06_Network_Lines.md

Network Lines

Let's learn about network lines

Reference: [Book] Network Introduction for IT Engineers

• To connect to a remote network, you need to use a WAN
  • Various types of network lines and technologies have been used in WAN to connect telecommunications carriers and users, but currently LAN-identical Ethernet is primarily used
• In addition to regular Ethernet, different technologies are used for special purposes such as low-speed line technologies based on voice circuits and high-speed communication between telecommunications carriers, or for subscriber identification/distinction

1. Internet Lines

• The lines connecting to telecommunications carriers for Internet access are called internet lines
• Simply connecting a cable to a telecommunications carrier does not enable connectivity; you must use a line connected to the Internet sold by the telecommunications carrier to access the Internet
• The technology used to connect homes to the Internet is slightly different from the leased line technology described later
  • The structure is not a direct connection between subscriber and telecommunications carrier, but uses shared transmission line technology
    • ex)
      • Although apartment optical LAN appears to be like a dedicated Internet line with direct internal wiring,
        • The line connecting from the apartment to the telecommunications carrier is shared among apartment subscribers
  • Since transmission lines are shared, the speed of general Internet lines is the maximum possible speed and is not guaranteed
    • In other words, speeds may slow down depending on surrounding usage

Types of General Internet Lines

• Optical LAN (Ethernet)
  • Giga ~ 100Mbps
• FTTH
  • Giga ~ 100Mbps
• Coaxial Cable Internet
  • Hundreds ~ Tens of Mbps
• xDSL (ADSL, VDSL, etc.)
  • Tens ~ Single Mbps

• The above Internet access technologies use existing telephone lines or are shared from a certain point onward
• Since not everyone is always using the Internet at maximum capacity, it is common to build shared sections in a way that does not guarantee maximum speed for each user

2. Leased Line

• Services that guarantee bandwidth between the subscriber and telecommunications carrier are generally called leased lines
• The subscriber and telecommunications carrier are connected by a dedicated cable, and inside the carrier, technology like TDM (Time Division Multiplexing) is used to guarantee communication quality as if directly connected
• Non-Internet dedicated leased lines are primarily used for headquarters-branch connections

Leased lines can be classified into low-speed lines based on voice transmission and high-speed lines called Metro Ethernet based on the transmission technology connecting with the subscriber

2-1. Low-speed Lines Based on Voice Transmission Technology

• Low-speed voice transmission technology is divided and used in 64kbps units
• It is a technology that has been used for a long time, developed by bundling small basic units to increase line access speed
• It was typically used when high speeds were not needed or when high reliability was required, but
  • Currently, Ethernet-based optical transmission technology has developed to a reliable level, so usage frequency is declining
    • However, low-speed lines are still often used for VAN (Value Added Network) for transmitting payment approvals such as Clear Text transmission and external connections
• To use low-speed voice transmission technology, a Router capable of converting to remote transmission technology is needed

2-2. Metro Ethernet High-speed Lines

• High-speed connections mostly use optical cable-based Ethernet
• The technology between subscriber and carrier is Ethernet, and inside the carrier, other high-speed communication technologies are used to bundle individual subscribers for communication
  • The reason communication techniques differ between subscriber and carrier internally is that carriers need to distinguish multiple subscribers and transmit subscriber traffic at high speed
  • Additionally, technology is used to unify various subscriber access technologies into a single technology
    • Why?
      • To carry low-speed communication lines, Ethernet, and voice on a single line!

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LLCF (Link Loss Carry Forward)

• LLCF is a feature that detects when one side's link goes down and brings down the opposite side's link as well
• When configuring a leased line with Ethernet, if the carrier does not configure LLCF, even if the leased line goes down at one site, the other site will appear to still have the line alive
  • After line provisioning, you should verify whether LLCF is configured!
• However, low-speed lines have a function to check Layer 2 protocol communication status, so Routers can detect when the other party's link is disconnected, making LLCF configuration unnecessary!

3. Internet Leased Line

• A product that guarantees communication bandwidth for Internet connection lines is called an Internet leased line
  • The subscriber connects to the telecommunications carrier, which then connects to the Internet
• Unlike the access technology used by subscribers in regular homes, Internet leased lines do not compete with other subscribers and guarantee connection quality between the telecommunications carrier and subscriber
• The most common recent Internet leased line technology is Ethernet
  • This technology called Metro Ethernet is based on recent Ethernet technology that can transmit several to tens of kilometers

4. VPN (Virtual Private Network)

• VPN is a network technology that creates the effect of a virtual connection even though it is not physically dedicated
• There are various VPN technologies, and technologies from the subscriber perspective and carrier perspective have developed separately

4-1. Carrier VPN

• Leased lines increase in cost as the connection distance grows
  • In-city leased line connections are not expensive, but cross-city or international leased lines are very expensive
  • Although available bandwidth is guaranteed, since subscribers do not always use 100% of the contracted bandwidth, wasted costs can be significant
• To reduce this cost waste, carriers directly distinguish subscribers using VPN technology to lower costs
  • The most representative VPN technology is MPLS VPN
    • MPLS VPN
      • Although multiple subscribers connect to a single MPLS network, technology to distinguish subscribers is applied so it can be used like a leased line
      • Using this technology, multiple subscribers access and communicate over a single network, sharing public lines and lowering costs
      • Leased line costs are affected by distance and speed, but MPLS VPN lines are primarily affected by speed,
        • So as distance increases, using public network technology like MPLS helps lower costs
      • Connections between headquarters-branches or branch-branch are mostly connected using MPLS VPN technology
        • From the user's perspective, there is nothing technically special to consider, so the same access technology as regular leased lines is used

4-2. Subscriber VPN

• If a general user uses VPN, it is mostly subscriber VPN technology
• Users can build their own virtual private network using regular Internet connections
• Connecting remote areas or overseas via leased lines is very expensive, and MPLS VPN lines are also more expensive than regular Internet connections
  • So VPN over regular Internet connections is used to lower costs

5. DWDM (Dense Wavelength Division Multiplex)

• Wavelength division multiplexing technology was developed to overcome the problem of high cable installation costs and difficulty in management when communicating over long distances

• Before WDM technology, only one communication per optical cable was possible

  • WDM
    • A device that divides the wavelength of the signal (light) used

• Telecommunications carriers needed to distinguish many subscribers and provide high bandwidth communication, so multiple cables had to be installed, and there were difficulties in installing such physical cables

  • With WDM and DWDM technology, different wavelengths of light on a single optical cable create multiple channels while simultaneously transmitting large amounts of data

• DWDM transmission technology uses more channels than conventional WDM

  • This technology is primarily used within telecommunications carriers for long-distance communication, but recently it is also being used for gigabit Internet in regular households