slide1 n.
Download
Skip this Video
Loading SlideShow in 5 Seconds..
Review: Medium Access Control Sublayer What is the problem to be addressed in this sublayer? Protocols that allow collis PowerPoint Presentation
Download Presentation
Review: Medium Access Control Sublayer What is the problem to be addressed in this sublayer? Protocols that allow collis

Loading in 2 Seconds...

play fullscreen
1 / 15

Review: Medium Access Control Sublayer What is the problem to be addressed in this sublayer? Protocols that allow collis - PowerPoint PPT Presentation


  • 293 Views
  • Uploaded on

Review: Medium Access Control Sublayer What is the problem to be addressed in this sublayer? Protocols that allow collision Pure ALOHA Slotted ALOHA CSMA CSMA/CD Collision free protocols: bitmap method, binary countdown and token. Collision free protocols: Token pass.

loader
I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
capcha
Download Presentation

PowerPoint Slideshow about 'Review: Medium Access Control Sublayer What is the problem to be addressed in this sublayer? Protocols that allow collis' - lyle


An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript
slide1
Review: Medium Access Control Sublayer
  • What is the problem to be addressed in this sublayer?
  • Protocols that allow collision
    • Pure ALOHA
    • Slotted ALOHA
    • CSMA
    • CSMA/CD
  • Collision free protocols:
    • bitmap method, binary countdown and token
slide2
Collision free protocols:
      • Token pass.
        • There is only one token in the network.
        • The token is passed through every node in the network.
        • Only the node that has the token can transfer data.
slide3
Limited contention protocols:
    • collision based protocols (ALOHA,CSMA/CD) are good when the network load is low.
    • collision free protocols (bit map, binary countdown) are good when load is high.
    • How about combining their advantages -- limited contention protocols.
      • Behave like the ALOHA scheme under light load
      • Behave like the bitmap scheme under heavy load.
slide4
Limited contention protocols:
    • adaptive tree walk protocol
      • trick: dynamic partition the stations into groups and limit the contention for each slot.
        • under light load, every one tries for each slot like ALOHA
        • under heavy load, only a small group can try for each slot
        • how do we do it
          • treat stations as the leaves of a binary tree.
          • first slot, all stations (under the root node) can try to get the slot.
          • if no conflict, repeat.
          • if conflict, use depth first search to traverse the tree, only nodes of a sub-tree get to try for the next slot.
slide5

Example:

0

2

1

3

6

4

5

D

A

B

C*

E*

F*

G

H*

Slot 0: C*, E*, F*, H* (all nodes under node 0 can try), conflict

slot 1: C* (all nodes under node 1 can try), C sends

slot 2: E*, F*, H*(all nodes under node 2 can try), conflict

slot 3: E*, F* (all nodes under node 5 can try), conflict

slot 4: E* (all nodes under E can try), E sends

slot 5: F* (all nodes under F can try), F sends

slot 6: H* (all nodes under node 6 can try), H sends.

slide6
Ethernet:
    • Invented at Xerox by Robert Metcalfe (founder of 3Com) and Dave Boggs
    • background:
      • ARPANet in late 60's, linking computers at different sites to central mainframe computers.
      • By early 70's, the cost of computers went down, introduction of mini-computers PDP, which means each school can have more than one computer!
        • Applications: share printers, share files, share cycles
      • Factory automation: many computers on factory floor
      • Need local area networks to link the computers
slide7
Ethernet:
    • Use shared medium instead of switched-based
        • cost: one adaptor/machine + link
        • performance: all hosts sharing one link.
    • first Ethernet:
      • 3 Mbps
        • PDP-11 0.25 MIPS, 0.1 Mbps peak
        • no all computers transmit at peak all the time meaning, easily support up to 100 computers at that time
    • Now:
        • 500MHz Pentium, around 200MIPS, 100Mbps
        • 10Mbps cannot support as many machines.
slide8
Medium Access Problem:
    • multiple stations may transmit on the medium at the same time, which may result in collisions
    • Two solutions
      • guarantee that only one station transfers at one time: (contention free protocol) FDDI, token ring, token bus use the first approach
      • try the luck and re-transmit if there is a collision (contention based protocol)
        • need algorithm to reduce the probability of collision
      • Ethernet uses CSMA/CD + binary exponential backoff to reduce the probability of collisions
slide9
CSMA/CD + binary exponential backoff
    • sense before send (CSMA)
    • abort sending upon detecting collision (CD).
    • adjust retransmission interval (binary exponential backoff)
      • each time slot to be 51.2 us
      • first collision, retransmission interval = random number between [0,1]
      • second collision, interval = random number between [0,1,2,3]
      • kth collision, interval = random number between [0, 2^k-1]
      • upper bound 1023 slots.
slide10
Important design parameters
    • Bandwidth: 10 Mbps
      • Propagation Delay: limit the frame size.
  • Physical medium
    • thin cable/thick cable/twisted pair/fiber

10Base5 500 meters thick (cable) Ethernet 100 nodes/seg

10Base2 200 meters thin (cable) Ethernet 30 nodes/seg

10BaseT 100 meters twist pair 1024 nodes/seg

10BaseF 2000 meters fiber optics 1024 nodes/seg

10Base5/10Base2, cable connected to each machine

10BaseT -- connecting to a hub

10BaseF -- between building Connecting

slide11
Multiple segments can be connected through the repeaters (hubs).
  • All segments connected by the repeaters are in the same collision domain.
    • constraint: no two transceivers may be 2.5km apart and separated by 4 repeaters.
    • frame format

| Preamble | Start| Dst Addr | Src Addr | length | Data |Pad |Checksum|

7 1 2/6 2/6 2 0-1500 0-46 4

      • Header: 14 Bytes, CRC: 4 Bytes
      • Minimum data (+ pad) length: 46 Bytes
      • Maximum data length: 1500 Bytes
slide12
Minimum frame size = ??
  • Why? To run CSMA/CD, each frame must be large enough to detect collision.
    • 2 * max propagation delay?
      • standard: 2500m, 500m per segment, 4 repeaters.
      • speed of light: 3*10^8m/s
      • speed of signal propagation: 2*10^8m/s
      • propagation delay: about 25us (on wire) +25 us in repeaters, total delay = 51.2us
      • How many bytes do we need in each frame?
  • Maximum frame size = ?
  • Why?
    • larger is better for bandwidth utilization
slide13
How to find out your Ethernet address: "arp”
      • /usr/sbin/arp xi --> xi (128.186.121.41) at 8:0:20:92:43:b1
    • Ethernet Switch: Increase the bandwidth, segments connected by switch have different collision domain.
      • Ethernet switch: data link layer device
      • Ethernet hub (repeater): physical layer device
  • Fast Ethernet
    • Keep everything in Ethernet, make the clock faster 100Mbps.
    • What are the problems?
      • Cable
        • 100Base-T4 100m category 3 UTP, 4 lines.
        • 100Base-Tx 100m category 5 twisted pair
        • 100Base-Fx 2000m Fiber optic
slide14
What are the problems?
    • Cable
    • CSMA/CD?
      • minimum frame size = 64byte = 512 bits,
      • 5.12us using 100Mbps transmission rate.
      • What can you do about this?
        • Increase the minimum frame size.
        • Reduce cable length
        • Faster Ethernet:
          • Reduce the cable length by a factor of 10, maximum length = 200 meters (100-Base-T, 100 meter cable).
      • Full duplex mode: point to point connection, no contention. No CSMA/CD needed, can have longer cable.
slide15
Gigabit Ethernet: make it even faster at 1Gbps.
    • Cable: mainly fiber optics.
    • CSMA/CD domain
      • Shortening the cable? 20 meters
    • Alternative: increase the minimum frame size to 512 bytes, CSMA/CD domain 200 meters (not much error margin)
      • Experimental studies say that typical frame size are 200 - 300 bytes.
    • backward compatibility:
      • carrier extension -- short packet, stuff extra bits to make to 512 bytes
    • improve performance: packet bursting -- transmit a burst of small frames, only the first one need carrier extension.