Chapter 3 – Fundamentals of WANs

  • 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.  

Leased Lines: 

  • 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. 

HDLC

  • 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. 

Ethernet as a WAN technology

  • 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. 

WAN over Ethernet

  • 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. 

Digital subscriber line (DSL): 

  • Creates short high speed link WAN between customer and ISP. 
  • To do so, uses same telephone line. 
  • 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 

Changes

  • 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 

Cable Internet

  • 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. 

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