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76 Cards in this Set
- Front
- Back
The Datalink Layer |
•The datalink layer providespoint-to-point connectivity between devices over the physical connectionsprovided by the underlying physical layer.
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Functions of the Data-Link layer
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•Transfer data between devices on the samenetwork
•Detect and possibly correct errors thatmay occur in the Physical Layer |
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A network’s architecture consists of a:
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Physical topology: physical structure of the network.
Logical topology: the way data passes through the network. Access methodology. |
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Ethernet |
Physical Topology: Historically—bus, currently—star Logical topology: Broadcast Access methodology: CSMA/CD |
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Topology: |
The shape of a communication system |
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Three most popular topologies for LAN: |
Bus Ring Star |
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LAN Logical Topology(Sequential} |
point-to-point link |
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LAN Logical Topology(Broadcast) |
Message is transmitted to all connected stations All computers on the network get the packet |
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Ethernet data reception |
In broadcast, the network does not direct the packet to its correct destination Instead, the packet is simply sent to every computer on the network Packet is sure to reach its destination Greatly simplifies the technology |
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Role of Destination Address |
Only intended receiver opens the message.
All other connected devices can ignore frame immediately upon based on the destination address. But increases the processing load on each computer on the network |
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Multiple Access protocols |
Distributed algorithm that determines how nodes share channel, i.e., determine when node can transmit |
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collision |
if node receives two or more signals at the same time |
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MAC Protocols |
Channel Partitioning Random Access “Taking turns” |
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Channel Partitioning |
divide channel into smaller “pieces” (time slots, frequency, code) allocate piece to node for exclusive use |
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Random Access |
channel not divided, allow collisions “recover” from collisions |
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“Taking turns” |
nodes take turns, but nodes with more to send can take longer turns |
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Channel Partitioning MAC protocols |
TDMA: time division multiple access FDMA: frequency division multiple access |
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TDMA: time division multiple access |
access to channel in "rounds“. each station gets fixed length slot (length = pkt trans time) in each round. |
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FDMA: frequency division multiple access |
channel spectrum divided into frequency bands each station assigned fixed frequency band |
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Random Access Protocols |
When node has packet to sendtransmit at full channel data rate R. no a priori coordination among nodes |
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random access MAC protocol specifies: |
how to detect collisionshow to recover from collisions (e.g., via delayed retransmissions) |
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Examples of random access MAC protocols: |
CSMA, CSMA/CD, CSMA/CA |
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CSMA/CD (Carrier Sense Multiple Access/Collision Detection) |
listen before transmit: If channel sensed idle: transmit entire frame If channel sensed busy, defer transmission human analogy: don’t interrupt others! |
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CSMA collisions |
entire packet transmission time wasted |
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Token passing: |
control token passed from one node to next sequentially. |
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Error detection techniques |
Parity (VRC)· Longitudinal Redundancy Checks (LRC) ·Checksums · Cyclic Redundancy Checks (CRC) |
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Parity |
the simplest error detection technique. |
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Longitudinal Redundancy Checks (LRC) |
seek to overcome the weakness of simple, bit-oriented one directional parity checking.. |
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A checksum is calculated by |
adding the decimal face values of all of the characters sent in a given data block sending only the least significant byte of that sum. |
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CRC sender procedure Step 1: |
User data is known, technology specifies a divisor with n + 1 bits |
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CRC sender procedure Step 2 |
At sender, add n zeros to the end of the data (divisor has n+1 bits) |
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CRC sender procedure Step 3 |
At sender, perform modulo-2 division of appended data with divisor |
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CRC sender procedure Step 4: |
At sender, append remainder to data as CRC and send to receiver |
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Modulo-2 division rules |
0 – 0 = 0 0 – 1 = 1 1 – 0 = 1 1 – 1 = 0 |
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Error Correction |
The receiving device detects the error and requests a re-transmission. The sending device then retransmits the portion that contained the error. |
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Ethernet frame structure |
Previous sections show destination address and CRC fields in Ethernet |
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Ethernet fields |
Preamble Start Frame Delimiter |
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Preamble |
Allows receiver to synchronize with sender |
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Start Frame Delimiter |
Indicates start of frame |
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Address |
48 bits in length All 1’s in the destination address is pre-defined to be the broadcast address on the LANAddresses may be universally administered |
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Hexadecimal notation |
Address broken up into 12 4-bit blocks Each 4-bit block is represented as a hexadecimal digit 0-f |
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Ethernet fields |
Length Data |
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Type of service |
Packets with higher value should get higher priority But generally not currently implemented |
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Total length |
Size of packet, including header and data Maximum packet size of 65,536 bytes |
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ID |
Used to re-assemble packet if it is fragmented by intermediate routers All fragments will have the same ID |
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Flags |
Indicates whether packet may be further fragmented, and whether it has in fact, been fragmented |
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Fragment offset |
Location of packet with respect to TCP datagram |
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Time to live |
Each router subtracts 1 from the field when a packet passes through it Packet is destroyed if TTL = 0 Helps clear stale packets from the network |
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Protocol |
Indicates protocol of IP user technology Specified in RFC 790 |
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Header Checksum |
Calculated on the header Ensures correctness of source and destination addresses throughout transmission Recomputed by every router because TTL changes |
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Options |
Can be used to indicate source routing |
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IP Addresses |
An address is a unique label that helps locate an entity on a network 32-bit values in source and destination address fields |
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IP Addresses - structure |
IP addresses are split into network part and host part |
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Network part |
identifies the network (autonomous system) to which the address belongs |
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Host part |
identifies the host within the network |
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CIDR |
Classless Inter-Domain Routing |
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Network Address Translation (NAT) |
Method by which IP addresses are mapped from one address block to another, providing transparent routing to end hosts. |
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Regional registries |
IP addresses distributed around the globe |
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Obtaining IP addresses |
Registries prefer allocating large address pools to large carriers |
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Why subnetting? |
Registries allocate large address blocks Subnetting enables the partition of a large address pool into multiple smaller blocks Subnetting allows organizations to distribute total pool of IP addresses in subnets in different ways |
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Subnet masks |
Sequence of 1’s followed by sequence of 0’s 1’s indicate network ID and subnet ID bits 0’s indicate host ID bits |
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What does a subnet mask do? |
The 0’s in the subnet mask block (mask) the corresponding bits in any destination address The 1’s in the subnet mask allow the corresponding bits to be seen |
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Use of subnet mask |
Subnet mask helps routers Subnet mask helps determine whether the destination host is on same subnet |
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Routing |
the process of moving data across network segments toward its final destination. |
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Switchers |
Switching on the second layer (data link layer). Using Mac addresses. Finding a right outgoing port based on the switching table. |
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Routers |
Switching on the third layer (network layer). Using IP addresses. Selecting a best route based on the routing table. |
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Routing table |
routers use it to decide where to send packets by looking up the destination address. |
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A routing table consists of: |
A series of destination networks The address of the local router that provides service to the destination network A cost associated with the route. |
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Route selection |
Routers use routing costs (metrics) to compare alternate paths to the same destination |
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Metric for a path |
the sum of metrics for all component paths. |
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Autonomous systems |
the unit of Internet routing |
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Exterior gateway protocols: |
each router advertises its connection with destination networks to all other routers. |
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Interior gateway protocols: |
each router distributes its connection information with destination networks to immediate neighbors. |
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MPLS |
Multi-protocol label switching (MPLS) eliminates unnecessary processing at routers. |
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MPLS simplifies routing in 2 ways |
Eliminate processing of unnecessary header fields Routing decision only made once per network per packet |
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IP version 6 overview |
Primarily expands source and destination address fields Also simplifies packet processing at routers |