Week 3: 1 Flashcards
What computer communication patterns do the principle of locality of reference help predict:
- Spatial locality of reference:
computers likely to communicate with other computers that are
located nearby - Temporal locality of reference:
computers are likely to communicate with the same computers
repeatedly
Thus - LANs are effective because of spatial locality of
reference, and temporal locality of reference may give insight into which computers should be on a LAN
What is ethernet?
- Single coaxial cable
- Widely used LAN technology
- Invented at Xerox PARC in 1970s
- One Ethernet cable is sometimes called a segment which is sypically limited to a maxium of 500 meters in length
What is CSMA?
- No central control managing when computers transmit
- Ethernet employs Carrier Sense Multiple Access to coordinate transmission among multiple attached computers
Multiple access:
* multiple computers are attached and any can be
transmitter
Carrier sense:
computer wanting to transmit tests for carrier before
transmitting
What does CD - Collision Detection do?
- Two computers may transmit simultaneously
- Signals from two computers will interfere with each other. Overlapping frames is called a collision
- Ethernet interfaces have hardware to detect transmission and monitor the outgoing signal
- A garbled signal is interpreted as a collision and if a collision is detected, a computer stops transmitting
List the steps of a collision recovery:
- Ethernet uses CSMA/CD to coordinate transmissions.
- Computer that detects a collision sends special signal to force all other interfaces to detect collision.
- Computer then waits for ether to be idle before transmitting. But if both computers wait same length of time, frames will collide
again. - Standard specifies maximum delay, and both computers choose random delay less than maximum
- After waiting, computers use carrier sense to avoid subsequent collision
How does exponential backoff work?
Ethernet uses Binary Exponential Backoff algorithm where computers double the delay with each subsequent collision.
What is ethernet 10Base5 - Thicknet?
- Uses thick coaxial cable
- Also called thicknet
- Operatess at 10 Mbps
- Uses digital signal
- 500 meters is the maximum segment length
- Up to 5 segments connected with repeaters
- uses AUI cables, transceivers and terminators
What is 10Base2 - Thinnet?
- Uses thin coaxial cable
- Also called thinnet
- Operates at 10 Mbps
- Uses digital signal
- 185 meters is the maximum segment length
- Uses BNC connectos and terminators
- 100Base2 is similar but operates at 100Mbps
What is ethernet 10BaseT?
- Also called Twisted Pair Ethernet
- Replaces the AUI cable with UTP
- Replaces thick coax with a hub
Many LAN technologies that use ring topology use token passing for synchronized access to the ring. What is a token?
It is a bit pattern that differs from normal frames.
Token ring hardware ensures that exactly one token exists in the ring and if it is lost it is regenerated.
How does a token ring operates?
A token circulates around the ring endlessly
CPU can’t process data at network speeds. What do computers use instead?
A card in the backplane Network Interface Card (NIC)
- Connector at back of computer then accepts cable to physical network
- NIC is built for one kind of physical network
- Some NICs can be used with different, similar hardware
What does LAN hardware do?
- ADDs hardware addresses, error detection codes, etc. to outgoing frames
- May use DMA to copy frame data directly from/into main memory
- Obeys access rules when transmitting
- Checks for errors on incoming frames
- Checks destination address on incoming frames
How are MAC addresses are assigned?
- They are usually a numeric value
- The size is selected by a specific network topology
- Hardware addreses must be unique on a LAN
- Static Addressing (hardware manufacturer assigns unique physical address)
