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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.
Functions of the Data-Link layer
•Transfer data between devices on the samenetwork



•Detect and possibly correct errors thatmay occur in the Physical Layer

A network’s architecture consists of a:
Physical topology: physical structure of the network.



Logical topology: the way data passes through the network.




Access methodology.

Ethernet

Physical Topology: Historically—bus, currently—star




Logical topology: Broadcast




Access methodology: CSMA/CD

Topology:

The shape of a communication system

Three most popular topologies for LAN:

Bus




Ring




Star

LAN Logical Topology(Sequential}

point-to-point link

LAN Logical Topology(Broadcast)

Message is transmitted to all connected stations




All computers on the network get the packet

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

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

Multiple Access protocols

Distributed algorithm that determines how nodes share channel,




i.e., determine when node can transmit

collision

if node receives two or more signals at the same time

MAC Protocols

Channel Partitioning




Random Access




“Taking turns”

Channel Partitioning

divide channel into smaller “pieces” (time slots, frequency, code)




allocate piece to node for exclusive use

Random Access

channel not divided, allow collisions


“recover” from collisions

“Taking turns”

nodes take turns, but nodes with more to send can take longer turns

Channel Partitioning MAC protocols

TDMA: time division multiple access




FDMA: frequency division multiple access

TDMA: time division multiple access

access to channel in "rounds“.




each station gets fixed length slot (length = pkt trans time) in each round.

FDMA: frequency division multiple access

channel spectrum divided into frequency bands




each station assigned fixed frequency band

Random Access Protocols

When node has packet to sendtransmit at full channel data rate R.


no a priori coordination among nodes

random access MAC protocol specifies:

how to detect collisionshow to recover from collisions (e.g., via delayed retransmissions)

Examples of random access MAC protocols:

CSMA,




CSMA/CD,




CSMA/CA

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!

CSMA collisions

entire packet transmission time wasted

Token passing:

control token passed from one node to next sequentially.

Error detection techniques

Parity (VRC)·




Longitudinal Redundancy Checks (LRC)




·Checksums ·




Cyclic Redundancy Checks (CRC)

Parity

the simplest error detection technique.

Longitudinal Redundancy Checks (LRC)

seek to overcome the weakness of simple, bit-oriented one directional parity checking..

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.

CRC sender procedure Step 1:

User data is known, technology specifies a divisor with n + 1 bits

CRC sender procedure Step 2

At sender, add n zeros to the end of the data (divisor has n+1 bits)

CRC sender procedure Step 3

At sender, perform modulo-2 division of


appended data with divisor

CRC sender procedure Step 4:

At sender, append remainder to data as CRC and send to receiver

Modulo-2 division rules

0 – 0 = 0


0 – 1 = 1


1 – 0 = 1


1 – 1 = 0

Error Correction

The receiving device detects the error and requests a re-transmission.




The sending device then retransmits the portion that contained the error.

Ethernet frame structure

Previous sections show destination address and CRC fields in Ethernet

Ethernet fields

Preamble




Start Frame Delimiter

Preamble

Allows receiver to synchronize with sender

Start Frame Delimiter

Indicates start of frame

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

Hexadecimal notation

Address broken up into 12 4-bit blocks




Each 4-bit block is represented as a hexadecimal digit 0-f

Ethernet fields

Length


Data

Type of service

Packets with higher value should get higher priority




But generally not currently implemented

Total length

Size of packet, including header and data




Maximum packet size of 65,536 bytes

ID

Used to re-assemble packet if it is fragmented by intermediate routers




All fragments will have the same ID

Flags

Indicates whether packet may be further fragmented, and whether it has in fact, been fragmented

Fragment offset

Location of packet with respect to TCP datagram

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

Protocol

Indicates protocol of IP user technology




Specified in RFC 790

Header Checksum

Calculated on the header




Ensures correctness of source and destination addresses throughout transmission




Recomputed by every router because TTL changes

Options

Can be used to indicate source routing

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

IP Addresses - structure

IP addresses are split into network part and host part

Network part

identifies the network (autonomous system) to which the address belongs

Host part

identifies the host within the network

CIDR

Classless Inter-Domain Routing

Network Address Translation (NAT)

Method by which IP addresses are mapped from one address block to another, providing transparent routing to end hosts.

Regional registries

IP addresses distributed around the globe

Obtaining IP addresses

Registries prefer allocating large address pools to large carriers

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

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

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

Use of subnet mask

Subnet mask helps routers




Subnet mask helps determine whether the destination host is on same subnet

Routing

the process of moving data across network segments toward its final destination.

Switchers

Switching on the second layer (data link layer).




Using Mac addresses.




Finding a right outgoing port based on the switching table.

Routers

Switching on the third layer (network layer).




Using IP addresses.




Selecting a best route based on the routing table.

Routing table

routers use it to decide where to send packets by looking up the destination address.

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.

Route selection

Routers use routing costs (metrics) to compare alternate paths to the same destination

Metric for a path

the sum of metrics for all component paths.

Autonomous systems

the unit of Internet routing

Exterior gateway protocols:

each router advertises its connection with destination networks to all other routers.

Interior gateway protocols:

each router distributes its connection information with destination networks to immediate neighbors.

MPLS

Multi-protocol label switching (MPLS) eliminates unnecessary processing at routers.

MPLS simplifies routing in 2 ways

Eliminate processing of unnecessary header fields




Routing decision only made once per network per packet

IP version 6 overview

Primarily expands source and destination address fields




Also simplifies packet processing at routers