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110 lines
3.6 KiB
Markdown
110 lines
3.6 KiB
Markdown
---
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title: "22-LAN-physical-layer"
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aliases:
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tags:
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- cosc203
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- lecture
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sr-due: 2022-10-22
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sr-interval: 3
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sr-ease: 250
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---
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# LAN vs WAN
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Local area network
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- a network connecting computers within a limited geographic area
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- e.g., building, school, home etc
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- wired or wireless
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Wide Area network
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- a network that extends over a large area such as town, country, region or the world
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- used to connec LANs and other types of networks together
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- often established with leased telecommunication circuits (e.g., chorus)
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## ethernet
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dominant wired tech
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- first used LAN tech
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- simple cheap
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- IEEE 802.3 standard
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- kept up with speed race: 10 Mbps - 400 Gbps
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- physical topology
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- bus: all nodes in the same collision domain can collide (old)
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- switched: active link-layer switch in center (new)
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frame structure
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- encapsulates IP datagram (or other network layer protocol packet) in ethernet frame
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- 
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- preamble: used to sync reciever and sender clock rates
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- 7 bytes of 10101010 followed by one byte of 10101011
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- addresses: 6 bytes source, destination MAC addr
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- if adapter recieves frame with matching MAC destination address, it passes data in frame to network layer protocol, otherwise adapter discards frame
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- type
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- most IP but other possible: e.g., Novell IPX, Appletalk
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- used to demultiplex up at reciever
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- CRC: cyclic redundancy check at reciever
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- error detected: frame is dropped
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- it can correct but doesn't because just dropping and resending is faster
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# ARP
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Address resolution protocol
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arp table: each IP node on lan has table
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- IP/MAC mappings for some LAN nodes
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- mapping is forgotten after TTL is exceeded (usually 20min)
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# Switch vs Router
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switch:
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- link layer deivce
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- takes an active role
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- store and foward ethernet frames
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- examine incoming frame's MAC addr, selectively forward frame to one-or-more outgoing links when frame is to be fowarded on segment, uses CSMA/CD
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- transparent
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- hosts unaware of presence of switches
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- plug and play, self-learning
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- switches do not need to be configured
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multiple simultaneous transmissions
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- hosts have dedicated direct connection to switch
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- switches can buffer packets
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- ethernet protocol used on each incoming link so:
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- no collisions: full duplex
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- A to A' and B to B' can transmit simultaneously, without collisions
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- each link is its own collision domain
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- A to A' and C to A' cannot happen simultaneously
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- each swtich has a switch table, each entry:
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- mac of host, inferface, time stamp
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- looks like a routing table
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self-learning
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- learns which hosts can be reached through which interface
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- when frame is recieved, switch records the sender/location pair in table
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- filtering/forwarding
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- if frame dest is unknown: flood
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- if known send just one link
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## vs routers
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- both store-and-foward
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- R: network layer
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- S: link layer
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- both have forwarding table
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- R: routing using IP and routing algorithms
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- S: learn forwarding table using flooding, learning, MAC
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# physical layer
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- transmit and recieve bit streams over a physical transmission medium
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- conversion betwen digital bits to analogue
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- define characteristics linke, voltage, data rate etc
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# All layers together
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- connect to internet:
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- need to get IP addr of:
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- itself
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- first hop router
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- DNS server
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- uses DHCP: dynamic host configuration protocol
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- DHCP is inside UDP inside IP inside Ethernet frame
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- frame is broadcast on LAN with dest FFFFFFFFFFF, recieved at router running DHCP server
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- DHCP creates a DHCP ACK with the info and sends it back to the client
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- can allocate temporaty IP address |