day6
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Networks
Instructor: Samir Datta
sdatta@cmi.ac.in
Semester: Fall 2005 Schedule: Tue &Thurs 5:00-6:15 pm
Recommended Text: Computer Networks
(Peterson & Davie 3rd ed.)
http://www.cmi.ac.in/~sdatta/networks/networks.html
Medium Access Control (MAC)
●Point to Point Link:
–
Two nodes coordinate
–Flow Control through Sliding Window
●Multiple Nodes share medium:
–Example, Nodes in LAN, Wireless LAN etc.
–How do they coordinate?
●
Centralized Controller
●Distributed Algorithm
Ethernet (CSMA/CD)
–
Carrier Sense Multiple Access with Collision Detection
–
Checks whether Medium is busy before sending –
If no, then send else wait
–
Collisions may still occur – what to do?–
Detect Collision. And...
–Wait. How long?
●
Deterministic wait – more collisions!●
Random wait. What if another collision?
●
Longer Random wait ●Exponential Backoff
WiFi (CSMA/CA)
WiFi
●Can we use the same protocol as Ethernet?
–Assumption: Tx hears all transmissions that Rx can –Tx may not be able to hear all that Rx hears
Tx may even hear something that Rx doesn't!
WiFi (Virtual Carrier Sense)
●RTS = Request To Send CTS = Clear To Send
–
Takes care of Hidden Terminal Problem
–Not Exposed Terminal Problem
WiFi (Timing Diagram)
Ethernet (contd.)
●Impl. on coaxial cable atmost 500m long
●4 repeaters may be present => tot. len < 2500m
●Frame format:
Ethernet Addresses
●Every NIC has unique Ethernet Address
●48b = 6B long
●First 3B are manufacturer specific
●Broadcast Address: all 1's
●Multicast Address: 1 followed by other than all 1's
●Promiscuous mode: Listens to all frames
Ethernet (contd.)
●An Ethernet frame must be at least 64B long ●Why?
–2500m @ 3 x 10^8 m/s = 8 x 10^-5 s = 80us
–80us @ 10Mbps = 800b = 100B!!!!
–2000m @ 3 x 10^8 m/s @ 10Mbps = 633b > 64B
–Somebody look at the standard!
802.5 Token Ring
●Nodes connected in ring
●Data flows in one direction
–Receive from upstream node
–Transmit to downstream node
●Token is a special sequence of bits
–Seized & Removed when a node wants to transmit –Reinserted when frame drained (delayed release)
–Or, when frame completely transmitted (early rel.)
Token Ring (contd.)
●Monitor “ensures health of ring”
–
corrupted token is replaced –
Corrupted/Orphaned frames taken care of
–Delay introduced if token longer than ring
●Monitor has to be elected –A node transmits “claim token” frame
–If more than one claims, some tie breaking rule –
Like, “highest address wins”
WPAN
●WPAN = Wireless Personal Area Network ●Two standards 802.15.1 (Bluetooth) and
802.15.3
●Bluetooth a “wire-replacement” technology
●Low bandwidth
●Can form piconet and scatternet
802.15.3
●High Bandwidth
●Piconet and PicoNet Controller (PNC)
●Essentially TDMA structure
●Nodes can request PNC for slots to transmit
Assignment I
●Questions?
Action Items
●Go through TCP RFCs
●Send me/ask questions about the assignment
●Start looking at IEEE Standards
●Prepare for classic paper presentation
●And the weekly presentation next week。
