Chapter 3 – Fundamentals of WANs – LinuxWheel

Leased line WANs, a type of WAN link that has been part of enterprise networks since 1960s
How Ethernet can be used to create WAN services by taking advantage of the longer cable length possibilities of modern fiber-optic Ethernet standards
Common WAN technology used to access the Internet
WAN is something given by a provider.
Usually, we would contact ISP and ask for WAN connection and choose which technology to use (MPLS, DSL, VPN, PPP…)

Leased Line WANs

Leased line WANs do forwarding data between two routers.
Business branches communicate (LANs) by using services and technologies of WANs and have to communicate over WANs.
WAN technologies include a large number of options for physical links and data-link protocols that control those links.

You don’t share bandwidth as with the broadband
Upload and download speeds are the same
Uses analog circuits
Uses the same path for a destination
Charges are fixed, whether use lease line or not, you pay the same
Permanent connection for the destination
Not scalable
ISPs put their equipment in buildings called central offices (CO)
ISP installs cables from the CO to almost every other building in the city, expecting to sell services there, one day.
Each site has customer premises equipment (CPE) which includes a router, serial interface card, and CSU/DSU.
Serial interface card on a router, acts as a NIC on a PC, sending and receiving data over a physical link.
Speeds are fixed (64kbps to 1.5Mbps)
Because leased lines are layer 1 service, people have come to an idea to control data-link details of leased lines. Therefore, they created HDLC and PPP protocols.

Data-link protocols perform a similar role: to control the correct delivery of data over a physical link!
For example, Ethernet data-link protocol uses the destination address field to identify the correct device that should receive data.
HDLC has less work to do because of the simple Point-to-Point topology of a point-to-point leased line.
When one router sends HDLC frame, it can go only on the other side of the leased line, to the other router.

At first, Ethernet was only appropriate for LANs
Restrictions on cable lengths and devices might allow a LAN that stretched a kilometer or two, to support campus LAN, and that was the limit.
As time passed, IEEE improved Ethernet standards.
Now, Ethernet is reasonable WAN technology.
For example, 1000BASE-LX supports 5km cable length, 1000BASE-ZX supports up to 70km cable length.
In the 21st century, many ISPs offer WAN services using Ethernet.
Different WAN technologies exist using Ethernet, but at the core, they all use the same technology (packets and Ethernet)

The picture explains how ISP create leased lines nowadays.
Customer connects to Ethernet link using router interface
Fiber Ethernet link leaves customer building and connects to nearby ISP location called a point of presence (PoP)
Inside ISP’s network, ISP uses any technology it wants to create Ethernet WAN services.
It is important to say that it uses Ethernet standards, packet-switched networks, and Ethernet router and switches together with IEEE Ethernet physical mediums.

Ethernet WAN service goes by two names: ethernet emulation and Ethernet over MPLS (EoMPLS).
Ethernet emulation means that the service acts like one Ethernet link
EoMPLS refers to Multiprotocol Label Switching MPLS. MPLS is technology used inside ISP’s cloud.

Voice cabling concept, without DSL
DSL on the other hand splits the line.
Voice switch at Telco CO forwards voice through the worldwide voice network, called Public Switched Telephone Network PSTN

A new DSL modem now connects to a phone outlet instead of a telephone in image before.
DSL modem follows physical and data-link standards to send data to/from the telco.
This is now a small LAN network, with SOHO router (switch, router, AP)
In order for router to send data to/from the Internet, ISP uses DSL access multiplexer (DSLAM).
DSLAM splits out the data over to the router at the lower right side of the image, which completes the connection to the Internet.
DSLAM also splits out voice data and sends them over PSTN

Has many similarities with DSL.
Cable Internet takes full advantage of existing cabling, using the existing cable TV to send data.
Uses asymmetric speeds, sending data faster (download), than upload speeds
Uses coaxial cable from CATV company
A DSL modem is replaced with a cable modem.
Difference in CATV side, where here we don’t have DSLAM, but we have a splitter that splits Ethernet data, and video data through a satellite dish.