ICOM 6115 – Computer Networks
and the WWW
Isidoro Couvertier, Ph.D.
Lecture 11
ICOM 6115
©Manuel Rodriguez-Martinez
Lecture Objectives
• Understand the properties of telephone
technologies used to implement the
physical layer
• Major technologies
– Modems (discussed in previous class)
– T1, T2, T3 and T4
– SONET
– ADSL
– Wireless Local Loops
ICOM 6115
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The Problem
Switching
Element
How to pass traffics from n slower lines into
a higher bandwidth line?
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Time division multiplexing
• Allows each slower line to put a piece of
data into higher speed link.
– Piece could be
• 1 one byte (T1 carrier)
• 1 bit (T2 carrier)
• Time using the high speed link is shared
• Frames on High speed link carry parts of
frames from slower links
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Time Division Multiplexing
Slower links
Packets
Faster Link Packets
Switching
Element
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Multiplexing and De-multiplexing
Switching
Element
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Switching
Element
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T1 Carrier
• Multiplex 24 64Kbps voice channels
– Also called DS1
• This is the first digital link on the phone system
– Codec –switching element that maps analog to digital
and vice-versa
• Bandwidth: 1.544 Mbps
– Each channel puts 8 bytes into frame
– Frame has size 193 bits
• 192 bits of data (24 channels x 8 bits)
• 1 bit for synchronization (alternates between 0 and 1)
– 1 frame is sent every 125 usec.
ICOM 6115
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Example of T1 Carrier
- Control bit is for synchronization of frames
- Successive frames should alternate the bit value
- Synchronization pattern 01010101
Bit Value
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Let’s carry the idea of multiplexing
• T2 carrier – Bandwidth of 6.312Mbps
– Multiplex 4 T1 links
– Multiplexing bits rather than bytes
• T3 carrier – Bandwidth of 44.736 Mbps
– Multiplex 7 T2 links
– Multiplexing bits
• T4 carrier – Bandwidth of 274.176 Mbps
– Multiplex 6 T3 links
– Multiplexing bits
• Most people lease T1 and T3 lines
– phone companies use T2 and T4 internally
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Multiplexing on T1, T2, T3 and T4
• The idea is to maximize usage of high
speed links
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Problem: How to standardize?
• T1, T2, etc. are used in North America and
Japan
• Europe and rest of the world used other
standards for multiplexing digital lines
• How can long distance carriers exchange
data and voice?
• Solution: Make up a new standard
– makes everyone more or less happy
– Not perfect but get everyone on board
ICOM 6115
©Manuel Rodriguez-Martinez
SONET/SDH
• Synchronous Optical NETwork
– Developed by Bellcore
• Synchronous Digital Hierarchy
– European amendments to SONET
• Standard for how phone companies
exchange data and voice on digital lines
– Long distance trunks use SONET
– T1, T2, …, T3 mainly for regional links
– Traffic = data or voice moved over the links
ICOM 6115
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Design Goals for SONET
• Interoperability
– Different carries (e.g. Sprint and AT&T) should be
able to exchange traffic
• Backward compatible
– Accept data from T1, …, T4 and from European
standards
• Support Multiplexing of Digital Links
– Must accommodate hierarchies of high speed links
• Built-in support for maintenance
– Piggyback maintenance data along with regular traffic
ICOM 6115
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What Synchronous means?
• Switching elements must be synchronized
to emit/receive frames
– Called Clock-based framing
• Need a master clock to which every other
switch synchronizes
• Every 125usec a SONET frame is sent
– It might be full of data
– It might be 50% filled with data
– It might be 0% filled with data
• Just padding
ICOM 6115
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SONET Frames
• First link in the hierarchy is STS-1 (OC-1)
– Synchronous Transport Signal 1 (Electrical carrier)
– OC denotes the optical carrier
– Bandwidth is 51.84Mbps
• Each frame can hold up to 810 bytes
• Logically it is viewed as a table
– 9 rows of 90 bytes (1 column is 1 byte)
– First 3 bytes in each row are management signals
• Begin of frame, begin of data, etc.
– First 2 bytes in the frame have a bit pattern indicating
begin of frame
ICOM 6115
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SONET STS-1(OC-1) Frame Format
Overhead
Payload
9 rows
80 columns
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Multiplexing of SONET
• STS-3 (OC-3) – Bandwidth – 155.52Mbps
– Multiplexes 3 OC-1 lines
– Frame is 810 x 3 = 2430 bytes long
• STS-9 (OC-9) - Bandwidth – 566.56Mbps
– Multiplexes 9 OC-1 lines
• STS-N (OC-N)
– Multiplexes N OC-1 lines
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STS, OC and SDH
• STS – denotes the electrical signal used
by the switching elements
• OC – denotes the actual optical carrier
moved the fibers
• SDH – hierarchy from the Europeans
– Their SDH-1 is equivalent to a OC-3 line
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SONET Hierarchy
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Switching on the Phone Lines
• Circuit Switching
– Dynamically establishes a physical path between
sender and receiver
• Must allocate lines at switches along path
• Message Switching
– Dynamically moves variable-sized blocks of data
between sender and receiver
• Virtual Circuit
• Packet Switching
– Dynamically move size-bound blocks of data between
sender and receiver
• Virtual Circuit with predictable packet size (enables QoS)
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Switching on the Phone Lines
Circuit
Switching
Packet
Switching
ICOM 6115
©Manuel Rodriguez-Martinez