As I progress in my CCNA studies I’ll be posting the notes I write up while reading or watching any CCNA training material. There’s a lot of fundamentals to understand and memorize for the CCNA exam but it also helps to actually know the material so you don’t have to look it up when you’re working on the job.
OSI physical layer defines the standards and protocols used to create the physical network and to send the bits across that network
Routers connect to a channel service unit/data service unit (CSU/DSU)
It is a four-wired cable from the telco that plugs into the CSU/DSU
Router cable and CSU/DSU are owned by customer
Wiring to the CO owned by telco
Demarc is one side where it is telco’s and one side that is customer’s responsibility. Typically, Demarc is where the telco physically terminates set of two twisted pairs inside customer building.
Cable between CSU/DSU and telco CO typically is RJ-48.
If router has built-in CSU/DSU, serial cable is not required. Typically just use RJ-48 cable.
To make point-to-point wan line work, devices need to synchronize their clocks to run at same speed (synchronization).
Device providing clocking, typically csu/dsu, is considered as the data communications equipment (dce).
Device recieving clocking, typically the router, referred to as data terminal equipment (dte).
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HDLC – High-Level Data Link Control – determines if data passed the link w/o any errors; discards frame if errors occurred. Also identifies the type of packet inside the HDLC frame so receiving devices knows the packet type.
HDLC defines framing. Includes an Address field and Protocol Type field in trailer containing a frame check sequence (FCS) field.
HDLC has a 1-byte address field
HDLC performs error detection using FCS in HDLC trailer. If frame has error it discards frame with no error recovery performed by HDLC
PPP very similar to HDLC
Frame Relay networks are multiaccess. More than two devices can attach to the network.
Leased line, called access links, is installed between each router and nearby Frame Relay switch.
Frame Relay examines data frames from routers that are defined by it’s own data-link header and trailer.
Frame Relay header holds address field called DLCI (data-link connection identifier).
WAN switch forwards frame based on the DLCI
Telco equipment can forward one frame to one remote site and other frame to another remote site – which is considered to be a form of packet switching – means that service provider chooses where to send each data packet sent into provider’s network, switching one packet to one device and next packet to another.
Logical path of a frame between each router is called a Virtual Circuit.
Multiple sites can connect to VC
Related posts:
- WAN Link Protocols
- CCNA Notes – IP Addressing and Routing
- Layers of the OSI Model
- Ethernet Cables & Speeds
- TCP/IP Model: Networking Basics
