Last week we gave you a crash course in what Fibre Optics is and how it works. 
This week we’ll explain the different types of Optical Networks along with GPON and EPON. 

Types of Optical Networks:

Active Optical Networks:

Active optical networks operate on a point-to-point architecture and relies on electrically powered network equipment such as routers and switches.  In an active optical network, each subscriber gets a dedicated fibre connection, this means one fibre cable for downstream traffic and another for upstream traffic.

Passive Optical Networks

Passive optical networks, on the other hand, works on a point-to-multipoint architecture where it uses passive, unpowered devices like optical splitters.  An optical splitter basically divides one incoming signal into multiple outgoing signals and subsequently reduces the number of fibre optic cables required in a passive optical network deployment.

Scenario: 

You want to deploy a fibre optic solution to either a newly built complex with 50 units.  When you use an Active Optical Network approach, you will have to provision at least 100 fibre connections to connect all of the units.  In comparison, using a Passive Optical Network approach will result in much fewer fibre cables provisioned as you will use optical splitters, and each customer only gets one fibre cable that handles both upstream and downstream traffic.

Passive Optical Networks can further be divided into two main types, namely GPON and EPON. 

GPON:

Gigabit Passive Optical Networks is a standard that is developed by the ITU-T, the body responsible for all telecommunication standards.  In terms of what we are more familiar with, the G.984 standard has some similarities and some major differences to the standard 802.3ah of Ethernet.

GPON’s provides asymmetrical data rates of 2.5Gbps downstream data and 1.25Gbps upstream data.  The usable, upstream data rate of GPON is 1.25Gbps, but, in terms of cost-efficiency, makes GPON based equipment more expensive than the standard Gigabit Ethernet port-based switches available today.

The most fundamental difference between GPON and EPON is perhaps in their architectural approach.  GPON uses 3, layer 2 networks:  ATM (Asynchronous Transfer Mode) for Voice, Ethernet for Data and a proprietary encapsulation for Video.

With ATM, the cell size is fixed in size, with a 5-byte header and a 48-byte payload.  This adds to the communication costs on both the OLT (found in the service provider’s central office) and the ONT (found at the subscribers’ premises).

GPON, having 3x layer 2 networks, also requires 3 management systems for the layer 2 protocols.  Services such as IP video are very bandwidth intensive on a GPON solution and add to the total cost of ownership to the service provider and the subscriber.

In short, GPON solutions provide higher data rates when compared to EPON, but the encapsulation for a GPON solution is complicated (in terms of data and the equipment) and far more expensive than an EPON solution.

EPON:

Ethernet Passive Optical Networks in comparison is based on the IEEE 802.3ah standard.  The IEEE is the body responsible for the development of all Ethernet, Wireless and Wi-Fi-based standards that we all are more familiar with.

EPON provides symmetrical data rates of 1.25Gbps (both upstream and downstream).  The actual usable bandwidth is 1Gbps, with a 250Mbps used for bandwidth encoding.  This makes the actual data rate of EPON the same as the rate of a gigabit Ethernet switch port, which is more cost-efficient in terms of equipment pricing.

EPON uses a single layer 2 networks for video, voice and data. It also uses variable-length packets of up to 1518 bytes in size that significantly reduces the overhead costs relative to the ATM cells used in GPON.

Because EPON has a single layer 2 networks it only requires one management system, again resulting in a lower cost of ownership to the service provider and to the subscriber.

Both EPON and GPON support AES based encryption which is applied both upstream and downstream.

Overall, EPON’s ease of use, installation, maintenance and cost efficiency (in terms of equipment pricing) makes it the best option for your FTTx network.

Bandwidth allocation in an EPON solution:

Downstream bandwidth allocation – data is broadcast downstream from the OLT (device found at the service providers central office) to all ONUs (devices found inside or near the subscriber’s premises) in variable-length packets.

Each packet carries a header uniquely identifies it as data intended for the specific ONU.

Upstream bandwidth allocation is managed, utilising Time Division Multiplexing.  TDM is a method of combining multiple signals into one single, signal by providing a dedicated time slot to each ONU.  These time slots are synchronised so that downstream and upstream packets from the ONU’s do not directly interfere with each other.

Don’t miss next week’s BDCOM 101 blog post when we explain the various components needed to create your own EPON network!