Datorntverk A lektion 10 Kapitel 13 Multiple access

Datornätverk A – lektion 10 Kapitel 13: Multiple access control. Local Are Networks. (CSMA/CD, Token Bus, Token Ring, Logical Link Control) Kapitel 14: Ethernet (Kapitel 15: Wireless LANs översiktligt. )

Chapter 13 Multiple Access

Figure 13. 1 Multiple-access protocols

Figure 13. 2 Evolution of random-access methods

Evolution of Contention Protocols Aloha Slotted Aloha CSMA/CD • Developed in 1970 to be used on radio LAN on Hawaiian islands. The access to the channel is random • Improvement to Aloha: Start transmission only at fixed time slots • Carrier Sense Multiple Access: Start transmission only if no transmission is ongoing • CD=Collision Detection: Stop ongoing transmission if collision is detected

Figure 13. 5 Collision in CSMA

Animeringar som illustrerar tystnadsdetektering i CSMA: ○ www. itm. mh. se/~mageri/animations/netbook/anim 06_2 -csma. mov ○ www. itm. mh. se/~mageri/animations/bjnil/anim 1 long. exe Animering som illustrerar kollisionshantering i CSMA/CD: ○ www. itm. mh. se/~mageri/animations/bjnil/anim 1. exe

Figure 13. 6 Persistence strategies

13. 7 CSMA/CD procedure

Figure 13. 8 CSMA/CA procedure

CSMA/CD • • • Sense for carrier. If carrier present, wait until carrier ends. Send packet and sense for collision. If no collision detected, consider packet delivered. Otherwise, abort immediately, perform “exponential back off” and send packet again. • CSMA/CD is used in traditional Ethernet LAN

Exponential Back-off • When a sender detects a collision, it sends a “jam signal”. ○ Jam signal is necessary to make sure that all nodes are aware of the collision ○ Length of the jam signal 48 bits • When collision is detected, the sender resends the signal after a random time ○ The random time is picked from an interval of 0 to 2 N x maximum propagation time ○ N is the number of attempted retransmission ○ Length of the interval increases with every retransmission

13. 3 Channelization FDMA TDMA CDMA

Note: In FDMA, the bandwidth is divided into channels.

Note: In TDMA, the bandwidth is just one channel that is timeshared.

Note: In CDMA, one channel carries all transmissions simultaneously.

Figure 13. 14 Chip sequences

Figure 13. 15 Encoding rules

Figure 13. 16 CDMA multiplexer

Figure 13. 17 CDMA demultiplexer

Chapter 14 Local Area Networks: Ethernet

Local Area Networks (LANs) • A computer network in a limited geographical area, a single building or several close to each other buildings • LANs are privately owned and built by the companies • Generally less expensive than WAN for comparable speed • LAN technologies use multiple access channels • Ethernet is the most common LAN technology

Figure 14. 1 Three generations of Ethernet

Traditional Ethernet • Work started back in 1973 by Bob Metcalfe and David Boggs from Xerox Palo Alto Research Center, as an improvement of the ALOHA • Experimental Ethernet implemented in 1975. • Cooperative effort between Digital, Intel, and Xerox produced Ethernet Version 1. 0 in 1980. • Ethernet was adopted with modifications by the standards committees IEEE 802. 3 and ANSI 8802/3. • Structure of Ethernet frame (Length)

Structure of Ethernet Frame • Preamble: ○ 7 bytes with pattern 1010 followed by one byte with pattern 10101011 ○ Used to synchronize receiver, sender clock rates • Addresses: 6 bytes, the frame is received by all adapters on a LAN and dropped if address does not match • Type: 2 bytes, is actually a length field in 802. 3 • CRC: 4 bytes, checked at receiver, if error is detected, the frame is simply dropped • Data payload: maximum 1500 bytes, minimum 46 bytes. If data is less than 46 bytes, pad with zeros to 46 bytes

Figure 14. 2 802. 3 MAC frame

Figure 14. 3 Minimum and maximum length

Network Interface Card (NIC) • • Each device on Ethernet network has its own interface card (NIC) to connect to the network The NIC is usually plugged into the device and has a 6 bytes (48 bits) physical address The physical address is normally written in hexadecimal notation 02 -11 -02 -2 C-4 D-1 B (example address) NIC for a desktop NIC for a laptop

Ethernet Addressing • Each station recognizes three classes of addresses. ○ Own address ○ Broadcast address (all 1's) ○ Optionally, one or more multicast addresses • Major reason for broadcast is address discovery. Brodcast Ethernet address is all 1 s, or in hexadecimal ○ FF : FF : FF • Multicast addresses are used for specialized link • layer functions. • Ethernet addresses are unique ○ First three bytes assigned to manufacturer by IEEE, the other three bytes assigned by the manufacturer

Figure 14. 5 Unicast and multicast addresses

Physical Layer of the Ethernet • PLS (Physical Layer Signaling) encodes and decodes data ○ Ethernet uses Manchester encoding • AUI (Attachment Unit Interface) – interface between PLS and medium dependent interface • MAU (Medium Attachment Unit) or transceiver • MDI (Medium Dependent Interface) is a piece of hardware connecting the transceiver to the medium

Figure 14. 6 Physical layer

Figure 14. 7 PLS

Figure 14. 8 AUI

Figure 14. 9 MAU (transceiver)

Figure 14. 10 Categories of traditional Ethernet

Classic 10 Mbps Ethernet • Four different implementation at the physical layer for the baseband 10 Mbps Ethernet ○ Thick Ethernet (10 base 5) – obsolete • Thick coaxial cable (0. 5” diameter) • 500 meter max length, bus physical topology ○ Thin Ethernet (10 base 2 802. 3 a) - obsolete • RG 58 coaxial cable • 185 meter max length, bus physical topology ○ Twisted Pair Ethernet (10 base. T 802. 3 i) • 4 pair UTP (unshielded twisted pair) cable • 100 meter max length, star physical topology ○ Fiber-link Ethernet (10 Base-FL) • Fiber cable connected to external transceiver • Star topology is used

Figure 14. 11 Connection of a station to the medium using 10 Base 5

Figure 14. 12 Connection of stations to the medium using 10 Base 2

Reflektioner Animering: Se www. itm. mh. se/~mageri/animations/ledningsreflex/

Figure 14. 13 Connection of stations to the medium using 10 Base-T

Figure 14. 14 Connection of stations to the medium using 10 Base-FL

Hub Concept • Separate transmit and receive pair of wires. • The hub retransmits the signal received on any input pair onto all output pairs. • Essentially the hub emulates a broadcast channel with collisions detected by receiving nodes.

Figure 14. 15 Sharing bandwidth

Figure 14. 16 A network with and without a bridge

Figure 14. 17 Collision domains in a nonbridged and bridged network

Ethernet Evolution • Introducing bridges ○ ○ Unlike a hub, a bridge is capable of filtering frames Each port of the bridge is connected to a single segment of LAN Capable of learning which the stations are connected to which ports Separates collision domains and therefore increases bandwidth • Introducing switches ○ Similar function as bridges ○ Contain bigger number of ports ○ A single device can be attached to a port

Figure 14. 18 Switched Ethernet

Figure 14. 19 Full-duplex switched Ethernet

Bridged vs. Switched Ethernet Bridge A B C D E F Switch

Fast Ethernet • Go from 10 mbit/s to 100 mbit/s • 3 competing standards: ○ 100 Base-TX ○ 100 Base-T 4 ○ 100 VG-Anylan • 100 Base-T 4 and 100 VG-Anylan are the losers (were not very well accepted). • 100 Base TX is the winner. It is almost a standard everywhere.

100 Base - TX • • • 100 Mbps over 2 pairs of wire (just like 10 base-T) Requires Category 5 UTP wiring or STP De facto standard today Very small price difference with 10 Mbps-only equipment Has clearly won over 100 base. T 4 and 100 VG-Anylan by now

Figure 14. 22 Fast Ethernet implementations

100 Base-FX • Fast Ethernet with fiber optic cables • Uses two optical fibers, one for transmission and one for reception

Gigabit Ethernet • Provides speeds of 1000 Mbps (i. e. , one billion bits per second capacity) for half-duplex and full-duplex operation. • Uses Ethernet frame format and MAC technology ○ CSMA/CD access method ○ Backward compatible with 10 Base-T, 100 Base-T and 100 Base. TX • Can be shared (hub) or switched

Figure 14. 29 Physical layer in Gigabit Ethernet

Gigabit Ethernet Implementations • Fiber ○ 1000 Base – SX • Short wavelengths, two fiber-optic cables ○ 1000 Base – LX • Long wavelengths, two fiber-optic cables • Copper ○ 1000 Base – CX • Uses shielded twisted pair copper jumpers ○ 1000 Base – TX • Uses category 5 twisted pair copper cable

Figure 14. 30 Gigabit Ethernet implementations

1000 Base - T • • • Four pairs of Category 5 UTP IEEE 802. 3 ab ratified in June 1999. Category 5, 6 and 7 copper up to 100 meters Uses encoding scheme 4 D-PAM 5 Five level of pulse amplitude modulation are used Complicated technique