Link Layer 3

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. 8 8 48 48 8 32 24 Start of Dest Src End of Control Body CRC Status frame addr addr frame 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

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. 1 0 Data stream: 1010 1 0 Random sequence: 0100101101011001 1 0 XOR of the two: 1011101110101001 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

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) • 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. A B C D Collisions Avoidance • Similar to Ethernet • Problem: hidden and exposed nodes 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 802.2 LLC frame format: • PPP assumes a duplex circuit • Note: PPP does not use addresses • Usual maximum frame size is 1500

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.