Week 5 Flashcards
What are the three components of CSMA/CD Media Access Control (MAC) protocols?
CS
MA
CD
What is the difference between shared ethernet and switched ethernet?
Shared ethernet has all hosts connected to the SAME bus and COMPETE for bandwidth
Switched ethernet has one or more direct, point-to-point connections between hosts/network segments.
What is the media access control mechanism of IEEE 802.11 wireless network, CSMA/CA?
CSMA works by checking that channels are clear before sending packets across a network.
This ensures there are no collisions of data.
What is LAN?
Local Access Network
A network of computers that are directly connected to each other.
What is WLAN?
Wireless local access network
A network of computers that are directly connected to each other.
What are properties of (W)LAN?
- limited area
- circuits owned by the org
- can be operated without obtaining license
Difference between a forwarding table and routing table?
- Forwarding table use MAC address; routing table uses IP address
- Routing table connects different subnets
What are the types of switches?
- Cut through
- Store and forward switching
- Fragment free switching
What is the ethernet frame structure?
Preabmle Destination Address Source Address Type/Length Data FCS
Characteristics of cut through switching?
- transmit as soon as destination address has been read
- low latency, but may transmit frames that have errors
Characteristics of Store and forward switching?
switch waits until entire frame is received, performs error control, then transmits
higher latency, but less capacity wasted with errors
Characteristics of fragment free switching?
- reads first 64 bytes (contains header)
- if OK, begin transmitting
- compromise between the other two approaches
Characteristics of fragment free switching?
- reads first 64 bytes (contains header)
- if OK, begin transmitting
- compromise between the other two approaches
What is throughougput?
The total amount of user data transmitted in a given period of time
What are bottlenecks?
A point in the network where congestion happens.
This is where the network or device cannot handle the demands of the network
Often resutls in dropped packets
How to find bottlenecks?
Check sever utilization
if high (>60%): server is bottleneck
if low (<40%): the circuit is the bottleneck
if moderate: both
Improving server performance?
Software improvements • fine tune network and NOS parameters, e.g. • memory used for disk cache • available buffer space • number of simultaneously open files
Hardware improvements • add second server (load balancing) • upgrade server's CPU and/or memory • add more hard disks • add second NIC to existing server
Improving disk performance?
Important!
• disks are the slowest components in a server
• many server tasks require disk access
RAID
• Redundant Array of Inexpensive Disks
• links several hard drives together
• can improve performance (read/write from/to several drives
simultaneously)
• can improve reliability (duplicate files across drives, when a
drive fails, another one still contains the files)
Improving circuit capacity?
Option 1: upgrade to better standard
• e.g. from 100BASE-T to 1000BASE-T
• requires new NICs and new switches, often new cables
• some switches provide e.g. 16 1000BASE-T ports plus 2 10GbE
ports: only upgrade path to the server
Option 2: segment the network
• add second NIC to server
• increase number of parallel paths to the server
How to segment a network?
- Add a second NIC
- Split into two LANs
- Add rounter to connect LANs
How to reduce network demand?
Move files to clients
• e.g. heavily used software packages
Install disk caching software on clients
• can store often used files locally
• reduces need to access file server
Spread out tasks to off-peak time
• let automated tasks such as backups run e.g. at night
What is CS?
CS = carrier sense
What is MA?
Multiple access =
Why CSMA/CD for wired?
Access point can detect which devices are talking and points to ONE device to talk at once. Then the next and so on.
What is CD?
Collision detection =
Why CSMA/CA for wireless?
Hidden node problem!
Collisions MUST be avoided
