ICOM 6115©Manuel Rodriguez-Martinez ICOM 6115 – Computer Networks and the WWW Isidoro Couvertier,...

ICOM 6115 ©Manuel Rodriguez-Martinez

ICOM 6115 – Computer Networksand the WWW

Isidoro Couvertier, Ph.D.

Lecture 11


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


The Problem

SwitchingElement

How to pass traffics from n slower lines into a higher bandwidth line?


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


Time Division Multiplexing

SwitchingElement

Slower linksPackets

Faster Link Packets


Multiplexing and De-multiplexing

SwitchingElement

SwitchingElement


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.


Example of T1 Carrier

- Control bit is for synchronization of frames- Successive frames should alternate the bit value- Synchronization pattern 01010101

Bit Value


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


Multiplexing on T1, T2, T3 and T4

• The idea is to maximize usage of high speed links


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


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


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


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


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


SONET STS-1(OC-1) Frame Format

9 rows

80 columns

Overhead Payload


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


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


SONET Hierarchy


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)


Switching on the Phone Lines

CircuitSwitching

PacketSwitching

ICOM 6115©Manuel Rodriguez-Martinez ICOM 6115 – Computer Networks and the WWW Isidoro Couvertier,...

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