26 - The last mile in WAN networks

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The old telephone network “PSTN” or “POTS” (Plain old telephone service) was all about voice. It used voice switches (centralitas locales, primarias, secundarias…). Now it’s gone. Current WAN networks are all-IP, and the technology used is mainly ethernet (using fiber optics or copper technologies for short-length cases (twinax, twisted pair).

It is not strange to use fiber optics in LAN ethernet networks. Take for example the use of SFP+ adapters for 10G-BASE-LX or media converters.

Remember these topologies:

Other backbone networks can use FDDI or other ring topologies. these are not often used since ring networks require a lot of planning and most WAN networks inherit the old locations used in the old PSTN.

So, let’s assume that the core network is ethernet, the same ethernet that you use in your home network. How do the clients connect to this network? This connection should be fast enough, steady (no shortages, latency or bandwith reduction), guarantee privacy between customers, and be able to travel the distance between:

Since ethernet is not good for long distances, so ISP use other technologies. Mainly:

Well that is now, but things started differently… let’s take a history lesson…

Circuit switching (PSTN) and packet switching (IP networks)

 Legacy dialup modems

Used since the beginning of the Internet era (1989) until the early-mid 2000s. They use the telephony bandwidth (0-4 kHz) to send and receive data instead of voice. No ethernet backbone, just a telephone network used for something it was not intended to.

As they use the same bandwidth, the line was not available for talk when connected to the Internet. Internet access had the cost of a local call, which could be a lot if you stayed connected for longs periods of time. Later, providers offered ”flat rates” (tarifas planas) (unlimited access from 18:00 until 6:00).

This is the sound that the dial-up modems made when connecting to the provider:

IMPORTANT: Maximum speed was 56 kbps (0,056 Mbps). Way too slow.

Oh, do you need access on-the-go? Then plug your 2G mobile phone to your laptop via RS232:

In the late 90s and early 2000s, the phone networks started to update and two new technologies appeared: ISND (RDSI) and DSL.

 DSL

DSL started as a variation of the old PSTN. It multiplexed voice and data in the same medium: a telephone pair. Now, DSL does not imply PSTN. ISPs can provide DSL access to anywhere (rural areas, or simply places where they do not want to install fiber optic)

ADSL (first iteration)
ADSL (first iteration)

The speed is proportional to the bandwidth allocated. It depends of the version: ADSL, ADSL2, VDSL, VDSL2… IMPORTANT: maximum speed: 100 Mbps (download). Typically 30 Mbps (download).

VDSL2
VDSL2

Maximum reach: 4 km.

IMPORTANT:

Transition to all-ip

DSL started as a PSTN technology, but then network backbones changed to all-ip (ethernet). How do ISPs keep offering DSL? Thanks to DSLAM (digital subscriber line access multiplexer).

DSLAM multiplex individual customer DSL connections into a single upstream link. This is:

More info (CCNA)

Even more info (CCNA)

When fibre arrives

Once there is fiber optics installed, ISPs want clients to leave their DSL connections. This is why:

Coaxial cable networks

Interesting fact: some countries as the USA have had coax networks since the 1950s. These networks were initially designed por pay CATV (cable TV). In the late 90s cable providers started transitioning to the DOCSIS system, that included both data and TV in the same cable.

DOCSIS does not support voice. Options offered:

Cable-modem-router Technicolor TC7210 used by "R". Keep in mind that R uses pair for voice, so the VoIP ATA is useless with this ISP
Cable-modem-router Technicolor TC7210 used by "R". Keep in mind that R uses pair for voice, so the VoIP ATA is useless with this ISP

IMPORTANT: Maximum DOCSIS speed: 10 Gbit/s (typically 300 Mbps symetrical1)

More info (CCNA)

HFC

Hybrid fibre-coaxial is a combination of fibre optics and coaxial in the last mile:

Redes de fibra óptica

Las redes de última milla sobre fibra utilizan el estándar GPON (Gigabit-capable Passive Optical Network). La fibra puede llegar hasta el hogar (FTTH), pero en otros países existen más posibilidades. Las redes FTTx pueden ser de varios tipos: FTTB (Fiber To The Bulding), FTTN (Fiber To The Node)… Más info. En España se ha optado por FTTH (Fiber to the Home).

GPON para redes FTTH

La última milla de FTTH tiene dos extremos:

Esquema ejemplo de red:

En la última milla GPON, el medio es compartido. Al igual que en las redes coaxiales, los repartidores de fibra son pasivos. Se encuentran dentro de las CTO (Cajas de Terminales Ópticos) que puedes ver en todas las fachadas. Típicamente son de 1x16 (1 conexión uplink, 16 downlink), aunque si se quiere exprimir el máximo de clientes, se pueden utilizar combinaciones de dos niveles de splitters (1x8 con 1x8) o bien 1x4 con 1x16.

CTO 1x16 de calle (izq y centro). CTO de RITI (derecha)
CTO 1x16 de calle (izq y centro). CTO de RITI (derecha)

CTO suberráneo (se instala en “cámaras de registro”): torpedo. Más info:

Parte lógica.

Voz: Usando VoIP mediante adaptadores ATA integrados en el router.

Más info en Redeszone: Topología de las redes de fibra óptica FTTx

Las longitudes de onda

En la fibra óptica se utilizan distintas lambdas para distintas funciones (igual que en RF se utilizan las frecuencias con multiplexación FDMA):

La tecnología de multplexación usada en fibra se llama WDM (Wavelength División Multiplexing) .

Más info sobre GPON:

Ejemplo de OLT didáctica

El siguiente rack contiene una cabecera GPON didáctica. Lo ensambló la empresa H24H (presentado en Xornadas de Electricidade e Electrónica 2017).

Contenido:

Manual de OLT2406 Series

Explicación en vídeo (activar subtítulos):

Para completar esta cabecera de red, faltaría:

Por mi parte me anda en la cabeza la OLT de Ubiquiti UF-OLT-4. Ubiquiti también vende ONTs: Loco, Nano G.

Ethernet sobre cableado típico de FTTH

Hemos visto GPON para FTTH, que en España utiliza fibra monomodo con conectores SC/APC. ¿Es posible usar ethernet sobre ese mismo tipo de fibra óptica? Por supuesto, se pueden usar convertidores de par trenzado a fibra, o bien transceptores de tipo SFP o SFP+.

En ethernet no hay ninguna norma, se puede usar el conector y el tipo de fibra que se quiera. Normalmente se utilizan a partir de 10 Gbps, o si las distancias exceden 100 m.

More info

Así de malos (o de buenos) son los routers que te da tu operadora (en Xataka)

Coming next, in season 3


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  1. same upload and download speeds ↩︎
  2. Se habla de 10 Gbps simétricos en nuevas OLT. ↩︎