1. Mozafar Bag-Mohammadi University of Ilam
Link Layer 3
Mozafar Bag-Mohammadi
University of Ilam

2. Token Ring Overview Examples
16Mbps IEEE (based on earlier IBM ring)
100Mbps Fiber Distributed Data Interface (FDDI)

3. Token Ring (cont) Idea Frame Format
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 24
CRC
Start of
frame
End of
Dest
addr
Body 48
Src
Status 32

4. Wireless LANs IEEE 802.11 Physical Media
spread spectrum radio (2.4GHz)
diffused infrared (10m)

5. Spread Spectrum Idea Frequency Hopping Direct Sequence
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
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
defines an 11-bit chipping code
Random sequence:
Data stream: 1010
XOR of the two: 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

10. Overview, 802.11 Architecture
ESS
Existing Wired LAN AP AP
STA
STA
STA
STA
BSS
BSS
Infrastructure Network
STA
STA
Ad Hoc Network
Ad Hoc Network
BSS
BSS
STA
STA

11. Mobility (cont) Scanning (selecting an AP) modes
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 A B C D
Hidden node
Exposed node
Sending node

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. 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

17. 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 LLC frame format:
PPP assumes a duplex circuit
Note: PPP does not use addresses
Usual maximum frame size is 1500
PPP suports an asynchronous link (8 data bits/no parity) or a bit-oriented synchronous link

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.