What is Backbone Cabling?
Backbone cabling is an important component in any structured cabling system for a building or campus. Like all parts of a structured cabling system, the minimum end goal is to serve the needs of the user for at least 3-10 years, so we need to be able to map out both current and future needs in order to build a compliant system.
Before jumping into components let’s first discuss how backbone cabling fits into a broader cabling solution, just so we have some context.
As the name suggests backbone cabling is like the backbone of your network connection, connecting the main ISP entrance point to the various enclosures, buildings, or subsystems that will make up the whole system.
Simple Backbone Cabling Diagram
Notice how individual outlets and work areas are not connected to the backbone? Horizontal cabling is used closer to network endpoints. Backbone cabling is used solely “...to provide interconnections between telecommunications rooms, equipment rooms, and entrance facilities.” (Commercial Building Telecommunications Cabling Standard).
Components
To handle these interconnections you might need:
Topology: Maximum of 3 (5?) Jumps between Users
Cross connects are a source of complexity, so ANSI / TIA / EIA 568-B standard keeps the system as simple as possible by determining a maximum of 2 hierarchical levels of cross connects within the backbone. These are the intermediate and main cross connections. In a system with both of these levels an incoming signal will hop the main, intermediate, and a final horizontal cross connection before being received by the end user. And any two end users within the system will be connected by a maximum of 5 jumps: 1 for the first horizontal connection, then one intermediate, then the main, then another intermediate, and a final horizontal connection.
Now for a trick question.
How many backbone cross connects can a signal jump in a compliant structured cabling system between user and ISP? Only two: The intermediate and then the main. The final hop is always a horizontal cross connect. It is not part of the backbone.
Cabling Requirements: Material and Length
Backbone cabling can use a 100 ohm twisted pair copper cable or a fiber solution. Each has its own maximum length per function, and function is defined by where the cable falls in the topology. them every day. Get their input on which solutions should be on the short list and what are the most important features. After the new solution has been installed, provide training on how to use it most effectively and efficiently. This training could come in many forms:
Material
Copper
Multimode Fiber
Singlemode Fiber
Main to Intermediate (m)
800
2,000
3,000
Main to Horizontal (m)
500
1,700
2,700
Intermediate to Horizontal (m)
300
300
300
Question: What is the longest physical distance of a single-mode backbone? How much longer is this than the largest compliant multimode fiber network?
Keep in mind that the maximum length is also directly related to price.
Multimode fiber is defined by its larger core diameter. At 50 micrometers, this diameter allows for a wider variance of wavelengths that can be produced by low-cost light sources such as LEDs or VCELs (Vertical Cavity Surface-Emitting Lasers). Wavelengths run from 850 nm to 1300 nm wavelengths. However, due to the larger core size, the light refracts against the edges of the cablemore frequently, leading to greater attenuation, or reduction of signal power. You will find some higher end multimode solutions employ longer wavelengths to reduce attenuation by as much as 2/3rds. Again, price rises as the quality of light source increases.
Single-mode fiber has a 9 micrometer diameter, much smaller than multimode fiber. The smaller size significantly reduces attenuation, but it also requires high quality light sources such as laser or laser diodes. These produce wavelengths between 1310 and 1550 nanometers.
Single-mode Vs. Multimode Fiber
Mode
Multimode
Singlemode
Wavelength (nm)
850
1300
1310
1550
Attenuation (dB/km)
3
1
.36
.22
This is why in installations where you are interconnecting buildings on a campus or large office building, the benefits of single-mode fiber can be necessary. Bandwidth is also a factor, as single-mode fiber can transmit larger bandwidths across greater distances, whereas multimode fiber encounters increasing difficulties the larger the bandwidth.
The various capabilities of single-mode versus multimode explain why you typically see singlemode fiber in carrier networks. Multimode is still widely deployed in enterprise and data centers where they will certainly meet the needs of their users for years to come.
