CCNA Complete Guide 2nd Edition.pdf - Cisco Learning Home

- DSL is a L1 service. It uses ATM as the L2 protocol for the communication between the DSL
modem (or router) and the ISP router. DSL uses PPP over ATM (PPPoA) protocol to
encapsulate PPP frames in the ATM AAL5 (ATM Adaptation Layer 5) cells. DSL transmit ATM
cells over DSL, instead of SONET. ATM cells are received and processed by the ISP router.
PPP and ATM are both L2 protocols, but they provide different functions in DSL connections.
PPP provides features such as address assignment, authentication, encryption, and compression.
- A DSL modem (or router) uses PPP over Ethernet (PPPoE) when performing bridging function
for a PC and a DSLAM when the PC connects to the DSL network using a PPPoE client.
The WAN IP address is dynamically assigned to the PC instead of the DSL router.
Cable Modems
- This remote access technology does not use a phone line for physical connectivity. It transmits
computer data in digital signals over cable TV coaxial cabling (analog). A splitter called
f-connector, is being used to split data received from drop cable to multiple cable TVs and the
cable modem in a subscriber house.
- Cable Modem Termination System (CMTS) is deployed at various concentration points or
hubs in the cable network to provide high-speed Internet access, voice, and other networking
services to home and business subscribers. The Cisco uBR (Universal Broadband Router) CMTS
Series is designed for deployments at MTUs (multi-tenant units), eg: apartments and hotels.
- The speed degrades as more subscribers are added to the network, as the bandwidth is shared
among subscribers.
- Downstream data referred to as the data going toward the home.
Upstream data referred to as the data going outward the home.
- Cable networks broadcast downstream traffic (logical bus networks). Security is a main concern.
- CATV (Community Antenna Television) standards use Multimedia Cable Network System
(MCNS) as the data link protocol for arbitration, addressing, and error detection.
- As the cables could be very long, the CSMA/CD algorithm used in Ethernet is not effective to
detect collisions in cable networks. MCNS defines some multiplexing methods – Time-Division
Multi Access (TDMA), where home users are allocated time slots per second to send upstream
data, and Frequency-Division Multiplexing (FDM) to receive downstream data.
FDM is a form of signal multiplexing where multiple baseband signals are modulated at different
frequency and bundled as a composite signal that could be transmit over a single physical circuit.
FDM was normally used in traditional analog telephone networks. Modern telephone networks
that employ digital transmission use Time-Division Multiplexing (TDM) instead of FDM.
- Multiplexing is the process of converting multiple logical signals into a single physical signal
for transmission across a physical channel.
- MCNS defines the following encoding schemes (QAM = Quadrature Amplitude Modulation):
i) QAM-64 6 bits per baud.
ii) QAM-256 8 bits per baud.
- MISC Notes: Analog modems and DSL support both symmetric and asymmetric speeds; while
ISDN, Frame Relay, and cable modems can only run across synchronous links or circuits.
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