Download Lektion 1-Introduktion

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
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
Animeringar som illustrerar tystnadsdetektering i CSMA:
○ www.itm.mh.se/~mageri/animations/netbook/anim06_2-csma.mov
○ www.itm.mh.se/~mageri/animations/bjnil/anim1long.exe
Animering som illustrerar kollisionshantering i CSMA/CD:
○ www.itm.mh.se/~mageri/animations/bjnil/anim1.exe
Figure 13.6
Persistence strategies
13.7
CSMA/CD procedure
Figure 13.8
CSMA/CA procedure
CSMA/CD
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•
•
•
•
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 2N 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
Mobiltelesystemens generationer
• 1G: Analog modulation – FDMA. T.ex. NMT. 80-talet.
• 2G: Digital modulation, TDMA + FDMA. T.ex. GSM. 90talet.
• 2.5G: GPRS, dvs paketförmedling.
• 3G: Edge (8PSK) eller WCDMA (spread spectrum). 2000talet.
• 3.5G: All-IP-infrastructure, inkl IP-telefoni istället för
kretskopplad telefoni? Asymmetrisk. HSDPA.
• 4G: Hybrid av många system? Heterogen täckning?
COFDM-modulation? Dynamisk kanalallokering? 2010talet?
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 10101010 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)
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•
•
•
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
02-11-02-2C-4D-1B (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 1s, or in hexadecimal
○ FF : FF : FF : 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
Figure 14.10
Categories of traditional Ethernet
Classic 10Mbps Ethernet
• Four different implementation at the physical layer for the baseband
10Mbps Ethernet
○ Thick Ethernet (10base5) – obsolete
• Thick coaxial cable (0.5” diameter)
• 500meter max length, bus physical topology
○ Thin Ethernet (10base2 802.3a) - obsolete
• RG58 coaxial cable
• 185 meter max length, bus physical topology
○ Twisted Pair Ethernet (10baseT 802.3i)
• 4 pair UTP (unshielded twisted pair) cable
• 100 meter max length, star physical topology
○ Fiber-link Ethernet (10Base-FL)
• Fiber cable connected to external transceiver
• Star topology is used
Figure 14.11 Connection of a station to the medium using 10Base5
Figure 14.12
Connection of stations to the medium using 10Base2
Reflektioner
Animering:
Se www.itm.mh.se/~mageri/animations/ledningsreflex/
Figure 14.13
Connection of stations to the medium using 10Base-T
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 10mbit/s to 100mbit/s
• 3 competing standards:
○ 100Base-TX
○ 100Base-T4
○ 100VG-Anylan
• 100Base-T4 and 100VG-Anylan are the losers (were not
very well accepted).
• 100Base TX is the winner. It is almost a standard
everywhere.
100Base - TX
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100 Mbps over 2 pairs of wire (just like 10base-T)
Requires Category 5 UTP wiring or STP
De facto standard today
Very small price difference with 10Mbps-only equipment
Has clearly won over 100baseT4 and 100VG-Anylan by
now
1000Base - T
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Four pairs of Category 5 UTP
IEEE 802.3ab ratified in June 1999.
Category 5, 6 and 7 copper up to 100 meters
Uses encoding scheme 4D-PAM5
Five level of pulse amplitude modulation are used
Complicated technique