dr. atiq ahmedcsit.uob.edu.pk/images/web/staff/lecture/doc-4.2019-10-4.no-5.pdf · 13/29 switching...

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Internet Architectures and Protocols

Dr. Atiq Ahmed

Department of Computer Science & Information TechnologyUniversity of Balochistan

Lecture

Dr. Atiq Ahmed (Dept. of CS & IT UoB) I A P August 30, 2019 1 / 29

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Outline

1 SwitchingPacket SwitchingCircuit SwitchingAddressing

2 Underlying technologiesUnderlying technologies


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Switching Packet Switching

Outline




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Packet Switching

Messages broken into smallerunits (packets)

Source & destination addresses inpacket header

Connectionless, packets routedindependently

Packet may arrive out of order

Pipelining of packets acrossnetwork can reduce delay,increase throughput

Lower delay than messageswitching, suitable for interactivetraffic


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Store-and-Forward Transmission

Packet switch must receive the entire packet before it can begin totransmit the first bit of the packet onto the outbound link


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Advantages of Packet Switching

Efficient use of Network

All the packets not follow same route in Packet Switching

High Data Transmission is very easy

Charges users only on the basis of duration of connectivity.

Uses digital network and enables digital data to be directlytransmitted toward destination


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Disadvantages of Packet Switching

In Packet Switching Packets arriving in wrong order

Takes Transmission delay

Requires Large amount RAM to handle large amount of datacommunication in packets

Switching Nodes require more processing power to reconstructpackets

Packets may be lost on their route, so sequence numbers arerequired to identify missing packets


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Packet Loss

A queue preceding a link has finite capacity that depends on:I Router designI Router cost

A packet can arrive to find a full queueI With no place to store such a packet, a router will drop that packet

i.e., the packet will be lostI Another reason for loss can be noise in any electronic circuit

Fraction of lost packets increases as the traffic intensity increases

A lost packet may be retransmitted on an end-to-end basis


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Throughput

Refers to the average data rate of successful data or messagedelivery over a specific communications link

Instantaneous Throughput at any instant of time is the rate (inbits/sec) at which any host is receiving the file

Average Throughput = FT bits/sec


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Switching Circuit Switching

Outline




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Circuit Switching

Resources needed to provide for communication between the endsystems are reserved for the duration of session

I Traditional telephone networks

Needs to first set up a dedicated, end-to-end path for theconnection before the information transfer takes place


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Circuit Switching


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Circuit Switching

Establish: source creates circuit to destinationI Node along the path store connection infoI Nodes may reserve resources for the connection

Transfer: source sends data over the circuitI No destination address, since nodes know path

Teardown: source tears down circuit when done


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Circuit Switching


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Advantages of Circuit Switching

Guaranteed bandwidthI Predictable communication performanceI Not ’best-effort’ delivery with no real guarantees

Simple abstractionI Reliable communication channel between hostsI No worries about lost or out-of-order packets


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Advantages of Circuit Switching

Simple forwardingI Forwarding based on time slot or frequencyI No need to inspect a packet header

Low per-packet overheadI Forwarding based on time slot or frequencyI No IP (and TCP/UDP) header on each packet


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Disadvantages of Circuit Switching

As the connection is dedicated it cannot be used to transmit anyother data even if the channel is free

It is inefficient in terms of utilization of system resources

As resources are allocated for the entire duration of connection,these are not available to other connections

Dedicated channels require more bandwidth

Prior to actual data transfer, the time required to establish aphysical link between the two stations is too long


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Switching Addressing

Outline




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Addressing

Four levels of addresses

I Physical addressI Logical addressI Port addressI Application-specific address


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Physical addresses

Also known as the link address, is the address of a node asdefined by its LAN or WAN

It is included in the frame used by the data link layer

The lowest level address and has authority over the link

The size and format of these addresses vary depending on thenetwork

Ethernet uses a 6-byte (48-bit) physical address that is imprintedon the network interface card (NIC)

LocalTalk (Apple) has a 1-byte dynamic address that changeseach time the station comes up


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Logical address

Necessary for universal communications that are independent ofunderlying physical networks

Uniquely identifies each host on a network regardless of theunderlying physical network

IP version 4 defines 32-bit address that can assign a uniqueidentifier to a host connected to the Internet

No two publicly addressed and visible hosts on the Internet canhave the same IP address


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Port address

A logical address is responsible for the arrival of packet at thedestination host

But each end-host runs multiple processes at the same time

In the TCP/IP architecture, the address assigned to a process iscalled a port address

Port address identifies a porcess inside a host

A port address/number in TCP/IP is 16 bits in length

The physical addresses change from hop to hop, but the logicaland port addresses usually remain the same


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Application-specific address

Some applications have user-friendly addresses designed for thatspecific application

Example: E-Mail address and the Universal Resource Locator(URL)

These addresses need to be converted into corresponding portand logical addresses for communication to occur


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Underlying technologies Underlying technologies

Outline




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Underlying technologies

TCP/IP protocol stack only defines protocols for the top three layer

Conceptually, the Internet is a set of switched WANs (backbones),LANs, point-to-point WANs, and connecting or switching devices

What comes below under these three layers is the topic ofdiscussion


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LAN

A local area network (LAN) is a computer network that is designedfor a limited geographic area such as a building or a campus

Can be used as an isolated network to connect computers in anorganization for the sole purpose of sharing resources, or can alsobe linked to a wide area network (WAN) or the Internet

Several technologies such as Ethernet, token ring, token bus,FDDI, and ATM LAN

Ethernet is the most dominant one


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IEEE Standard Project 802

To set standards to enable intercommunication amongequipments from a variety of manufacturers

To specify functions of the physical layer and the data link layer ofmajor LAN protocols

Division of the data link layer into two sub-layers: logical linkcontrol (LLC) and media access control (MAC)

Creation of several physical layer standards for different LANprotocols (http://www.ieee802.org/)


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Ethernet Frame

PreambleI The first field of the 802.3 frame contains 7 bytes (56 bits) of

alternating 0s and 1sI Alerts the receiving system to the coming frameI Enables it to synchronize its input timingI The 56-bit pattern allows the stations to miss some bits at the

beginning of the frameI Preamble is actually added at the physical layer and is not

(formally) part of the frame


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Ethernet Frame

Start frame delimiter (SFD):I Second field (1 byte: 10101011) signals the beginning of the frameI SFD warns the station or stations that this is the last chance for

synchronizationI Last 2 bits are 11 and alert the receiver that the next field is the

destination addressI SFD is also added at the physical layer


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