1 10Base-T Ethernet LAN

2 LANs u Local Area Networks u Limited Geographical Area –Single office –Single building –University campus or industrial park u Generally, high speeds –Now, most operate at around 10 Mbps –100 Mbps is emerging as the new “base speed” u Most Data Traffic is Local

3 Standards Setting u LANs are Subnets (single networks) u Subnet technology is Dominated by OSI Standards (true for LANs) u IEEE Creates most LAN Standards –Institute for Electrical and Electronic Engineers –Submits its standards to ISO and ITU-T for ratification u IEEE 802 Committee –LAN standards are set by the IEEE 802 Standards Committee. –802.3 for Ethernet Standards –802.5 for Token-Ring LAN Standards – for Radio and Infrared Wireless LANs

4 LANs and OSI Architecture u OSI is a 7-layer architecture u LAN transmission only uses Layers 1 and 2 u Layer 1: Physical Layer –Connectors, Media, Electrical signaling u Layer 2: Data Link Layer –Packaging data into frames –Managing transmission over link (error handling, etc.) –Access control: when each station may transmit

5 OSI Physical and Data Link Layers F4-1 Station A Station A Station B Station B Connector Plug Connector Plug Transmission Medium (telephone wire, etc.) Electrical Signal Physical Layer (OSI Layer 1) Physical (plugs, media, etc.); Electrical (voltages, timing, etc.)

6 OSI Physical and Data Link Layers Data Link Layer (OSI Layer 2) Frame 2 Frame 1 Station A Station A Station B Station B

7 Data Link Layer u For point-to-point transmission –A point-to-point connection is a data link –So is a transmission system shared by multiple devices, only one of which can transmit at a time because of collisions Transmission

8 Data Link Layer u First function: Packaging of Data (1s and 0s) –PDU at Data Link Layer is called a frame u Second Function: Access Control –Only one station can transmit at any time –If another transmitted, their signals would scramble one another –Must control access to (transmission into) the transmission medium

9 Data Link Layer Logical Link Control Layer Media Access Control (MAC) Layer Base-T Base-T Base Base Other Physical Layer Other Physical Layer Physical Layer 4 Mbps Physical Layer 4 Mbps Physical Layer 16 Mbps Physical Layer 16 Mbps Other Physical Layer Other Physical Layer OSI Data Link Layer (Layer 2) OSI Physical Layer (Layer 1) OSI Data Link Layer is subdivided into two layers Media access control Logical link control OSI Data Link Layer is subdivided into two layers Media access control Logical link control

10 Media Access Control (MAC) Layer OSI Physical (Layer 1) Media Access Control MAC Layer Base-T Base-T Base Base Other Physical Layer Other Physical Layer Physical Layer 4 Mbps Physical Layer 4 Mbps Physical Layer 16 Mbps Physical Layer 16 Mbps Other Physical Layer Other Physical Layer MAC layer implements media access control: When a station may transmit Controls the framing of data along the wire MAC layer implements media access control: When a station may transmit Controls the framing of data along the wire

11 OSI Physical (Layer 1) LLC Logical Link Control Layer Logical Link Control Layer Media Access Control (MAC Layer) Media Access Control (MAC Layer) MAC 4 Mbps MAC 4 Mbps MAC 16 Mbps MAC 16 Mbps Other MAC Other MAC Base-T Base-T Base Base Other Physical Layer Other Physical Layer Physical Layer 4 Mbps Physical Layer 4 Mbps Physical Layer 16 Mbps Physical Layer 16 Mbps Other Physical Layer Other Physical Layer Provides Control Function Begin/end connections between stations Error correction (optional) Provides Control Function Begin/end connections between stations Error correction (optional)

12 Simple LAN Using Ethernet 10Base-T RJ-45 jacks 10Base-T Hub (Multiport Repeater) Unshielded Twisted Pair (UTP) Wiring (4-Pair Bundle) PC RJ-45 Jack NIC Network Interface Card 10Base-T UTP Wiring Bundles: 4 Pairs EIA Category 3, 4, or 5

13 Ethernet 10Base-T (802.3u) u Physical Layer Standard –10 Mbps (10 in 10Base-T) –Baseband signaling: Injects voltage changes directly into the wires (Base in 10Base-T) u Hubs (Multiport Repeaters) –Connect the stations together 10Base-T Hub

14 NICs u Network Interface Cards –Implement Physical Layer t Plug and Electrical Signaling –Implements the Data Link Layer (data packaging, access control, etc.) t LLC (802.2) t MAC (802.3 MAC)

15 Wiring u Unshielded Twisted Pair (UTP) –Twisted several times per foot to reduce interference, T in 10Base- and Unshielded t No protection except for plastic coating t Distance limitation: 100 meters (attenuation, distortion, noise and interference, crosstalk) -- propagation u Categories of UTP Wiring –Category 5: The best. Good for 100 Mbps –Category 3 and 4: lower. May be OK for 100 Mbps u Wiring Plugs: RJ-45 Standard –Similar to home (RJ-11) jacks, but wider

16 Attenuation u As signal travels, gets weaker –If too weak, cannot tell 1s and 0s Distance

17 Distortion u As signal travels, it become distorted –Changes shape –Successive bits may merge, making reception difficult Distance

18 Noise u Unwanted energy on line. Always present u Noise floor is average level u Noise spikes will cause problems Signal Strength Time Signal Noise Spike Noise Floor

19 Signal to Noise Ratio (SNR) u Ratio of Signal Power to Noise Power –If SNR is high, few noise errors SNR Error Rate 100% 0% 1

20 Interference u Unwanted signal from outside sources –Often intermittent, difficult to diagnose Signal Strength Signal Interference

21 Physical Layer Transmission in 10Base-T F4-4 10Base-T Hub Step 1 Station A Transmits on Upstream Pair (Wires 1&2) Step 1 Station A Transmits on Upstream Pair (Wires 1&2) Station A Station B Station C

22 Physical Layer Transmission in 10Base-T 10Base-T Hub Step 2 Hub Repeats (Broadcasts) The Message To All Stations On Downstream Pairs (Wires 3&6) Step 2 Hub Repeats (Broadcasts) The Message To All Stations On Downstream Pairs (Wires 3&6) Station A Station B Station C Bus transmission means broadcasting

23 Manchester Encoding u Inefficient –Transmits 10 million bits per second –Must change line state 20 million times per second –20 Mbaud (baud is a change in the line state) –Technology limits the baud rate –Only 10 Mbps for 20 Mbaud –Bit rate is less than the baud rate (opposite in modems)

24 MAC Layer CSMA/CD Media Access Control u Controls when stations may transmit –If two transmit at once, signals will be scrambled Collision X Collisions will grow rapidly above 30% of line utilization. Keep traffic moderate, or throughput will be affected.

25 CSMA/CD Media Access Control u CS: Carrier Sense –Each NIC always listens for traffic on the line –This lets it recognize messages sent to its address –This also lets it know if the line is free u CSMA: Carrier Sense Media Access –A station may transmit if it hears no traffic on the network

26 CSMA/CD Media Access Control in Ethernet Station A is Transmitting Station B must wait Station A is Transmitting Station B must wait 10Base-T Hub Station A Station B Station C Must Wait

27 CSMA/CD Media Access Control u CD: Collision Detection –If two stations transmit at once …. –Their signals collide, scrambling one another –Because each sender listens (senses the carrier), both know that there has been a collision –Both stop and wait a random amount of time. X Collision

MAC Layer Frame Preamble Start of Frame Delimiter Destination Address Source Address Length Data Pad Frame Check Sequence In asynchronous transmission, each byte is sent separately, in a 10-bit frame. In MAC Layer frames, transmission is synchronous. Many data bytes are sent in each frame of variable length. Frames must be between 64 octets and 1518 octets long, divided into groups of bits called fields. Field lengths are measured in “octets”, eight bits. “Octet” is a synonym for “byte”

29 Error Detection u MAC Layer Process Provides error detection –Determines that an error exists u Does not provide error correction –Merely discards the frame –No request for retransmission if there is an error u Logical Link Control Layer may do error correction –Receiving LLC process detects discarded frames –Receiving LLC process asks for retransmissions

30 Logical Link Control (LLC) Layer Next Higher Layer (Usually Internet) Next Higher Layer (Usually Internet) LLC Layer Process LLC Layer Process MAC Layer Process MAC Layer Process Station A Next Higher Layer (Usually Internet) Next Higher Layer (Usually Internet) LLC Layer Process LLC Layer Process MAC Layer Process MAC Layer Process Station B LLC Frame MAC Frame can ask for retransmission of lost MAC frames. Optional. LLC frame placed within MAC frame

31 Ethernet 10Base-T Network with Two Hubs Hub 1 Hub 2 Station AStation B Station C Station A Transmits to Hub 1 Hub 1 Transmits Out All Ports, Including the Port to Hub 2 Hub 2 Broadcasts the Message Out All of Its Ports Station C Receives the Message 100 m Segment Maximum 100 m Segment Maximum 100 m Segment Maximum

32 Ethernet 10Base-T LAN with Multiple Hubs Hub 1 Hub 2Hub 3 Hub 4 Station A Station B Station C Station D UTP Wire UTP Wire UTP Wire UTP Wire UTP Wire Daisy chain, no Loops allowed! Daisy chain, no Loops allowed! Maximum distance between farthest Stations is 4 Hubs/5 100 meter segments

33 Speed and Distance u Transmission speed worsens problems –Error rates increase because bit periods are smaller and are more likely to be damaged by brief noise spikes and interference –High speeds create high-frequency components in the signal that attenuate more rapidly than lower-frequency components –In general, as speed increases, maximum distance decreases, although improving technology can lessen the decrease

34 Dealing with Propagation Effects u Use High-Quality, High-Cost Media –Use media designed for long-distance propagation –Optical fiber, coaxial cable –Too expensive for runs to many desktops, good for hub-to-hub u Use Inexpensive Media to the Desktop –Improve the technology (allows 100 Mbps+ on UTP) –Accept distance limitations (100 meters for UTP) –More popular alternative to desktop because of low cost