Leased lines
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Leased Lines. Leased Lines are Circuits (From Chapter 1) Often goes through multiple switches and trunk lines Looks to user like a simple direct link. Trunk Line. Switch. Leased Line. Leased Lines. Leased lines Limited to point-to-point communication Limits who you can talk to

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Leased Lines

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Leased lines

Leased Lines

  • Leased Lines are Circuits (From Chapter 1)

    • Often goes through multiple switches and trunk lines

    • Looks to user like a simple direct link

Trunk

Line

Switch

Leased

Line


Leased lines1

Leased Lines

  • Leased lines

    • Limited to point-to-point communication

      • Limits who you can talk to

    • Carriers offer leased lines at an attractive price per bit sent to keep high-volume customers

Leased Line


Leased line meshes

Leased Line Meshes

  • If you have several sites, you need a mesh of leased lines among sites

Mesh

Leased Line


Leased line speeds

Leased Line Speeds

  • Largest Demand is 56 kbps to a few Mbps

  • 56 kbps (sometimes 64 kbps) digital leased lines

    • DS0 signaling

  • T1 (1.544 Mbps) digital leased lines

    • 24 times effective capacity of 56 kbps

    • Only about 3-5 times cost of 56 kbps

    • DS1 signaling

  • Fractional T1

    • Fraction of T1’s speed and price

    • Often 128, 256, 384 kbps


Leased line speeds1

Leased Line Speeds

  • T3: is the next step

    • 44.7 Mbps in U.S.

  • Europe has E Series

    • E1: 2.048 Mbps

    • E3: 34 Mbps

  • SONET/SDH lines offer very high speeds

    • 156 Mbps, 622 Mbps, 2.5 Gbps, 10 Gbps


Sonet sdh

SONET/SDH

  • Created as Trunk Lines for Internal Carrier Traffic

    • As were other leased lines

  • The Trunk Line Breakage Problem

    • Problem: unrelated construction products often break carrier trunk lines, producing service disruptions

    • The most common cause of disruptions

X


Sonet sdh uses a dual ring

SONET/SDH Uses a Dual Ring

  • Normally, Traffic Travels in One Direction on One Ring

  • If Trunk Line Breakage, Ring is Wrapped; Still a Ring, So Service Continues

Switch

Normal Operation

Wrapped


Digital subscriber lines dsls

Digital Subscriber Lines (DSLs)

  • Saw DSLs in Chapter 5

  • Can Use Instead of Traditional Leased Lines

    • Less expensive

  • HDSL (High-Speed DSL)

    • Symmetrical: Same speed in each direction

    • HDSL: 768 kbps (Half a T1) on a single twisted pair

    • HDSL2: 1.544 Mbps (T1) on a single twisted pair


Digital subscriber line

Digital Subscriber Line

  • Normal Leased Lines Used Data Grade Wires

    • High-quality, high-cost

    • Two pairs (one in each direction)

  • DSLs Normally Use Voice Grade Copper

    • Not designed for high-speed data

    • So sometimes works poorly

    • Usually one pair (ADSL, HDSL)

    • Sometimes two pairs (HDSL2)


Problems of leased lines

Problems of Leased Lines

  • With many sites, meshes are expensive and difficult to manage

  • With N sites, N*(N-1)/2 leased lines for a mesh

    • May not need all links, but usually use many

Sites

Lines

5

10

10

45

25

300


Problems of leased lines1

Problems of Leased Lines

  • User firm must handle switching and ongoing management

    • Expensive because this requires planning and the hiring, training, and retention of a WAN staff


T1 leased lines

T1 Leased Lines

  • Voice Requirements

    • Analog voice signal is encoded as a 64 kbps data stream (see Chapter 5)

    • 8 bits per sample

    • 8,000 samples per second


T1 leased lines1

T1 Leased Lines

  • T1 lines are designed to multiplex 24 voice channels of 64 kbps each

  • T1 lines use time division multiplexing (TDM)

    • Time is divided into 8,000 frames per second

      • One frame for each sampling period

    • Each frame is divided into 24 8-bit slots

      • One for each channel’s sample in that time period

      • (24 x 8) 192 bits

      • Plus one framing bit for 193 bits per frame


T1 leased lines2

T1 Leased Lines

  • Speed Calculation

    • 193 bits per frame

    • 8,000 frames per second

    • 1.544 Mbps

  • Framing Bit

    • One per frame

    • 8,000 per second

    • Used to carry supervisory information (in groups of 12 or 24 framing bits)


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