Broadband: copper to fibre

Copper to fibre

Talk of fibre to the premises (FTTP) started in the 1980s. Although FTTP became common place for leased lines and ISDN30 services in businesses, despite a few trials focused on cable TV services it wasn't until the 2010s, with demand for internet bandwidth surging, that FTTP started to take off for residential customers.

Technologies such as Asymmetric Digital Subscriber Line (ADSL) and its derivative Very high speed Digital Subscriber Line (VDSL) were able to provide speeds of up to a few tens of megabits – speeds which had previously been thought unobtainable over copper phone lines. ADSL started to be rolled out in the early 2000s, following trials in the late 90s, and VDSL followed from the end of the 2000s.

You can see this story, from the Strowger years through to the fibre being laid today, in our Flowers to Fibre exhibition.

The access network

The access network is the part of the telephone network between the local exchange and the customer. It is sometimes called the local loop, or the last mile. For most of the twentieth century it was made entirely of copper: every customer had their own pair of copper wires running all the way back to the exchange.

In the exchange, each line arrives on the main distribution frame (MDF), where it is cross-connected to the switching equipment. Outside cables arrive on one side of the frame, referred to as the Line side, and lines are connected to cables leading to the switching equipment on the opposite (Exchange Side) of the frame.

From the Line side of the MDF, large cables, carrying hundreds of pairs, run underground through ducts to the green roadside cabinets known as primary cross-connection points (PCPs). A PCP cabinet typically serves around 1000 customers although in rural areas smaller ones serve around 300.

At the cabinet, the cables arriving from the exchange are referred to as exchange-side (E-side) cables, the pairs are cross-connected again onto smaller distribution-side (D-side) cables, which run along each street to a distribution point (DP). The DP is usually at the top of a telegraph pole, or in an underground joint box. From there a single-pair drop wire runs to each home, ending at the master socket. A DP typically serves 10-20 customers

This structure still matters today. When broadband arrived it had to work over these same copper pairs, and the green cabinets later became the place where fibre met copper.

xDSL

ADSL and VDSL work by utilising higher frequencies than the analogue phone system, allowing both the analogue phone and data to use the line at the same time. The range of frequencies available on any line is determined by its length – the shorter the line, the higher the maximum frequency signal it can sustain.

ADSL uses equipment in the exchange called a DSLAM (DSL Access Multiplexer) to provide service. In the UK this was typically an Alcatel 7300 or a Fujitsu FDX.

ADSL can provide speeds up to 8Mbits/sec downstream, dependent on line length. ADSL2+ can provide speeds up to 24Mbits/sec.

VDSL uses higher frequencies to achieve higher speeds. It can do this by moving the DSLAM from the exchange building into the roadside cabinet.

In the UK, VDSL DSLAMs are typically Huawei or ECI.

G.fast is an evolution of VDSL technology which pushed copper further still, using even higher frequencies over even shorter distances. This means its deployment costs are much higher as the DSLAM needs to be situated very close to the customer. In the UK Openreach deployed G.fast at a small number of locations before halting rollout in favour of FTTP

FTTP

There are two approaches to Fibre to the Premesis (FTTP). One uses a dedicated fibre strand, or multiple strands, from the exchange to each customer; this is called point to point. The alternative is a passive optical network (PON). PON uses a single fibre strand from the exchange to a device called a splitter. From the splitter, separate fibres lead to each customer.

Point to point is used for higher end services like leased lines, and by some suppliers for residential broadband services. However, most suppliers, including Openreach, use PON for residential broadband services.

The splitter is entirely passive – it needs no power and simply divides the light from one fibre between many. That means it can be placed out in the street, close to the homes it serves, often in a small enclosure mounted on a telegraph pole.

GPON and beyond

The current most popular system is Gigabit PON (GPON), which provides 2.4Gbits/sec downstream and 1.2Gbits/sec upstream, shared between the customers on a splitter. Typical split ratios are 1:16 or 1:32, however the GPON specification allows a split of up to 1:128.

XGPON uses 10Gbits/sec downstream and 2.4Gbits/sec upstream, however it is more common to find XGSPON, which is symmetric – that is, 10Gbits/sec in both directions.

Speeds are shared between all the customers on a splitter.

In the exchange: the ODF and OLT

Fibres coming in from the street arrive at an optical distribution frame (ODF). Here each fibre is joined to a patch lead that connects it to the equipment, much as copper lines were cross-connected on the main distribution frame.

From the ODF the fibres run to a device called an Optical Line Terminal (OLT), which terminates the fibre lines and switches the traffic onto uplinks towards the internet.

An OLT typically takes 16 line cards, each one serving 16 lines. With a 1:32 split arrangement that means one OLT can serve 8,192 customers.