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In Packet Tracer this is how we draw a WAN link, or a T1 link:

enter image description here

Does the "red line" here literally means there is a single cable connecting the two routers in real life?

If no, then what are the exact devices and their topology in a T1 link?

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No. There may be a dedicated circuit between the routers and the nearest telco central office, but between offices they are switched and multiplexed onto other, higher capacity circuits.

Today, most T1 circuits are emulated over a packet-switched (IP) network. They are rapidly becoming obsolete, and are being replaced by SIP over the IP network.

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  • "most T1 circuits are emulated over a packet-switched (IP) network" Really? Don't they have super low latency and jitter? I'd surprised if that would work over IP. – AndreKR Apr 15 at 20:37
  • Voice works just fine over IP – Ron Trunk Apr 15 at 22:44
  • Not necessarily. The transport network simply needs to be sufficiently fast and reliable to get data to the endpoint at a rate that doesn't result in "frame slips" -- i.e. the contents of a timeslot arriving too late. Let's just say circuit emulation is voodoo. – Ricky Apr 16 at 0:26
  • But why does T1 need to be emulated? And if it is emulated over IP network, then what layer 1 & layer 2 is used? When we say "IP network", do we mean "IP over Ethernet"? If yes, then when we say "T1 emulated over IP", do we mean "T1 over IP over Ethernet"? – Noob_Guy Apr 17 at 6:36
  • Because all circuit switched equipment has been replaced with packet switched equipment. It’s more efficient and flexible. Ethernet is becoming the standard, but there are others that are fading into history. – Ron Trunk Apr 17 at 13:09
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Literally? No. Unless the devices were very close, it's very unlikely they would ever have been directly connected. Even 20-30 years ago, in the era of T1's, there were repeaters, digital cross-connects, and multiplexing into larger T-carriers.

As Ron has said, today the T1 is a relic. It will be emulated and carried as packets like every thing else "internet". In fact, the 4-wire T1 interface is all but extinct. Every T1 I've seen for decades is physically an HDSL circuit -- single pair to the smartjack where it becomes the 4-wire T1. At the CO, it's packetized and routed to the destination end, where the process is reversed -- fully (back to a 4-wire T1, like the two router example pictured), or partially (channelized high bitrate interface [T3, OC3, etc.])

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