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The Physical Layer May 2005 Barry Irwin


Objectives • Gain understanding of various cabling types • Understand what Layer 1 transmission media are commonly used • Understand Ethernet Wiring


Some Physical Level Basics • All network connections are at some level a circuit • Data is transmitted as 1 or 0 • This is done by varying the voltage present on the transmission media (encoding otherwise for RF based protocols)


Layer 1 Media • Copper – – – – – –

Varying Grades Shielded Unshielded Twisted Straight Single/dual/multi-core

• Fibre – Glass – Plastic

• Wireless (Air)


Common Layer One Communications • • • • • • • •

Ethernet (802.3) ADSL/SDSL ISDN Frame Relay X.25 SONET ATM GSM/TDMA/CDMA


ADSL/SDSL • Asymmetric Digital Subscriber line – Used for ‘last mile’ solution – Uses a spread spectrum transmission – Provides up to 7 Mbit/s transfer – Down is always faster than Up

• Symmetric Digital Subscriber Line – Similar use to ADSL – Provide symmetric communications – Usually limited to 2Mbit/s in both directions


POTS • • • • •

Plain Old Telephone System Uses some plain copper cabling Allows for 4Khz of Bandwidth on the line All communications over this are Analog Digital communications require some kind of Modulation and demodulation – Enter the MO DEM

• Limited in Speed best achievable is 56Kbit/s down and 33.6Kbit/s up


ISDN • A ‘high speed’ solution for running over standard telephone copper • Provides Digital transmission – differing from POTS • Basic Rate (BRI) – Single copper pair carries 2 Data Channels (B) ( 64Kbit/sec each) and a Signalling channel (D) – ISDN device can bond these to form a single 128Kbit/sec virtual connection

• Primary Rate (PRI) – Single copper pair carries 30 Data Channels (B) and a single 64K D Channel


Frame Relay • The frame relay network provides a number of virtual circuits • These form the basis for connections between stations attached to the same frame relay network. (cloud) • The resulting set of interconnected devices forms a private frame relay group, which may be either fully interconnected with a complete mesh of • Frame relay is documented in RFC 2427.


X.25 • Relatively Simple Packet based protocol • Previously used quite extensively by Telco’s • Being replaced by newer technologies


ATM • Designed for high speed ( 155Mbit) communications • Provides low latency and fair handling of multiple streams • Has true provision of QoS • Widely adopted by Telco’s


Wireless Technologies • • • • •

GSM CDMA TDMA 3GPP 802.11

• Use radio frequencies for data transmission • Placement is key • Frequencies may be regulated


Ethernet Family •

ThickNet (10Base5) – First variant of Ethernet – Core cable with stations hanging off – 500 Meters per segment – 10 Mbit/second

ThinNet (10Base2) – – – –

Much cheaper variant Stations connected to bus 185 meters 10Mbit/second

Fiber ( 100BaseFX) – Allowed for Ethernet networks to be propagated over fibre optics – Allowed for much longer distances 2km+


Putting it Together


Ethernet Family • Twisted Pair (10BaseT) – Uses Category 4 and 5 cabling – 10 Mbit/sec – Limited to 100m/cable , 0.5M/patch cable

• Twisted Pair (100BaseT) – Category 5 Cable – using all 8 conductors – 100Mbit/s

• Twisted Pair (1000BaseT) – Gigabit Ethernet – 1000Mbit/sec


Network Cards


Network Cards


Network Cards


Network Cards


Building an Ethernet UTP cable • What you need – – – –

Cable Crimping Tool RJ45 Jacks Boots

• The process – – – – –

Measure & Cut Strip Check Crimp Check


The RJ45 Connector


Standard Ethernet Cable


Crossover Ethernet Cable


Right and Wrong


Coming Up • Building your own network…


How NOT to build a high performance network


How NOT to build a high performance network

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