Chapter 4 Flash Cards - WANs
Explain the Bit Rate for a DS0
64 kbps
Explain the Bit Rate for a DS1 (T1)
1.544 Mbps (24 DS0s, plus 8 kbps overhead)
Explain the Bit Rate for a DS3 (T3)
44.736 Mbps (28 DS1s, plus management overhead)
Explain the Bit Rate for a E1
2.048 Mbps (32 DS0s)
Explain the Bit Rate for a E3
34.368 Mbps (16 E1s, plus management overhead)
Explain the Bit Rate for a J1 (Y1)
2.048 Mbps (32 DS0s; Japanese Standard)
Describe an HDLC frame.
1 byte - Flag 1 byte - Address 1 byte - Control Data 2 Byte - FCS
Describe the Proprietary Cisco HDLC Frame.
1 byte - Flag 1 byte - Address 1 byte - Control 2 byte - Type Data 2 Byte - FCS
Define Synchronous
The imposition of time ordering on a bit stream. Practically, a device tries to use the same speed as another device on the other end of a serial link. However, by examining transitions between voltage states on the link, the device can notice slight variations in the speed on each end and can adjust its speed accordingly.
Define Clock Source
The device to which the other devices on the link adjust their speed when using synchronous links.
Define CSU/DSU
Channel Service Unit/Data Service Unit. Used on digital links as an interface to the telephone company in the United States. Routers typically use a short cable from a serial interface to a CSU/DSU, which is attached to the line from the telco with a similar configuration at the other router on the other end of the link.
Define Telco
Telephone Company
Define Four-wire circuit
A line from the telco with four wires, composed of two twisted-pair wires. Each pair is used to send in one direction, so a four-wire circuit allows full-duplex communication.
Define T1
A line from the telco that allows transmission of data a 1.544 Mbps
Define E1
Similar to a T1, but used in Europe. It uses a rate of 2.048 Mbps and 32 64-kbps channels.
List some synonyms for a point to point leased line.
leased line, leased circuit, link, serial link, serial line, point-to-point link, circuit
Define LAPF
Link Access Procedure - Frame. Defines the Frame Relay header and trailer.
Define 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.
Define 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.
Define 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.
Define 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.
Define 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.
Define DCE (Layer 1)
Data Communications Equipment. 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.
Define 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.
Define 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
Define Frame Relay
An international 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.
Define HDLC
High-Level Data Link Control. A bit-oriented synchonous data link layer protocol developed by the International Organization for Standardization (ISO).
Define Leased Line
A serial communications circuit between two points, provided by some service provider, typically a telephone company (telco). Because the telco does not sell a physical cable between the two endpoints, instead charging a monthly fee for the ability to send bits between the two sites, the service is considered to be a leased service.
Define 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. This term is mainly used to contrast with the WAN term circuit switching, in which the provider sets up a (Layer 1) circuit between two devices, and the provider makes no attempt to interpret the meaning of the bits.
Define 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.
Define 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.
Define 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 (leased circuit), but doing so without a physical circuit. This term is meant as a contrast with a leased line or leased circuit.
