Networking and Telecommunications 4 Lan Local Area Networking

Why Lans? l Resources Sharing – – l Communications – l 2 File sharing

Lan Topology l 4 Layout of the systems along communications channel 3/8/2021

Bus Topology l l 5 Single cable run where all systems directly connect Original

Ring Topology l l l messages circulate in a loop logically a ring can

Star Topology l l l 7 Systems connected to a central switch/hub (or mau)

Channel Access Methods l 10 "Party Analogy" 3/8/2021

Polling l l l 11 Server polls each client One at a time Round

Contention l l Multiple systems contending for the same shared channel How do collisions

Aloha (contention) l l l 14 U. of Hawaii, use radio signals between islands,

CSMA / CD (contention) l l l 16 Carrier Sense Multiple Access with Collision

CSMA / CD (cont. ) l NIC, before transmitting a packet, compares voltage on

CSMA / CA (contention) l l 19 Carrier Sense Multiple Access with Collision Avoidance

Token Passing l l l l 20 Pass a special small packet, called a

Client-Server l l 23 Client systems loaded with same NOS (specialized client software, minimal).

Peer to Peer l l l 25 All systems loaded with the same NOS

Other Models Exist l l 27 Diskless workstations Thin clients Images. Create image of

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Networking and Telecommunications 4. Lan (Local Area Networking) 1 3/8/2021

Why Lans? l Resources Sharing – – l Communications – l 2 File sharing Data Printers etc. . Email central administration 3/8/2021

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Lan Topology l 4 Layout of the systems along communications channel 3/8/2021

Bus Topology l l 5 Single cable run where all systems directly connect Original technology, often used coax cable. Bidirectional signal. If cable breaks, all systems down Used within lans in the past. . . sometimes still used as a backbone today. 3/8/2021

Ring Topology l l l messages circulate in a loop logically a ring can be physically setup as a: – – – 6 Ring Star (typical) Bus 3/8/2021

Star Topology l l l 7 Systems connected to a central switch/hub (or mau) Most common today, uses TP cable In some sense, this is what the old mainframes/terminals used, also used by PBX/phone systems, electrical systems Each system has its own dedicated cable to central switch/hub If cable breaks, only one system is down Hub/switch becomes central point of failure 3/8/2021

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Channel Access Methods l 10 "Party Analogy" 3/8/2021

Polling l l l 11 Server polls each client One at a time Round robin Similar to mainframe/terminals Slow Not used often in lans 3/8/2021

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Contention l l Multiple systems contending for the same shared channel How do collisions occur? – – l 13 2 or more systems attempt to transmit at same time Propagation delay There are 3 contention schemes 3/8/2021

Aloha (contention) l l l 14 U. of Hawaii, use radio signals between islands, 2 frequencies (send and receive) “Talk whenever you want, even if someone else is talking" Send message, if no ACK, then resend When traffic was heavy, the system failed. Works best under light load (20% or less usage) 3/8/2021

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CSMA / CD (contention) l l l 16 Carrier Sense Multiple Access with Collision Detection multiple systems access the same communication channel, and will try to detect collisions most often used today Ethernet is based on this method Based on above Aloha scheme, "listen before talking" 3/8/2021

CSMA / CD (cont. ) l NIC, before transmitting a packet, compares voltage on network with a reference voltage: – – 17 if high - collision is occurring, back off and wait a random amount of time, and then try again if medium - someone is talking, don't talk, back off and wait a random amount of time and try again if low - channel is free, start transmitting message, but keep on listening (should hear a medium signal) If the signal jumps to high, then a collision has occurred, continue to transmit for a few more seconds (jam the network), and the back off, wait a random amount of time and resend packet 3/8/2021

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CSMA / CA (contention) l l 19 Carrier Sense Multiple Access with Collision Avoidance Used in old Mac Appletalk lans and some wireless lans today Client gets permission to talk to server, from server, before transmitting message Avoid collisions 3/8/2021

Token Passing l l l l 20 Pass a special small packet, called a token, around the network. Only when a system held the token, could it transmit. If you didn’t have the token, you didn’t transmit. Token would be passed around in a logical ring fashion, one system after another. If a system didn’t need to transmit, it just passed the token on to the next system. No contention, no collisions Method used by IBM's Token Ring network architecture Deterministic (CSMA/CD is non-deterministic). 3/8/2021

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Client-Server l l 23 Client systems loaded with same NOS (specialized client software, minimal). The full NOS is loaded on the server. Clients request files, servers share files over network (server has the applications and data files to be shared on servers hard drive) Applications run on clients (RAM) Mid-sized to large lans 3/8/2021

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Peer to Peer l l l 25 All systems loaded with the same NOS Each system can share files and request files Small lans 3/8/2021

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Other Models Exist l l 27 Diskless workstations Thin clients Images. Create image of a “good” client, containing o. s, all applications/data files. Store image on server. Download images to clients. (altiris or norton ghost) Hybrids, part client/server and part peer-topeer. Or part client/server and part images. 3/8/2021

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