slides of unit-2 adsland x.25

8/3/2019 Slides of Unit-2 ADSLand X.25

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ADSL

Asymmetric Digital Subscriber Line

Lecture slides by M.A.Rasheed


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WAN Technology Operates at 3 layer OSI model as below:

1. PHY

2. Data Link

3. Network

Most of WAN technology are packet-switched

network categorized as Switched Virtual circuitNetwork ( 3-phase, connection oriented)


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WAN technology are

PPP-ADSL (Point-to-point protocol) forADSL

ISDN(PPP-ADSL took place) X.25

Frame Layer

ATM (famous in Malaysia/Singapore) MPLS (multi-protocol label switched)

Sonet as WAN backbone


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Term of packets used at each 3 layer

PHY layer bits

Data link layer frame or PDU

Network layer packet or datagram


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Introduction

ADSL is a form of DSL, a data communications

technology that enables faster data transmission over

copper telephone lines

ADSL is capable of providing up to 50 Mbps, and supports

voice, video and data.

ADSL is the #1 Broadband Choice in the World with over

60% market share

ADSL is now available in every region of the world


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What does ADSL mean

Asymmetric - The data can flow faster in one direction than theother. Data transmission has faster downstream to thesubscriber than upstream

Digital - No type of communication is transferred in an analogmethod. All data is purely digital, and only at the end,modulated to be carried over the line.

Subscriber Line - The data is carried over a single twisted paircopper loop to the subscriber premises


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How does ADSL work

ADSL exploits the unused analogue bandwidth available inthe wires

ADSL works by using a frequency splitter device to split a

traditional voice telephone line into two frequencies

4 25,875 138 1104 KHz

PSTN DownstreamUpstream


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ADSL Modulation

Modulation is the overlaying of information (or the signal)

onto an electronic or optical carrier waveform

There are two competing and incompatible standards for

modulating the ADSL signal:

Carrierless Amplitude Phase (CAP)

Discrete Multi-Tone (DMT)


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Carrierless Amplitude Phase

Carrierless Amplitude Phase (CAP) is an encoding methodthat divides the signals into two distinct bands:

1. The upstream data channel (to the service provider), which is carried

in the band between 25 and 160kHz

1. The downstream data channel (to the user), which is carried in the

band from 200kHz to 1.1MHz .

These channels are widely separated in order to minimize thepossibility of interference between the channels.


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X.25 and Frame relay


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X.25 X.25 is a packet-switching wide area network

developed by ITU-T in 1976.

X.25 defines how a packet-mode terminal can be

connected to a packet network for the exchange ofdata.

X.25 is what is known as subscriber network

interface (SNI) protocol.

It defines how the users DTE communicates withthe network and how packets are sent over thatnetwork using DCEs.


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X.25


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X.25 network is a packet switching network thatused X.25 protocol.

X.25 is a standard packet switching protocol thathas been widely used in WAN.

X.25 is a standard for interface between the hostsystem with the packet switching network inwhich it defines how DTE is connected andcommunicates with packet switching network.

It uses a virtual circuit approach to packetswitching (SVC and PVC) and usesasynchronous (statistical) TDM to multiplexpackets.


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Figure 17-2

X.25 Layers in Relation to the OSI Layers


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X.25 protocol specifies three layers:

i. Physical Layer (X.21)

ii. Frame Layer (LAPB)

iii. Packet Layer (PLP) (Packet Layer Protocol)

X.25 Layers


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X.21 hardware interface


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Frame Relay

Packet-switching with virtual-circuit

technology

Improvement of previous technology X.25

Operate only at the PHY and Data link

layer.


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Frame Relay: Why it is needed?

Higher Data Rate at Lower Cost

Allow Bursty Data

Less Overhead Due to Improved

Transmission Media (compared to prev.

tech X.25)


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Higher Data Rate at Lower Cost

Fig. Frame Relay versus Pure Mesh T-Line Network

To connect all the highspeed LANs, it is better used frame-relay

network rather than T-Line Network which cost a lot of money

and impractical


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a. X.25 traffic

b. Frame Relay traffic

Fi 18 13


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Figure 18-13

Comparing Layers in

Frame Relay and X.25


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Figure 12.3 Frame Relay frame

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