Flashcards
Ethernet II
Cards in this deck (55)
- 1Timing (Ethernet)Worst case scenario: 2 stations at furthest points in network
FOR 802.3:
10Mbps
Max distance 2500m
Max repeaters 4
Round trip delay 51.2us
THEREFORE: frame must be at least 512 bits long or station may finish transmission before a collision is detected - 2Standard Ethernet10BASET
Normally wired as a star network (logically a bus topology) and uses a central hub
In case of collision, hub sends jamming signal and station use back off algorithm to determine when to send again - 3HubUsed as a concentrator that accepts signals on one line and transmits them to all ports except the one which the signal came in on
- 4BridgeDevice that creates a single aggregate network from multiple comm networks or segments
Used to overcome the shared media problem aka high rate of collision by create a BRIDGE (two port switch).
Contains MAC addresses from either side of bridge. If MAC addr in table, wont pass over frame to other side (means its local) - 5MAC and IP BroadcastMAC Broadcast - FFFFFFFF
IP Boradcast: If host portion all 1's then it is broadcast addr.
aka x.x.255.255 - 6Fast Ethernet I (802.u)Different from standard Ethernet in aspects of cabling options, encoding and network spans.
=>Transfers data in nibbles
=>25Mhz clock speed = 100Mbps
=>Supports 10 and 100 Mbps via reconcilation sub layer that maps between nibble and bit transfer - 7Fast Ethernet II (802.u)Structure:
Media Independant Interface
+Separates the MAC layer from PHYS layer
=====>Permits MAC to operate unchanged over various media - 8Fast Ethernet III (802.u)Five sub layers in phys layer:
1)PCS Physical coding SL: encoding, transmit/receive collision detect; encoding diff each media cos unique PCS
2)PMA: serialisation and extracts timing info
3)PMD: Electrical optical specifications - 9Fast Ethernet IV (802.u)Functionality:
>Allows interoperability between 10 and 100 networks
>Auto-negotiate func
>CSMA/CD is sensitive to round trip prop delay to fram trans time ratio - should be small for good performance.
>SO Tf > Tprop otherwise entire frame may be transmitted before collision detected.
>Increasing rate by 10 either means: min frame size increases to 640 bytes OR network length decreases to 200m - 10AUTO NEGOTIATION I10BaseT includes link testing feature
In absence of traffic, sends 100ns pulse every 16ms.
-->LINK FAIL = no pulse within 50-150ms
-->Link is re-established when two consecutive link test pulses/single data packet received
-->Encoded in pulse train called FLP fast link pulse burst. - 11AUTO NEGOTIATION IIFLP consists of 33 pulse position that provide clock and data info.
-17 odd numbered pulses = clock info
-16 even number pulses = data info - 12Data EncodingProblem with sending raw 0's and 1's as long stings tend to loose synchronicity
Use MANCHESTER encoding or DIFFERENTIAL MANCHESTER ENCODING - 13Manchester Encoding ISelf-clocking encoding method which always transitions at the middle of each bit period
-0 expressed as low-high transition and 1 by high-to low
-0 or 1 signify 0 or 1 occur at midpoint
-Transitions at start of period are overhead, don't signify data - 14Manchester Encoding IIAD:
-Self clocking and very easy to synch
DIS:
-Confusion in data aka if all 0's does that mean idle or no data
-Uses double the bandwidth - 15Differential Manchester Encoding I-Transition is guaranteed at least once every bit for robust clock recovery
-Unlike Manchester, presence of transition is important, not polarity of it.
-0 = transition in middle of the bit period
-1 = no transition - 16Differential Manchester Encoding IIAD:
-Coding scheme works exactly the same if inverse
-No confusion with meaning of 0's
-More immunity to noise
DIS:
-Takes up twice the bandwidth - 17NRZI0 = transition in middle of period
1 = remain at same level
Transition in middle only when data = 1 - 188BSTSends 8 bits every 6 ternary signals
- 19LAN Addresses IPv432 Bit IP Address:
-network layer addr
-Used to get datagram to destIP network
LAN MAC addr:
-48bit address embedded in NIC
-Used to get one datagram from one interface to another phys connected int - 20LAN addresses IIEach NIC has a unique MAC address
MAC addr allocation handled by IEEE
Analogy: MAC = student num, IP = postal code
>MAC has flat addr = portability; can move LAN card from one LAN to another
>IP hierarchical addr NOT portable = depends on IP network which node is attached - 21ROUTINGThis is how we get packets and frames to their destinations
Protocols determine best know path to dest, who is closer based on IP addr
==>Determine if A and B are on same network by comparing subnet mask with dest IP addr. If not on same network, so the MAC addr of router will become the MAC dest. Router then receives it, looks at IP, says they know where that is, then sends out with source MAC as MAC of router, and dest MAC as either that of the next router or phys connected PC. - 22ARPAddress Resolution Protocol
>Each IP node on LAN an ARP table
>ARP table: IP/MAC addr mapping for a LAN node;
>If don't know MAC addr of dest, look at table, and if not there, sends an ARP request out to everyone who either accepts or denies. The one that replies, will send an ARP reply back to the sender via unicast containing is MAC addr to be used as the destination - 23TTL Time TO LiveTime after which address mapping will be forgotten (20 min)
- 24Trace RouteSend out echo request, and give back answer. Decrements TTL every time pass a location
- 25Routing To Another LANA creates datagrame with source A, dest B
A uses ARP to get R's MAC
A creates linke layer frame with R's MAC addr as dest, frame contains A-to-B IP datagram
R data link layer receives frame
R removes IP datagram from Ethernet frame, sees that it is destined for B
R uses ARP to get B's physical location addr
R creates frame containing A-B datagram and sends to B or router that my know B - 26Characteristics of wireless LANAD:
-Very flexible within reception area
-AD-HOC networks w/o prev planning is possible
-almost no wiring required
-more robust against disasters
DIS:
-typically very low bandwidth (1-54Mbits) due to shared medium
-products have to follow national restrictions - 27IEEE 802.11 Wireless LANDesign Goals:
-Global, seamless operation
-Low power for battery use
-No special permissions or licenses to use
-Robust transmission tech
-Security (no one should be able to read data), privacy and safety - 28802.11 DefinitionsBSS Base Service Set
>Group of stations that connect together through an access point
>Stations in BSS can communicate with each other (located within Basic Service Area BSA)
ESS Extended Service Set
Multiple inter-connected BSS's via Distributed System DS - portals in DS provide internet access - 29Architecture of infrastructure networkSTA Station: terminal with access mechanisms to the wireless and radio contact to access point
BSS: group os stations using same radio freq
Access Point: Station integrated into wireless LAN and DS
DS: interconnected network to form one logical network - 30Architecture of ad-hoc networkStation STA - terminal access to mechanisms to the wireless medium
Independant Basic Service Set (IBSS) - group of stations using same radio freq - 31Distribution ServicesStations within BSS can comm directly with eachother
DS:
Transfer MAC SDU's between AP's and ESS
-Transfer MSDU's between portals and BSS's in ESS
-Transfer MSDU between station in same BSS (broadcast/multicast) - 32Infrastructure ServicesSelect AP and establish association with AP
-> then can send receive frames via AP and DS
RE-ASSOCIATION: move from one AP to another AP
DISASSOCIATION: terminate association
Authentication service to establish identity of other stations
Privacy service to keep contents secret - 33Hidden Node ProblemCollision can occur that no one can detect because Node A is out of signal of Node B so neither can detect when they are sending
- 34CSMA/CAIf send something, way to overcome hidden node issue is that one node sends a 'request to send' message to the dest node. When it receives that, it will put up a clear to send. If the hidden node sees that, it will not send that as the dest computer has signified it is going to receive something already
- 35802.11 Layers and FunctionsMAC - access mechanism, fragmentation, encryption
MAC Management - synchronisation, roaming
MIB - power management
PLCP - Physical Layer Convergence Protocol, clears channel for assessment signal.
PMD - physical medium dependant, modulation and coding
PHY Management - channel selection
Station Management -coord of all management functions - 36802.11 (Legacy)FHSS - Frequency hopping spread spectrum: spreading and de-spreading, signal strength 1mbit, min hops 2.5
DSSS Direct Sequence Spectrum: DBPSK modulation for 1Mbit, DQPSK for 2/bit - 37MAC ServicesContention Service - best effort
Contention Free Service - time bounded transfer
MAC can alternate between contention periods CPs and Contention Free Periods CFPs
DIS: Can end up with a long ass waiting queue, with heavy loaded networks however, this becomes useful - 38802.11 MAC Layer 1 - DFWMACDFWMAC-DCF CSMA/CA collision avoidance via randomised back off mechanism
DFWMAC-DCF with RST/CST
Distributed Foundation Wireless MAC, avoids hidden terminal problem - 39MAC Layer IIProperties: defined through different inter frame spaces
SIFS: short inter frame spacing; highest priority for ACK, CST, polling response
PIFS: Medium priority
DIFS: lowest priority for asynched data services - 40802.11 CSMA/CA Access Method I-If station ready sends carier sense to start sensing medium
-If medium free for duration of Inter-frame space, station can start sending
-if medium bust, station waits for free IFS, then the station must additionally wait a random backoff time
-If another station occupies medium during back off time, backoff timer stops - 41802.11 CSMA/CA access method IISending Unicast Packets:
-Station sends RTS with reservation param after waiting for DIFS (determines amount ogf time packet needs the medium)
-ACK via CST after SIFS by receiver
-Sender can now send data at once, ACK
-Other stations make reservations via RTS and CTS - 42Physical Carrier SensingAnalyse all detected frames
Monitor relative signal strength from other sources - 43Virtual Carrier Sensing at MAC sublayerSource stations informs other stations of transmission time
Carried in duration field of RTS and CTS
Stations adjust Network Allocation Vector to indicate when channel becomes idle - 44Collision, Error, LossesCollision Avoidance
>When station senses channel busy, it waits until channel becomes idle for DIFS period and then begins random backoff.
>Station transmits frame when backoff timer expires
>If collision occurs, recompute backoff over interval twice as long
Receiving station of error free frame ACKS back -> sending station interprets non arrival of ACK as a loss - 45PCF Point Coordination FunctionPCF provides connection oriented, contention free services through polling. A PC point coordinator in AP performs PCF
- 46CFP - Contention Free PeriodCFP
Determine freq which CFP occurs
Initiated by beacon frame transmitted by PC in AP
During CFP stations may only transmit to respond to a poll from PC or to send ACK - 47Frame TypesManagement Frames: Deals with station association and de-association, timing and synch, authentication
Control Frames: hand shaking and ACKS during data transfer
Data Frames: Data transfer - 48FRAME STRUCTUREMAC header - 30 bytes
Frame body - 0-2312 bytes
CRC - 4 bytes - 49802.11 MAC ManagementSynchronisation - try to find LAN and stay within LAN via timer
Power Management - sleep-mode without missing a message; periodic sleep, frame buffering
Association and Re-association- integration into LAN, roaming (change networks by changing access points)/scanning
MIB Management Info Base - managing, read, write - 50Beacons I>Beacon frames is management frame
>Contains info about work and transmitted periodically to announce presence of wireless LAN and to synch members of service set
>Beacon frames transmitted by access point in BSS basic service set - 51Beacons IIBeacon Interval - amnt of time between beacon transmissions. Before station enters power save mode, need beacon interval to know when to wake up and receive beacon
Time stamp - adter receiving beacon, timestamp value used to update local clock - process enables synch all stations associated with same access point - 52Beacons IIISSID Service Set Identifier - identifies specific wireless LAN. Station mus have same SSID as access point
Supported Rates - each beacon carries info that describe rates particular wireless LANS can support - 53Beacons IVParam Sets - beacon includes info about specific signalling methods e.g. beacon belonging to frew hopping network would indicate hopping pattern and dwell time
Capability info - signifies requirements of station that wish to belong to wireless LAN that beacon represents
Traffic Indication Map - access point periodically sends TIM within beacon to identify which stations are using power saving mode have data frames waitinrg for them in the access points buffer. TIM identifies a station via association ID that the access point used during the association process - 54Power MangementSwitch off station when not needed
TSF Timing Synchronisation Function > stations wake up at same time
Infrastructure: TIM - lists unicast receivers transmitted by AP; DTIM - list of broadcast/multicast receivers
Adhoc: Adhoc TIM - announcement of receivers by stations buffering frames, more complicated, collision of ATIMs possible - 55802.11 RoamingScanning - scan environment i.e listen to medium for beacon signals
Re-association Request - station sends a request to one or several APs
Re-association Response:
success (AP has answered and station can now participate)
failure (continue scanning)
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- Flashcards on Ethernet II, created by Eric Wojcik on 11/06/2018.
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- Eric Wojcik
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