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HDLC (High-level Data Link Control)
Based on IBM's SDLC protocol, HDLC is one of the oldest WAN
protocol frame types. Cisco's version of
HDLC protocol is called Cisco HDLC and it is the default frame
type for serial interfaces on Cisco routers.
Many other WAN protocol frames are based on the HDLC protocol
and frame including:
Protocols Based on HDLC
Protocol WAN Technology
LAPB - Link Access Procedure Balanced X.25
LAPD - Link Access Procedure D Channel ISDN
LAPF - Link Access Procedure Frame Frame Relay
PPP - Point-To-Point Protocol Serial Switched
Cisco HDLC - High-level Data Link Control Cisco Serial
HDLC Frame Fields - (Cisco HDLC adds the protocol field)
FlagHeader
Data FCS FlagAddress Control Protocol
8 bit delimiter
01111110
8 or 16 bits Variable length, 0
or more bits
16 or
32 bits
8 bit delimiter
01111110
Circuit Switched Networks
Circuit switched networks like phone lines PSTN (public switched
telephone network) or POTS (plain old
telephone system) and ISDN (Integrated Services Digital Network)
have a continuous connection or
circuit from one end point to another. The circuit can be
divided into multiple conversations through
multiplexing. Modems provide the ability to convert the traffic
to digital information. Typically circuit
switched networks have slow speeds and a low cost.
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Packet Switched Networks
Packet switched wide area networks (WANs) such as V.35, Frame
Relay, and ATM use network relay
switches to create paths to destination networks. The paths
across the networks are called virtual circuits.
Virtual circuits can be permanent virtual circuits (PVC) or
switched virtual circuits (SVC) that are created
temporarily to send data to destination networks and then are
tore down and erased.
X.25 - is an older outdated technology, a precursor to Frame
Relay and is still used in some countries.
Frame Relay - cost effective to implement in hardware because
multiple circuits can go out of the same
interface. Uses DLCIs to create switched paths to destination
networks. DLCIs are locally significant only,
not unique. Frame Relay is capable of high speeds.
ATM - expensive to implement in hardware, because it uses
multiple interfaces and hardware devices.
Uses cells instead of frames. ATM cells are a fixed size of 53
bytes, 48 of which is for data. ATM is
capable of high speeds, and can delivering video and voice, as
well as data.
Dedicated Line Networks
T1 and E1 - bandwidth is dedicated and therefore is always
available and delivers consistent speeds.
Dedicated lines have expensive installation and month to month
costs. A T1 line is 1.5 Mbps.
Public Internet Networks
DSL
Cable modem
Wireless - WiMAX
