1 Link Layer 3 Mozafar Bag-Mohammadi University of Ilam.
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Transcript of 1 Link Layer 3 Mozafar Bag-Mohammadi University of Ilam.
1
Link Layer 3
Mozafar Bag-Mohammadi
University of Ilam
2
Token Ring Overview
Examples 16Mbps IEEE 802.5 (based on earlier IBM ring) 100Mbps Fiber Distributed Data Interface (FDDI)
3
Token Ring (cont) Idea
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
Control
8 8 8 24
CRCStart offrame
End offrame
Destaddr
Body
4848
Srcaddr
Status
32
4
Wireless LANs IEEE 802.11 Physical Media
spread spectrum radio (2.4GHz) diffused infrared (10m)
5
Spread Spectrum
Idea spread signal over wider frequency band than required originally designed to thwart jamming
Frequency Hopping Direct Sequence
6
Frequency Hopping transmit over random
sequence of frequencies
sender and receiver share… pseudorandom
number generator seed
802.11 uses 79 x 1MHz-wide frequency bands
7
Direct Sequence 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
Random sequence: 0100101101011001
Data stream: 1010
XOR of the two: 1011101110101001
0
0
0
1
1
1
8
Glossary of 802.11 Wireless Terms
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.
9
Supporting Mobility
Case 1: ad hoc networking Case 2: access points (AP)
tethered each mobile node associates with an AP
B
H
A
F
G
D
AP-2
AP-3AP-1
EC
Distribution system
10
Overview, 802.11 Architecture
STASTA
STA STA
STASTASTA STA
APAP
ESS
BSS
BSSBSS
BSS
Existing Wired LAN
Infrastructure Network
Ad Hoc Network
Ad Hoc Network
11
Mobility (cont)
Scanning (selecting an AP) 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
modes active: when join or move passive: AP periodically sends Beacon frame
12
MACAW Sender transmits
RequestToSend (RTS) frame Receiver replies with
ClearToSend (CTS) frame Neighbors…
see CTS: keep quiet see RTS but not CTS: ok to
transmit Receiver sends ACK when it
has frame neighbors silent until see ACK
Collisions no collisions detection known when don’t receive CTS exponential backoff
13
Hidden & Exposed nodes
14
Collisions Avoidance
Similar to Ethernet Problem: hidden and exposed nodes
Hidden node
Exposed node
Sending node
A B C D
15
Point to Point Data Link Control 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.
16
Dial-Up Access
AccessRouter
Modems
Point-to-Point (serial) links
Many data link connections are point-to-point serial links: 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 point-
to-point links are simple: Main role is encapsulation of IP
datagrams No media access control needed
Point-to-Point Links
Router
Router
Router Router
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Data Link Protocols for Point-to-Point links
SLIP (Serial Line IP) First protocol for sending IP datagrams over dial-up links
(from 1988) Encapsulation, not much else
PPP (Point-to-Point Protocol):• Successor to SLIP (1992), with added functionality• Used for dial-in and for high-speed routers
HDLC (High-Level Data Link) :• Widely used and influential standard (1979)• Default protocol for serial links on Cisco routers• Actually, PPP is based on a variant of HDLC
18
PPP - IP encapsulation The frame format of PPP is similar to HDLC and the 802.2 LLC frame
format:
PPP assumes a duplex circuit Note: PPP does not use addresses Usual maximum frame size is 1500
7E
flag
1
FF
addr
1
03
ctrl
1 2
protocol
<= 1500
data
2
CRC
7E
flag
1
0021 IP datagram
C021 link control data
8021 network control data
19
Additional PPP functionality
In addition to encapsulation, PPP supports: multiple network layer protocols (protocol multiplexing) Link configuration Link quality testing Error detection Option negotiation Address notification Authentication
The above functions are supported by helper protocols: LCP PAP, CHAP NCP
20
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.