Leased Lines

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

Trunk Line
Switch
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

Mesh
Leased Line
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 T1s 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 Operation
Wrapped
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

Sites
Lines
5
10
10
45
25
300
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 channels 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)