Unit3_Link - Data Link Layer 3-1 Unit 3 Data Link Layer and...

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Unformatted text preview: Data Link Layer 3-1 Unit 3 Data Link Layer and LAN After studying this unit, you will be able to 1. understand the principles behind data link layer services: 2. explain the principles of LANs. Data Link Layer 3-2 Unit 3: Outline ¡ 3.1 Introduction and services ¡ 3.2 Flow Control and Retransmission Protocols ¡ 3.3 PPP and HDLC ¡ 3.4 Multiple access protocols ¡ 3.5 Ethernet ¡ 3.6 Hubs and switches ¡ 3.7 Wireless LAN Data Link Layer 3-3 Unit 3.1 Introduction Data Link Layer 3-4 Link Layer: Introduction Some terminology: ¡ hosts and routers are nodes ¡ communication channels that connect adjacent nodes along communication path are links ¢ wired links ¢ wireless links ¢ LANs ¡ layer-2 packet is a frame , encapsulates datagram ¢ datagram is a layer-3 packet. “link” data-link layer has responsibility of transferring datagram from one node to adjacent node over a link Data Link Layer 3-5 Link layer: context ¡ Datagram transferred by different link layer protocols over different links: ¢ e.g., Ethernet on first link, frame relay on intermediate links, 802.11 on last link ¡ Each link layer protocol provides different services ¢ e.g., may or may not provide reliable data transfer (rdt) over link • “Reliable” means no error, no packet loss. Data Link Layer 3-6 Link Layer Services ¡ Framing ¢ encapsulate datagram into frame, adding header and trailer ¡ Link Access ¢ channel access if shared medium ¢ “MAC” addresses used in frame headers to identify source, dest • different from IP address! Data Link Layer 3-7 Link Layer Services (more) ¡ Flow Control ¢ pacing between adjacent sending and receiving nodes • if Tx sends too fast, buffer overflows at Rx. ¢ To solve the problem, feedback from Rx can be used. • Rx might say: “You may send me n frames now, but after they have been sent, don’t send any more until I have told you to continue.” ¡ Error Detection and Correction ¢ errors caused by signal attenuation, noise. ¢ receiver detects presence of errors: ask sender for retransmission or drops frame ¢ receiver identifies and corrects bit error(s) without resorting to retransmission ¢ studied in Unit 2. Data Link Layer 3-8 Link Layer Services (more) ¡ Reliable delivery between adjacent nodes ¢ seldom used on low-bit-error link (fiber, some twisted pair) ¢ wireless links: high error rates ¢ if error or packet loss is detected, Rx may ask Tx to retransmit. ¡ Remark: Flow control and rdt are also services provided by transport layer. ¢ We will discuss TCP flow control in Unit 5. Data Link Layer 3-9 Adaptors Communicating ¡ link layer implemented in “adaptor” ¢ a.k.a. Network Interface Card (NIC) ¢ Ethernet card, PCMCIA card, 802.11 card ¡ sending side: ¢ encapsulates datagram in a frame ¢ adds error checking bits, rdt, flow control, etc....
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This note was uploaded on 05/01/2011 for the course ELECTRICAL EE3015 taught by Professor Albertsun during the Spring '11 term at City University of Hong Kong.

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Unit3_Link - Data Link Layer 3-1 Unit 3 Data Link Layer and...

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