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

Token Ring Overview Examples 16Mbps IEEE 802.5 (based on earlier IBM ring) 100Mbps Fiber Distributed Data Interface (FDDI)

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

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

Spread Spectrum Idea Frequency Hopping Direct Sequence spread signal over wider frequency band than required originally designed to thwart jamming Frequency Hopping Direct Sequence

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

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 1

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.

Supporting Mobility Case 1: ad hoc networking Case 2: access points (AP) tethered each mobile node associates with an AP

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

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

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

Hidden & Exposed nodes

Collisions Avoidance Similar to Ethernet Problem: hidden and exposed nodes A B C D Hidden node Exposed node Sending node

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.

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

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

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 PPP suports an asynchronous link (8 data bits/no parity) or a bit-oriented synchronous link

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

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.