Network Fundamentals / Ch. 4 - Fundamentals of WANs
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| access link | In Frame Relay, the physical serial link that connects a Frame Relay DTE device, usually a router, to a Frame Relay switch. The access link uses the same physical layer standards as do point-to-point leased lines.
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| back-to-back link | A serial link between two routers, created without CSU/DSUs, by connecting a DTE cable to one router and a DCE cable to the other. Typically used in labs to build serial links without the expense of an actual leased line from the telco.
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| clocking | The process of supplying a signal over a cable, either on a separate pin on a serial cable or as part of the signal transitions in the transmitted signal, so that the receiving device can keep synchronization with the sending device.
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| DTE (Layer 1) | Data terminal equipment. From a Layer 1 perspective, the DTE synchronizes its clock based on the clock sent by the DCE. From a packet-switching perspective, the DTE is the device outside the service provider’s network, typically a router.
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| CSU/DSU | Channel service unit/data service unit. A device that understands the Layer 1 details of serial links installed by a telco and how to use a serial cable to communicate with networking equipment such as routers.
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| DCE (Layer 1) | From a physical layer perspective, the device providing the clocking on a WAN link, typically a CSU/DSU, is the DCE. From a packet-switching perspective, the service provider’s switch, to which a router might connect, is considered the DCE.
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| DS0 | Digital signal level 0. A 64-kbps line or channel of a faster line inside a telco whose origins are to support a single voice call using the original voice (PCM) codecs.
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| DS1 | Digital signal level 1. A 1.544-Mbps line from the telco, with 24 DS0 channels of 64 kbps each, plus an 8-kbps management and framing channel. Also called a T1.
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| Frame Relay | An standard data-link protocol that defines the capabilities to create a frame-switched (packet-switched) service, allowing DTE devices (typically routers) to send data to many other devices using a single physical connection to the Frame Relay service.
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| HDLC | High-Level Data Link Control. A bit-oriented synchronous data link layer protocol developed by the International Organization for Standardization (ISO).
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| leased line | A serial communications circuit between two points, provided by some service provider. The telco charges a monthly fee for the ability to send bits between the two sites, the service is considered to be a leased service.
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| packet switching | A generic reference to network services, typically WAN services, in which the service examines the contents of the transmitted data to make some type of forwarding decision. Contrasts with the term circuit switching, where bits are not interpreted.
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| PPP | Point-to-Point Protocol. A protocol that provides router-to-router and host-to-network connections over synchronous point-to-point and asynchronous point-to-point circuits.
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| serial cable | A type of cable with many different styles of connectors used to connect a router to an external CSU/DSU on a leased-line installation.
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| synchronous | The imposition of time ordering on a bit stream. A device will try to use the same speed as another device on the other end of a serial link. Each device can notice slight variations in the speed on each end and can adjust its speed accordingly.
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| T1 | A line from the telco that allows transmission of data at 1.544 Mbps, with the ability to treat the line as 24 different 64-kbps DS0 channels (plus 8 kbps of overhead).
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| virtual circuit | In packet-switched services like Frame Relay, VC refers to the ability of two DTE devices (typically routers) to send and receive data directly to each other, which supplies the same function as a physical leased line,
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