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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 • 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 • Carriers offer leased lines at an attractive price per bit sent to keep high-volume customers Leased Line
Leased Line Meshes • If you have several sites, you need a mesh of leased lines among sites Mesh Leased Line
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 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 • 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 • 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) • 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 • 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 • 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 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 • 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 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 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)