables and they are called MTP- LC cables.

How is 10 Gbps SFP different from 40 Gbps SFP?

Note: SFP means Small form-factor pluggable (SFP) is a specification for a new generation of optical modular transceivers. The devices are designed for use with small form factor (SFF) connectors, and offer high speed and physical compactness. They are hot-swappable.

• SFP -> 1G and SFP+ -> 10G
• But, QSFP -> 40G (technically, it is QSFP+)

10G:

• The transceivers support 10 Gbps bandwidth
• It uses a single strand of line
• We use LC cables as it supports 10G transmission and reception on single cable

40G:

• It operates as 4x10G or 40G- both methods
• If 4x10G, each pair acts as separate 10G cables and can be used individually as 10G links
• If 40G, the entire link together acts as 40G link.
• We use MTP- 12 for 40G

Universal Transceivers?

• Uses the same port and cable, if we only change the transceiver, we can get universal transceiver which supports both 10G and 40G
• Supports both single mode and multimode depending on our requirements of distance

Console vs Management:

Console	Management
No ip address	Has ip address
Needs physical access	Allows remote access using SSH, telnet,..
Serial	Ethernet (RJ-45)
Very slow (128 kbps). But, when running in command line we won’t notice	Fast (1 Gbps bandwidth)
We use rollover cables	We use normal ethernet cables
Blue and grey flat cables are used for console	Yellow flat cables are used for management port

Breakout Cables:

• Each 40G MTP-12 is split into 4x10G LC cables. Thus, it can operate only at 10G
• But, it gives higher port density and takes less space
• Thus, by using only one physical port, we get 4 ports of 10G each

Note:

• Cat6 and Cat7 can support 10G .ie. we can get 10G speeds on normal ethernet cables
• If a transceiver is attached to a wire, only then, it is called cable

Difference between OM3 and OM4

OM stands for optical multi-mode. Both OM3 and OM4 are  50/125 core fiber but they have different internal construction that allows the OM4 fiber to provide the same performance as OM3 but for longer distances. The reason for this is the difference in bandwidth, OM3 has 2500 megahertz bandwidth , OM4 has 4700 megahertz bandwidth. What this translates to is longer transmission distances for the OM4 fiber. OM4 cable has a lower attenuation and operates at a higher modal bandwidth. Both these factors mean that less power is lost in the transmission of the signal and that means the signal can be transmitted further or through more connector pairs (which add to the losses).

Universal transceiver- how does it support different speeds or bandwidth?

https://www.rp-photonics.com/bandwidth.html

Bandwidth is given by:

In SM fibres, the wavelength is varied from 1490 - 1625 nm

In SM fibres, the wavelength is varied from 850 - 1300 nm

Why is SM fibre expensive than MM fibre? Which of these should be used inside a data center?

SM fibre is more expensive as it is made of pure glass.

Inside a datacenter, MM fibres can be used and in between two data centers, we can use SM. This is because, MMF fiber cabling has a shorter distance, thus easier to manufacture, more commonly used and less expensive.