Data Link Layer
Frame Relay switches
Frame Relay switch
Time Division Multiplexing (TDM)
Integrated Services Digital Network (ISDN)
Public Switched Telephone Network (PSTN)
Support for voice and video
Data bearer channels
Switched virtual circuits
Number of subscribers
Committed information rate
Distance from the central office of the provider
Dedicated Layer 2 links
T1 Leased Line
Frame Relay switch
Dedicated switched lines
The purpose of the access layer is to provide very high bandwidth communications between network devices.
Most security screening to prevent unauthorized entry to the network happens at the core layer.
Untrusted external connections are segmented from the rest of the network at all three levels.
The core layer uses high speed equipment to provide policy-based connectivity.
The distribution layer aggregates WAN connections at the edge of the campus.
Normally the CSU/DSU is the designated demarcation point for the service provider but not the customer.
The segment between the demarcation point and the central office is known as the "last mile."
The local loop is the segment between the CSU/DSU and the serial port on a router.
Putting data on the local loop is the responsibility of the DTE.
WANs generally support higher bandwidth than LANs support.
A WAN link typically traverses shorter geographic distances than a LAN link traverses.
A WAN often relies on the services of carriers, such as telephone or cable companies, but a LAN does not.
All WAN implementations generally use the same Layer 2 protocol but there are many accepted LAN Layer 2 protocols in use.
Routing protocols are incompatible with this function.
It restricts the communication sent to voice traffic only.
A telephone must be used to initially start transferring data.
Data cannot be transferred until a circuit has been established.
ATM differs from other WAN protocols in that it uses variably sized packets.
Most WAN protocols use HDLC or a variant of HDLC as a framing mechanism.
The frame header consists of the frame check sequence and cyclic redundancy check.
ISDN differs from Frame Relay, HDLC, and ATM in that it is packet-switched rather than circuit-switched technology.
Packet-switched networks are less susceptible to jitter than circuit-switched networks are.
Packet-switched networks can efficiently use multiple routes inside a service provider network.
Packet-switched networks do not require an expensive permanent connection to each endpoint.
Packet-switched networks usually experience lower latency than circuit-switched networks experience.