CCENT Concepts Pt. 3 Flashcards
Bit Rate of DS0
64 kps
Bit Rate of DS1 (T1)
1.544 Mbps (24 DS0s, plus 8 kpbs
overhead)
Bit Rate of DS3 (T3)
44.736 Mbps (28 DS1s, plus
management overhead)
Bit Rate of E1
2.048 Mbps (32 DS0s)
Bit Rate of E3
34.368 Mbps (16 E1s, plus
management overhead)
Bit Rate of J1 (Y1)
2.048 Mbps (32 DS0s; Japanese
standard)
HDLC Framing
Standard HDLC (No Type Field) Flag - 1 Byte Address - 1 Byte Control - 1 Byte Data - Variable FCS - 4 Bytes Proprietary Cisco HDLC (Adds Type Field)
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 variation in the speed on each end and can adjust its
speed accordingly.
Clock source
The device to which the other devices
on the link adjust their speed when
using synchronous links.
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.
Telco
Telephone Company.
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 ciruit allows full-duplex
communications.
T1
A line from the telco that allows
transmission of data at 1.544
Mbps.
E1
Similar to a T1, but used in Europe. It
uses a rate of 2.048 Mbps and 32
64-kbps channels.
Access Link
In Frame Relay, the physical serial link that connects a Frame Relay DTE
device, usually a router, to a Frame Relay switch. This access link uses the
same physical layer standards as do point-to-point leased lines.
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 a actual leased line from the telco.
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.
DCE (Layer 1)
Data communication 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.
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.
HDLC
High-Level Data Link Control. A bitoriented
synchronous data link layer
protocol developed by the International
Organization Standardization (ISO).
Leased Line
A serial communication 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 bit between the two sites, the service is considered
to be a leased service.
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.
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.
