COMPUTER NETWORKS Lecture9 Mr Husnain Sherazi Review Lecture

COMPUTER NETWORKS Lecture-9 Mr. Husnain Sherazi

Review Lecture 8 w w w CSMA/CD Wireless LANs and CSMA/CA Token Ring FDDI ATM

Chapter 9 Hardware Addressing and Frame Type Identification

Introduction w Need to devise technique for delivering message through LAN medium to single, specific destination computer w w Sending computer uses a Hardware Address to identify the intended destination of a frame w w Sending computer also identifies Type of data carried in the frame

Specifying a Destination w Data sent across a shared network reaches all attached stations - for all LAN topologies w Interface hardware detects delivery of frame and extracts frame from medium w But. . . most applications want data to be delivered to One specific application on another computer - not All computers

Hardware Addressing w Most network technologies have a hardware addressing scheme that identifies stations on the network w Each station is assigned a numeric Hardware Address or Physical Address

Hardware Addressing w Sender includes hardware address in each transmitted frame w Only station identified in frame receives copy of frame w Most LAN technologies include sender's hardware address in frame, too

LAN Hardware and Packet Filtering

LAN Interface w LAN interface handles all details of frame transmission and reception w Adds hardware addresses, error detection codes, etc. to outgoing frames w w May use DMA to copy frame data directly from main memory w w Obeys access rules (e. g. , CSMA/CD) when transmitting

LAN Interface – Checks error detection codes on incoming frames – May use DMA to copy data directly into main memory – Checks destination address on incoming frames • Station's address, a copy of the frame is passed to the attached computer – Frames not addressed to the local computer are ignored and don't affect the local computer in any way

Format of Hardware Addresses w Numeric value w Size selected for specific network technology w Length is one to six bytes

Assigning Hardware Addresses w Hardware addresses must be unique on a LAN w How can those address be assigned and who is responsible for uniqueness?

Assigning Hardware Addresses Static Configurable Dynamic Hardwar e manufacturer assigns permanent physical address to each network interface. Address can be set by end user, either manually (e. g. , switches or jumpers on the inte rface ) or electronically (e. g. , through software Interfac e automatically assigns physical address each time it is powered up Manufacturer must ensure every interface has a unique address System administrators must coordinate to avoid conflict Automatic scheme must be reliable to prevent conflicts

Broadcasting

Problems with Broadcasting w For every broadcast frame on the network, each computer uses computational resources: – Places contents into memory – Interrupts the CPU – Allows system software to make a decision whether to discard or use the frame

Problems with Broadcasting – If a pair of computers, use broadcasting, instead of sending them directly, all other computers waste CPU time while discarding the frames.

Multicasting w Solution: The restricted form of broadcasting: Multicasting: – It works like broadcasting – However it does not forward frames automatically to the CPU – The interface hardware is programmed in advance to accept certain frames that have multicast address as the destination address

Summary w w w Hardware Addressing Specifying a Destination Packet Filtering Assigning Hardware Addresses Broadcasting Multicasting