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
– 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
5
10
Lines
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)