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Link Layer 3
Mozafar Bag-Mohammadi
University of Ilam
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Token Ring Overview

Examples
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16Mbps IEEE 802.5 (based on earlier IBM ring)
100Mbps Fiber Distributed Data Interface (FDDI)
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Token Ring (cont)
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Idea
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Frames flow in one direction: upstream to downstream
special bit pattern (token) rotates around ring
must capture token before transmitting
release token after done transmitting
 immediate release
 delayed release
remove your frame when it comes back around
stations get round-robin service
Frame Format
8
8
48
48
Start of
frame
Control
Dest
addr
Src
addr
32
Body
8
CRC End of
frame
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Status
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Timed Token Algorithm
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Token Holding Time (THT)
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Token Rotation Time (TRT)
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upper limit on how long a station can hold the token
how long it takes the token to traverse the ring.
TRT <= ActiveNodes x THT + RingLatency
Target Token Rotation Time (TTRT)
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agreed-upon upper bound on TRT
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Algorithm (cont)
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Each node measures TRT between successive
tokens
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if measured-TRT > TTRT: token is late so don’t send
if measured-TRT < TTRT: token is early so OK to send
Two classes of traffic
 synchronous: can always send
 asynchronous: can send only if token is early
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Token Maintenance
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Lost Token
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no token when initializing ring
bit error corrupts token pattern
node holding token crashes
Generating a Token (and agreeing on TTRT)
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execute when join ring or suspect a failure
send a claim frame that includes the node’s TTRT bid
when receive claim frame, update the bid and forward
if your claim frame makes it all the way around the ring:
 your bid was the lowest
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everyone knows TTRT
you insert new token
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Maintenance (cont)
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Monitoring for a Valid Token
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should periodically see valid transmission (frame
or token)
maximum gap = ring latency + max frame < =
2.5ms
set timer at 2.5ms and send claim frame if it fires
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Wireless LANs
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IEEE 802.11
Physical Media
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spread spectrum radio (2.4GHz)
diffused infrared (10m)
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Spread Spectrum
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Idea
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spread signal over wider frequency band than required
originally designed to thwart jamming
Frequency Hopping
Direct Sequence
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Frequency Hopping
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transmit over random
sequence of
frequencies
sender and receiver
share…
 pseudorandom
number generator
 seed
802.11 uses 79 x
1MHz-wide frequency
bands
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Direct Sequence
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for each bit, send XOR of that bit and n
random bits
random sequence known to both sender and
receiver
called n-bit chipping code
802.11 defines an 11-bit chipping code
1
0
Data stream: 1010
1
0
Random sequence: 0100101101011001
1
0
XOR of the two: 1011101110101001
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Glossary of 802.11 Wireless Terms
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Station (STA): A computer or device with a wireless network
interface.
Access Point (AP): Device used to bridge the wireless-wired
boundary, or to increase distance as a wireless packet repeater.
Ad Hoc Network: A temporary one made up of stations in mutual
range.
Infrastructure Network: One with one or more Access Points.
Channel: A radio frequency band, or Infrared, used for shared
communication.
Basic Service Set (BSS): A set of stations communicating
wirelessly on the same channel in the same area, Ad Hoc or
Infrastructure.
Extended Service Set (ESS): A set BSSs and wired LANs with
Access Points that appear as a single logical BSS.
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Supporting Mobility
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Case 1: ad hoc networking
Case 2: access points (AP)
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tethered
each mobile node associates with an AP
Distribution system
AP-1
AP-3
F
AP-2
A
B
G
H
C
E
D
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Overview, 802.11 Architecture
ESS
Existing
Wired LAN
AP
STA
BSS
AP
STA
STA
BSS
STA
Infrastructure
Network
STA
Ad Hoc
Network
STA
BSS
BSS
STA
Ad Hoc
Network
STA
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Mobility (cont)
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Scanning (selecting an AP)
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node sends Probe frame
all AP’s w/in reach reply with ProbeResponse frame
node selects one AP; sends it AssociateRequest frame
AP replies with AssociationResponse frame
new AP informs old AP via tethered network
When
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active: when join or move
passive: AP periodically sends Beacon frame
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MACAW
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Sender transmits
RequestToSend (RTS) frame
Receiver replies with
ClearToSend (CTS) frame
Neighbors…
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Receiver sends ACK when it
has frame
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see CTS: keep quiet
see RTS but not CTS: ok to
transmit
neighbors silent until see ACK
Collisions
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no collisions detection
known when don’t receive CTS
exponential backoff
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Hidden & Exposed nodes
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Collisions Avoidance
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Similar to Ethernet
Problem: hidden and exposed nodes
Hidden node
Exposed
node
A
B
C
D
Sending
node
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Point to Point Data Link Control
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one sender, one receiver, one link: easier than broadcast link:
 no Media Access Control
no need for explicit MAC addressing
 e.g., dialup link, ISDN line
popular point-to-point Data Link Control protocols:
 PPP (point-to-point protocol)
 Protocol choice for dialup link.
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Point-to-Point (serial) links
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Many data link connections are
point-to-point serial links:
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Dial-in or DSL access connects hosts to
access routers
Routers are connected by
high-speed point-to-point links
IP hosts and routers are connected
by a serial cable
 Data link layer protocols for pointto-point links are simple:
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Access
Router
Modems
Dial-Up Access
Router
Main role is encapsulation of IP
datagrams
No media access control needed
Router
Router
Point-to-Point Links
Router
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Data Link Protocols for Point-to-Point links
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SLIP (Serial Line IP)
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PPP (Point-to-Point Protocol):
•
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First protocol for sending IP datagrams over dial-up
links (from 1988)
Encapsulation, not much else
Successor to SLIP (1992), with added functionality
Used for dial-in and for high-speed routers
HDLC (High-Level Data Link) :
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Widely used and influential standard (1979)
Default protocol for serial links on Cisco routers
Actually, PPP is based on a variant of HDLC
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PPP - IP encapsulation
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The frame format of PPP is similar to HDLC and the 802.2 LLC frame
format:
flag
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addr ctrl
7E
FF
03
1
1
1
protocol
data
CRC
flag
7E
2
<= 1500
0021
IP datagram
C021
link control data
8021
network control data
PPP assumes a duplex circuit
Note: PPP does not use addresses
Usual maximum frame size is 1500
2
1
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Additional PPP functionality
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In addition to encapsulation, PPP supports:
 multiple network layer protocols (protocol multiplexing)
 Link configuration
 Link quality testing
 Error detection
 Option negotiation
 Address notification
 Authentication
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The above functions are supported by helper protocols:
 LCP
 PAP, CHAP
 NCP
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PPP Support protocols
 Link
management: The link control protocol (LCP) is
responsible for establishing, configuring, and negotiating a
data-link connection. LCP also monitors the link quality and is
used to terminate the link.
 Authentication:
Authentication is optional. PPP supports two
authentication protocols: Password Authentication Protocol
(PAP) and Challenge Handshake Authentication Protocol
(CHAP).
 Network
protocol configuration: PPP has network control
protocols (NCPs) for numerous network layer protocols. The IP
control protocol (IPCP) negotiates IP address assignments
and other parameters when IP is used as network layer.
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