Leased Lines Leased Lines are Circuits From Chapter

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 Switch Line 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 – DS 0 signaling • T 1 (1. 544 Mbps) digital leased lines – 24 times effective capacity of 56 kbps – Only about 3 -5 times cost of 56 kbps – DS 1 signaling • Fractional T 1 – Fraction of T 1’s speed and price – Often 128, 256, 384 kbps

Leased Line Speeds • T 3: is the next step – 44. 7 Mbps in U. S. • Europe has E Series – E 1: 2. 048 Mbps – E 3: 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 T 1) on a single twisted pair – HDSL 2: 1. 544 Mbps (T 1) 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 (HDSL 2)

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 5 10 25 Lines 10 45 300

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

T 1 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

T 1 Leased Lines • T 1 lines are designed to multiplex 24 voice channels of 64 kbps each • T 1 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

T 1 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)