A: WAN Basics

By certskills September 27, 2018 09:05

What happens over an Ethernet WAN link? What Data Link addresses appear in the frame that the routers forward over the WAN? What data link headers exist on the WAN link? What types of cabling must be used? The latest question asks about these kinds of details – answers below the fold.

The Answers:



Background: Ethernet WAN Links

Ethernet WAN links include a connection from each customer site to some service provider device, usually an Ethernet switch, as shown in Figure 2. Because the service provider keeps their gear in a different building than most of their customers, the Ethernet link between the customer and SP runs far longer than the 100-meter maximum for most copper-based Ethernet cabling.

Figure 2: Ethernet WAN Links with Fiber Cabling and the SP PoP

The link does use Ethernet as the Data Link protocol. As a result, the routers connected to the Ethernet WAN act much the same as they would if connected to a simple Ethernet crossover cable in a lab. In particular, they would use their own Ethernet MAC addresses when creating and forwarding frames, as shown in Figure 3.

Figure 3: Data Link Addresses PC1 Towards PC2


Why the Right Answer is Right

The single correct answer mentions that the links use fiber optic cabling. Some Ethernet standards define the use of Unshielded Twisted Pair (UTP) copper cabling, but those have a maximum distance of 100 meters. The link must use fiber optic cabling to meet the longer distance requirements.


Why the Wrong Answers are Wrong

One answer mentions the use of UTP copper cabling, which would be unrealistic. In theory, the customer might be within 100 meters of the Service Provider’s device in their PoP, but such a case would be rare.

One answer mentions the use of headers that include parts of the HDLC and Ethernet header; Ethernet WAN links do not use HDLC physical or data link standards at all.

Finally, one answer mentions the use of Ethernet MAC addresses, which is true. However, it mentions the PCs’ MAC addresses, used on the frame sent between the routers over the WAN. On that link, as shown earlier in Figure 3, the frame includes the two routers’ MAC addresses instead.

Q: WAN Basics
Q: TCP/IP Model Lingo
By certskills September 27, 2018 09:05
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  1. Anthony July 11, 19:46

    This answer explanation for “D) The WAN link uses a header that combines HDLC and Ethernet” is making me curious about the physical links in HDLC.

    As I understand it HDLC and Ethernet are both Layer 2 protocols. So what kind of physical link does a WAN using HDLC use? Would it not be a Ethernet cable? And if it is an Ethernet cable, how do engineers decide whether to use Ethernet protocol or HDLC protocol?

    Reply to this comment
    • certskills Author July 12, 09:50

      The links on which you use HDLC are not ethernet cables aka not Ethernet at layer 1. They follow some different standards, but generally fall into what we call time division multiplexing (TDM). In the most recent 10-15 years, the router has a card that does a function called CSU/DSU, and the cable has an RJ-48 connector (same size as RJ-45), which connects to a box installed by the Telco. The lines inside the Telco, well, it’s a ton more complexity. If you’re heard of T1, T1, DS0, DS1, DS3, and so on – that is, the digital point-to-point WAN circuits introduced in the 1960’s, popular in the 1970s- 1990s, waning in the 2000’s, almost not used in the 2010s, those are the ones.
      Hope this helps…

      Reply to this comment
  2. Hoang February 18, 05:19

    You mentioned in your book that leased line uses either HDLC or P2P protocols. But why doesn’t leased line use Ethernet layer 2 protocol because leased line consists of multiple switches connected together installed in different buildings. I think that switches would use layer 2 ethernet protocols?

    Reply to this comment
    • certskills Author February 28, 16:06

      In the figure in the book that shows a serial link, with the switches owned by the telephone company, those are not Ethernet switches. They use other networking standards to create serial links. Those standards create a layer 1 service, that is, the delivery of bits from one end to the other. The devices on the end of the link can then choose among a small set of data link protocols that were created to work on serial links, including HDLC and PPP. Those Wide Area Network (WAN) switches use Time Division Multiplexing (TDM) to create various layer 1 services that run at speeds like T1 (1.544 Mbps) and T3 (around 44Mbps.)
      So… no Ethernet data link on those serial links. Hope this helps.

      Reply to this comment
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