EE450-U7-DataLinkLayer-PartI-Nazarian-Spring10

EE450-U7-DataLinkLayer-PartI-Nazarian-Spring10 - University...

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University of Southern California Viterbi School of Engineering EE450 omputer Networks Computer Networks ata Link Layer Data Link Layer Shahin Nazarian Spring 2010
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Data Link Layer (DLL) – a Link to Link Protocol Every layer provides a set of services and provides it to the yer above it layer above it DLL (Layer 2) is a link-to-link basis (not end-to-end basis) protocol Receiver side needs to synch frame (to know where it starts where it ends) receiver is the other side of the link hysical layer transmits the bit Physical layer transmits the bit and makes sure no duplicate or missing bit exists, but DLL andles the errors (caused by Shahin Nazarian/EE450/Spring 2010 handles the errors (caused by noise or signal attenuation) 2
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Where Is The Link Layer (LL) Implemented? LL is implemented in each and every node uch of LL controller’s functionality Example: host link layer Much of LL controller s functionality is implemented in hardware link layer is implemented in “adaptor” ka r IC host schematic application transport (aka Network Interface Card or NIC card) E.g., Ethernet card (Intel’s 254 controller) PCM IA cpu memory host us network link 8254x controller), PCM-CIA card (plug-in to PC’s PCI card slot), 802.11 card (Atheros R 006 controller physical transmission bus (e.g., PCI) link physical AR5006) Implements link, physical layer Attaches into node’s system buses network adapter card (aka network Interface card) Shahin Nazarian/EE450/Spring 2010 Combination of hardware, software, and firmware 3
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Some DLL Services 1. Framing Frame synchronization, adding header and trailer and ncapsulating acket into frame encapsulating packet into frame 2. Error Error detection detection, done by the receiver node, using parity checks, FCS ( Frame Frame Check Check Sequence Sequence ,) etc. 3. Error control, Error control, done by receiver node, using one of the following: Drop the Frame eceiver simply drops the frame Receiver simply drops the frame ARQ ( Automatic Repeat Automatic Repeat reQuest reQuest ) protocols Error correction 4. Flow Flow control control (to prevent buffer overflow at the receiver, i.e., to ensure the receiver is not overwhelmed by the sender) top & ait Shahin Nazarian/EE450/Spring 2010 Stop & Wait Sliding Window Protocol 4
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Error Control, Flow Control Error Control 1 st technique [simplest approach]: In case error is detected, receiver will drop the frame. so automatically packet is dropped as well. Note that the transport layer still does error correction 2 nd technique: Receiver will request from the sender (Note: not the end node, but the node on the other side of the link) to retransmit using Automatic Repeat reQuest (ARQ) 3 rd technique: Receiver is not only able to detect the error, but also able to locate (i.e., correct) it (Most of the networking applications use dropping and ARQ instead of this technique) Flow Control A procedure that lets the receiver inform the sender to ot overwhelm it by too many frames (so sender would slow Shahin Nazarian/EE450/Spring 2010 not overwhelm it by too many frames (so sender would slow down) 5
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EE450-U7-DataLinkLayer-PartI-Nazarian-Spring10 - University...

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