getting connected (chapter 2 part 4)diesburg/courses/cs3470_fa14/sessions/s11/s11.pdf · csma...
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Getting Connected (Chapter 2 Part 4)
Networking CS 3470, Section 1
Sarah Diesburg
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Five Problems
Encoding/decoding Framing Error Detection Error Correction Media Access
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Five Problems
Encoding/decoding Framing Error Detection Error Correction Media Access
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Core Ethernet
Ethernet standard is 802.3 Legacy multiple-access network Set of nodes sends and receives frames over
shared link Kind of like a bus
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Core Ethernet
Ethernet is CSMA/CD, which stands for “Carrier Sense, Multiple Access with Collision Detection.” Carrier sense: all nodes can distinguish between
idle and busy link Collision detection: node listens as it transmits
can tell if it collides with another frame
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Core Ethernet
By standard, Ethernet is implemented using Coax, on segments limited to 500m
Hosts joined by “tapping into” segment Multiple Ethernet segments joined together
by repeaters What do you think a repeater does?
No more than four repeaters allowed.
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Core Ethernet
Total reach: 2500m All physical limitations considered, an
Ethernet is limited to 1024 hosts maximum. Any signal emitted by a host on the ethernet
is broadcast over the entire network Terminators attached to the end absorb the
signal and prevent bounce-back.
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Ethernet Technologies: 10Base2
10: 10Mbps; 2: under 200 meters max cable length thin coaxial cable in a bus topology
repeaters used to connect up to 5 multiple segments repeater repeats bits it hears on one interface to its
other interfaces: physical layer device only!
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Core Ethernet
Cable Types: 10Base2 10Mbps peak, Baseband, with 200m limit
10Base5 10Mbps peak, Baseband, with 500m limit
10BaseT 10Mbps peak, Baseband, Twisted-Pair cable, 100m
limit
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Core Ethernet
Encoding schemes Original specification used Manchester encoding Higher-speed Ethernet uses 4B/5B or the similar
8B/10B encoding
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Shared Access
Everyone speaks at the same time Competition for the same link, speaking at
the same time produces the notion of a collision domain.
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Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble: 7 bytes with pattern 10101010 followed by one byte
with pattern 10101011 used to synchronize receiver, sender clock rates
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Ethernet Frame Structure (more)
Addresses: 6 bytes Type: indicates the higher layer protocol,
mostly IP but others may be supported CRC: checked at receiver, if error is
detected, the frame is simply dropped
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MAC Addresses
Every Ethernet adapter in the world has a unique address Media Access Control (MAC)
Burned into ROM in hardware Sequence of six numbers separated by
colons Six binary numbers Usually displayed in hex for humans
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MAC Addresses
Binary representation 00001000 00000000 00101011 11100100
10110001 00000010 Human (hex) representation 8:0:2b:e4:b1:2
How do we go from binary to hex (and back)?
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Hex Exercise
What is the following hex mac address in binary?
E0:DB:55:E7:AD:1D What is the following binary mac address in
hex? 11001010:00101011:11001101 (first half) 00000111:00101111:01010100 (second half)
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MAC Addresses
How do we find them on our machines? Windows cmd -> “ipconfig /all”
Linux $> /sbin/ifconfig –a
Can a machine have more than one MAC address?
Is it changeable?
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MAC Addresses
Ethernet adapter receives all frames and accepts frames that are addressed to Its own address Broadcast address (all 1’s) Multicast address that it has been programmed to
accept Can also be put into promiscuous mode to
accept all frames Only used in network debugging and hacking
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MAC Addresses
So if a MAC address is unique, can every packet sent be traced back to a unique user?
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CSMA (Carrier Sense Multiple Access) Ethernet is CSMA/CD, which stands for
“Carrier Sense, Multiple Access with Collision Detection.” Carrier sense: all nodes can distinguish between
idle and busy link Collision detection: node listens as it transmits
can tell if it collides with another frame
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CSMA (Carrier Sense Multiple Access) CSMA: listen before transmit: If channel sensed idle: transmit entire frame If channel sensed busy, defer transmission
Human analogy: don’t interrupt others!
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CSMA collisions
collisions can still occur: propagation delay means two nodes may not hear each other’s transmission
collision: entire packet transmission time wasted
spatial layout of nodes
note: role of distance & propagation delay in determining collision probability
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CSMA/CD (Collision Detection)
CSMA/CD: carrier sensing, deferral as in CSMA collisions detected within short time colliding transmissions aborted, reducing channel
wastage
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CSMA/CD collision detection
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Collisions
When an adapter detects its frame in a collision, it transmits a 32-bit jamming sequence and stops Will minimally send 96 bits – 64 bit preamble and
32 bits of jamming Also called a runt frame
What sort of configuration will cause the most runt frames?
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Transmitter Algorithm
So that’s great that we can detect collisions, but lot’s of collisions are probably bad
How can we minimize collisions? Exponential backoff
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Transmitter Algorithm
Maximum distance between any to end hosts is 2500m
At most four repeaters in any Ethernet segment Transmitters are required to send 512 bits per frame.
Consider A and B at opposite ends of the network. A sends at time “t.” One link latency, d, is required to
reach host B First bit of A's transmission reaches B at time “t+d.” B
still sees an idle line and begins to transmit B's frame immediately collides with A's. B sends a 96-bit
jamming sequence (a runt frame), arriving at time t +2d
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Some Physical Specifications to the 802 Standard ... ... So A must still be talking when the runt
frame comes back. Max distance = 2500m Round-trip delay assuming four repeaters and “low”
propagation speed is 51.2µs. At 10-Mbps, this “pipe” is exactly 512 bits.
Falloff: 51.2µs to send. Whoa! Collision. Either send next frame or not... (Backoff 0 or 51.2µs) Still busy? Double your choices and pick randomly
(e.g., pick randomly from 0µs, 51.2µs, 102.4µs, 153.6µs)
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“Taking Turns” MAC protocols
Polling master node “invites”
slave nodes to transmit in turn
concerns: polling overhead latency single point of failure
(master)
Token Passing control token passed from
one node to next sequentially.
token message concerns:
token overhead latency single point of failure (token)
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IEEE 802.11 Wireless LAN
802.11b 2.4-5 GHz unlicensed radio spectrum up to 11 Mbps direct sequence spread spectrum (DSSS) in
physical layer Greater tolerance of interference by adding more
redundancy Each bit is represented by multiple bits – if some bits
are damaged, can recover All hosts use same chipping code for decoding
widely deployed, using base stations
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IEEE 802.11 Wireless LAN
802.11a 5-6 GHz range up to 54 Mbps
802.11g 2.4-5 GHz range up to 54 Mbps
All use CSMA/CA for multiple access
Cannot sense collision detection! All have base-station and ad-hoc network
versions
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Base station approch
Wireless host communicates with a base station base station = access point (AP)
Basic Service Set (BSS) (a.k.a. “cell”) contains: wireless hosts access point (AP): base station
BSS’s combined to form distribution system (DS)
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Ad Hoc Network approach
No AP (i.e., base station) wireless hosts communicate with each other
to get packet from wireless host A to B may need to route through wireless hosts X,Y,Z
Applications: “laptop” meeting in conference room, car interconnection of “personal” devices battlefield
IETF MANET (Mobile Ad hoc Networks) working group
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IEEE 802.11: multiple access
Collision if 2 or more nodes transmit at same time CSMA makes sense:
get all the bandwidth if you’re the only one transmitting shouldn’t cause a collision if you sense another
transmission Collision detection doesn’t work: hidden terminal
problem
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IEEE 802.11 MAC Protocol: CSMA/CA 802.11 CSMA: sender
if sense channel idle for DIFS (distribute inter-frame space) sec.
then transmit entire frame (no collision detection)
if sense channel busy then binary backoff
802.11 CSMA: receiver if received OK return ACK after SIFS
(short inter-frame space)
SIFS required because of hidden terminal problem
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Collision avoidance mechanisms
Problem: Two nodes, hidden from each other, transmit
complete frames to base station Wasted bandwidth for long duration !
Solution: Small reservation packets Nodes track reservation interval with internal
“network allocation vector” (NAV)
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Collision Avoidance: RTS-CTS exchange Sender transmits short RTS (request to send)
packet: indicates duration of transmission Receiver replies with short CTS (clear to
send) packet Notifying (possibly
hidden) nodes Hidden nodes will not
transmit for specified duration: NAV
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Collision Avoidance: RTS-CTS exchange RTS and CTS short: collisions less likely, of shorter duration end result similar to collision detection
IEEE 802.11 allows: CSMA CSMA/CA: reservations polling from AP
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Summary of MAC protocols
What do you do with a shared media? Channel Partitioning, by time, frequency or code Random partitioning (dynamic), Taking Turns
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End of Notes for Exam 1
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