1. 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 Switch Trunk Line Leased Line

2. 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

3. Leased Line Meshes If you have several sites, you need a mesh of leased lines among sites Leased Line Mesh

4. 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

5. 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

6. 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

7. 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 OperationWrapped

8. 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

9. 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)

10. 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 SitesLines 510 45 25300

11. 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

12. 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

13. 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

14. 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)