Summary of MAC protocols

1
Summary of MAC protocols

• What do you do with a shared media?
• Channel Partitioning, by time, frequency
• Time Division, Frequency Division
• Random partitioning (dynamic),
• ALOHA, S-ALOHA, CSMA, CSMA/CD
• carrier sensing easy in some technologies
  (wire), hard in others (wireless)
• CSMA/CD used in Ethernet
• Taking Turns
• polling from a central site, token passing

2
Link Layer

• 5.1 Introduction and services
• 5.2 Error detection and correction
• 5.3Multiple access protocols
• 5.4 Link-Layer Addressing
• 5.5 Ethernet

• 5.6 Hubs and switches
• 5.7 PPP
• 5.8 Link Virtualization ATM

3
Ethernet

• Connection topology
• Frame structure
• CSMA/CD (Exponential Backoff)
• Link and physical layers

4
CSMA (Carrier Sense Multiple Access)

• CSMA listen before transmit - Carrier Sensing.
• If channel sensed idle transmit entire frame
• If channel sensed busy, defer transmission
• Human analogy dont interrupt others!

5
CSMA collisions
spatial layout of nodes
collisions can still occur propagation delay
means two nodes may not hear each others
transmission
d propagation delay
collision entire packet transmission time wasted
note distance propagation delay causes
possible collision
6
CSMA/CD (Collision Detection)

• CSMA/CD
• carrier sensing, deferral as in CSMA
• collision detection during transmission
• collisions could be detected within short time
• colliding transmissions aborted, reducing channel
  wastage
• human analogy the polite conversationalist

7
Exercise with Ethernet CSMA/CD

• The protocol
• Analyze the time sequence of events
• Time is measured as
• Bit time the time to send one bit
• Calculation 0.1 microsec/bit for 10 Mbps
  Ethernet
• What about 100Mbps Ethernet?

8
Ethernet uses CSMA/CD

• No slots
• adapter doesnt transmit if it senses that some
  other adapter is transmitting, that is, carrier
  sense
• transmitting adapter aborts when it senses that
  another adapter is transmitting, that is,
  collision detection

• Before attempting a retransmission, adapter waits
  a random time, that is, random access
• Adapt the length of random time to reduce
  collisions, exponential backoff

9

• Time is measured as
• Bit time the time to send one bit
• Calculation
• 0.1 microsec/bit for 10 Mbps Ethernet
• What about 100Mbps Ethernet?

10
Ethernet CSMA/CD algorithm

• 1. Adaptor receives datagram from net layer
  creates frame
• 2. If adapter senses channel idle (lasting for 96
  bit times), it starts to transmit frame. If it
  senses channel busy, waits until channel idle and
  then transmits
• 3. If adapter transmits entire frame without
  detecting another transmission, the adapter is
  done with frame !

• 4. If adapter detects another transmission while
  transmitting, aborts and sends jam signal.
• Jam Signal make sure all other transmitters are
  aware of collision 48 bits
• 5. After aborting, adapter enters a random defer
  period called exponential backoff.
• exponential backoff after the mth
  collision, adapter chooses a K at random from
  0,1,2,,2m-1. Adapter waits K?512 bit times and
  returns to Step 2. The maximum, m can be 10.
• Bit time 0.1 microsec/bitThe longest backoff
  time is at K1023, about 50 msec

11
More on Ethernets CSMA/CD

• Exponential Backoff
• first collision choose K from 0,1 delay is K?
  512 bit transmission times
• after second collision choose K from 0,1,2,3
• after ten collisions, choose K from
  0,1,2,3,4,,1023
• heavy load random wait will be longer
• advantage adapt retransmission attempts to
  estimated current load, and reduce collisions.

See/interact with Java applet on AWL Web
site highly recommended !
12
Exercise with Ethernet CSMA/CD

• Events at one transmitting node.
• given a K 10, how long the node wait?
• Events at two transmitting nodes.
• Examples, trace the protocol
• Analyze the time sequence of events

13
Ethernet

• Connection topology
• Frame structure
• CSMA/CD (Exponential Backoff)
• Link and physical layers

14
Ethernet

• dominant wired LAN technology
• first widely used LAN technology
• Simpler, cheaper than token LANs and ATM
• Kept up with speed race 10 Mbps 10 Gbps

Metcalfes Ethernet sketch
15
Star topology

• bus topology popular through mid 90s
• all nodes in same collision domain (can collide
  with each other)
• today star topology prevails
• active switch in center
• each spoke runs a (separate) Ethernet protocol
  (nodes do not collide with each other)

• switch

• bus coaxial cable

• star

16
Not an atypical LAN (IP network)
Dedicated
Shared
17
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

18
Ethernet Frame Structure (more)

• Addresses 6 bytes
• if adapter receives frame with matching
  destination address, or with broadcast address
  (eg ARP packet), it passes data in frame to
  net-layer protocol
• otherwise, adapter discards frame
• Type indicates the higher layer protocol (mostly
  IP but others may be supported such as Novell IPX
  and AppleTalk)
• CRC checked at receiver, if error is detected,
  the frame is simply dropped

19
Unreliable, connectionless service

• Connectionless No handshaking between sending
  and receiving adapter.
• Unreliable receiving adapter doesnt send acks
  or nacks to sending adapter
• stream of datagrams passed to network layer can
  have gaps (missing datagrams)
• gaps will be filled if app is using TCP
• otherwise, app will see the gaps
• Ethernets MAC protocol unslotted CSMA/CD

20
Ethernet uses CSMA/CD

• No slots
• adapter doesnt transmit if it senses that some
  other adapter is transmitting, that is, carrier
  sense
• transmitting adapter aborts when it senses that
  another adapter is transmitting, that is,
  collision detection

• Before attempting a retransmission, adapter waits
  a random time, that is, random access

21
Ethernet CSMA/CD algorithm

• Adaptor receives datagram from net layer
  creates frame
• 2. If adapter senses channel idle (lasting for 96
  bit times), it starts to transmit frame. If it
  senses channel busy, waits until channel idle and
  then transmits
• 3. If adapter transmits entire frame without
  detecting another transmission, the adapter is
  done with frame !

• 4. If adapter detects another transmission while
  transmitting, aborts and sends jam signal
• Jam Signal make sure all other transmitters are
  aware of collision 48 bits
• 5. After aborting, adapter enters exponential
  backoff after the mth collision, adapter chooses
  a K at random from 0,1,2,,2m-1. Adapter waits
  K?512 bit times and returns to Step 2
• Bit time 0.1 microsec for 10 Mbps Ethernet
  for K1023, wait time is about 50 msec

22
Ethernets CSMA/CD (more)

• Jam Signal make sure all other transmitters are
  aware of collision 48 bits
• Bit time .1 microsec for 10 Mbps Ethernet for
  K1023, wait time is about 50 msec

• Exponential Backoff
• Goal adapt retransmission attempts to estimated
  current load
• heavy load random wait will be longer
• first collision choose K from 0,1 delay is K?
  512 bit transmission times
• after second collision choose K from 0,1,2,3
• after ten collisions, choose K from
  0,1,2,3,4,,1023

• See/interact with Java
• applet on AWL Web site
• highly recommended !

23
CSMA/CD efficiency

• Tprop max propagation delay between 2 nodes in
  LAN (e.g 400m)
• ttrans time to transmit max-size frame
• Efficiency goes to 1 as tprop goes to 0
• Goes to 1 as ttrans goes to infinity
• Much better than ALOHA, but still decentralized,
  simple, and cheap

24
802.3 Ethernet Standards Link Physical Layers

• many different Ethernet standards
• common MAC protocol and frame format
• different speeds 2 Mbps, 10 Mbps, 100 Mbps,
  1Gbps, 10G bps
• different physical layer media fiber, cable

• MAC protocol
• and frame format

• 100BASE-TX

• 100BASE-FX

• 100BASE-T2

• 100BASE-T4

• 100BASE-SX

• 100BASE-BX

25
Manchester encoding

• Used in 10BaseT
• Each bit has a transition
• Allows clocks in sending and receiving nodes to
  synchronize to each other
• no need for a centralized, global clock among
  nodes!
• Hey, this is physical-layer stuff!

26
Gbit Ethernet

• uses standard Ethernet frame format
• allows for point-to-point links and shared
  broadcast channels
• in shared mode, CSMA/CD is used short distances
  between nodes required for efficiency
• uses hubs, called here Buffered Distributors
• Full-Duplex at 1 Gbps for point-to-point links
• 10 Gbps now !