THE NETWORK CORE

Mesh of interconnected Routers The fundamental question: how

is data transferred through network? circuit switching

dedicated circuit per call: telephone net

packet-switching data sent through net in discrete

“chunks”

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NETWORK CORE

Long distance transmission is typically done over a network of switched nodes

Nodes not concerned with content of data End devices are stations

Computer, terminal, phone, etc. A collection of nodes and connections is a

communications network Data routed by being switched from node to node Node to node links usually multiplexed

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NETWORK CORE: CIRCUIT SWITCHING End-to-end resources reserved for “call”

link bandwidth, switch capacity dedicated resources: no sharing circuit-like (guaranteed) performance call setup required

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NETWORK CORE – CIRCUIT SWITCHING

Switched circuits allow data connections that can be initiated when needed and terminated when communication is complete

Circuit switched network - a network in which a dedicated circuit is established between sender and receiver and all data passes over this circuit.

The telephone system is a common example.

The connection is dedicated until one party or another terminates the connection.

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CIRCUIT SWITCHING

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NETWORK CORE – CIRCUIT SWITCHING Dedicated communication path between two stations Three phases (Establish, Transfer, Disconnect) Inefficient (for data traffic)

Channel capacity dedicated for duration of connection Much of the time a data connection is idle If no data, capacity wasted

Set up (connection) takes time Once connected, transfer is transparent Circuit switching designed for voice Constant Data rate (Both ends must operate at the same

rate)

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NETWORK CORE - CIRCUIT SWITCHING

Multiplexing in Circuit Switched Networks Multiplexing is a technique, in which a single

transmission medium is being shared among multiple users.

Types of Multiplexing Frequency Division Multiplexing FDM Time Division Multiplexing TDM

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CIRCUIT SWITCHING: FDM AND TDM

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Output Stream generated by a synchronous time division multiplexer

MULTIPLEXER TRANSMISSION STREAM WITH ONE INPUT DEVICE TRANSMITTING DATA.

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TWO STATIONS OUT OF FOUR TRANSMITTING VIA A STATISTICAL MULTIPLEXER

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NETWORK CORE: PACKET SWITCHING Packet switched network

A network in which data is transmitted in the form of packets

Multiple users share network resources No dedicated bandwidth is allocated No resources are reserved, resources used as needed Each packet uses full link bandwidth Good for bursty traffic, simpler, no call setup Packets queued and transmitted as fast as possible Packets are accepted even when network is busy, which

causes the delivery to slow down12

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PACKET SWITCHING: STATISTICAL MULTIPLEXING

Sequence of A & B packets does not have fixed pattern statistical multiplexing

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A

B

C10 Mb/sEthernet

1.5 Mb/s

D E

statistical multiplexing

queue of packetswaiting for output

link

NETWORK CORE: PACKET SWITCHING

The goal of packet switching is to move packets through routers from source to destination

Packets sent one at a time to the network Two approaches are used:

Datagram Approach Virtual Circuits Approach

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PACKETS FORWARDING Two broad classes of packet switched networks are:

Datagram Networks Any network that forwards the packet according to the destination

address is called a datagram network The routers in the Internet forwards packets according to host

destination addresses; hence the Internet is a datagram network. Virtual Circuit Networks

Any network that forwards the packet according to the virtual circuit identifier is called a virtual circuit network

Examples are X.25, Frame Relay, ATM technologies

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PACKET SWITCHING - DATAGRAM Datagram Approach:

Each packet is treated independently No reference to packets that have gone before Each node chooses next node on path using destination

address Packets with same destination address may not follow same

route Packets may arrive out of sequence, may be lost It is up to receiver to re-order packets and recover from lost

packets No Call setup For an exchange of a few packets, datagram quicker Analogy: driving, asking directions 16

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PACKET SWITCHING - DATAGRAM The Internet is a Datagram network

Datagram network is not either connection-oriented or connectionless.

Internet provides both connection-oriented (TCP) and connectionless services (UDP) to applications.

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DATAGRAM NETWORKS A datagram network is not either a connectionless or a

connection oriented network. It can provide connectionless service to some of its

applications and connection-oriented service to other applications.

Example The Internet, which is a datagram network, provides

both connectionless (UDP) and connection oriented (TCP) services to its applications

Networks with Virtual Circuits are, however, always connection-oriented.

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PACKET SWITCHING - DATAGRAM

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PACKET SWITCHING: DATAGRAM APPROACH

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PACKET SWITCHING – VIRTUAL CIRCUITS Virtual Circuit Approach:

Virtual circuit packet switched network create a logical path through the subnet

Call request and call accept packets establish a virtual connection

Virtual route remains fixed through the call. All packets from one connection follow this path. Each packet contains a virtual circuit identifier

instead of destination address to determines the next hop

Not a dedicated path No routing decisions required for each packet

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SWITCHING TECHNIQUE – VIRTUAL CIRCUIT Preplanned route established before packets sent All packets follow same route Similar to circuit in circuit-switching network

Hence virtual circuit Each packet has virtual circuit identifier

Nodes on route know where to direct packets No routing decisions

Not dedicated path, as in circuit switching Packet still buffered at node and queued for output Routing decision made on before that virtual circuit

Network may provide services related to virtual circuit Sequencing and error control

Packets should transit more rapidly If node fails, all virtual circuits through node lost

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PACKET SWITCHING: VC APPROACH

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VIRTUAL CIRCUITS VS. DATAGRAMU

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Network can provide sequencing and error control

Packets are forwarded more quickly No routing decisions to

make Less reliable

Loss of a node looses all circuits through that node

Less Processing Delay at a node

No call setup phase Better if few packets

More flexible Routing can be used to

avoid congested parts of the network

More reliable If a node fails, packets may

find an alternate route that bypass that node

More Processing Delay at a node

VC Datagram

CIRCUIT SWITCHING VS. VIRTUAL CIRCUITS

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Path A dedicated path is

established between two devices for the duration of session.

Reserved Resources The link (multiplexed / not

multiplexed) that makes the path are dedicated, and cannot be used by other connections

constant data rates

Route No dedicated path is

established. Only a route is defined. Each switch creates an entry in its routing table for the duration of virtual circuit

Shared Links The link that makes a route

can be shard by other connections

CS VC

FEATURES OF CIRCUIT AND PACKET SWITCHING

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NETWORK TAXONOMY

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NETWORK ACCESS Network Access:

The physical link that connects an end system to its Edge Router, which is the first router on a path from the end system to any other distant end system.

Classification of Network Access: Residential Access

Connecting a home end system to an edge router Dial-up modems, DSL, HFC system

Company Access Switched Ethernet LANs

Mobile Access Wireless LAN (802.11b) Wide Area Wireless Access Networks (GPRS, 3G, WAP) Note: these categories are not hard and fast

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PHYSICAL MEDIA Twisted Pair Cable

UTP Cat 5 Coaxial Cable

Baseband and Broadband Cable Fiber Optics

Multimode and single mode Terrestrial Radio Channels

Local Area Radio Channels (Wireless LANs)Wide Area Radio Channels (WAP, I-mode, 3G)

Satellite Radio ChannelsGeostationary Satellites (36000 km)Low Altitude Satellites

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DELAY PACKET SWITCHED NETWORKS

Considering what can happen to a packet as it travels from its source to its destination. As a packet travels from one node to other node

(host or end system), it suffers from several types of delays at each node along the path

Most important types of delays are: Processing Delay Queuing Delay Transmission Delay Propagation Delay

TYPES OF DELAY

Processing Delay The time required to process (examine the

packet’s header and determine where to direct the packet) is part of the processing delay

Processing delay in high-speed routers is typically on the order of microseconds or less.

After this nodal processing, the router directs the packet to the queue that precedes the link to the next router.

Processing Delay depends on the processing speed of a router.

TYPES OF DELAY Queuing Delay

At the queue, the packet experiences a queuing delay as it waits to be transmitted onto the link.

The queuing delay of a packet will depend on the number of earlier-arriving packets that are queued and waiting for transmission across the link

If queue is empty, and no other packet is being transmitted, the queuing delay will be zero

If traffic is heavy and many other packets are waiting to be transmitted, the queuing delay will be long

Thus, queuing delay depends on the intensity and nature of traffic arriving at the queue.

Queuing delays can be in the order of microseconds to milliseconds in practice

TYPES OF DELAY

Transmission Delay It is the amount of time required to push an

entire packet into the link The time taken by a transmitter to send out all

the bits of a packet onto the medium Also called Store and Forward Delay Node receives complete packet before

forwarding Transmission Delay is directly proportional to

the length of the packet Transmission delays are typically in the order of

microseconds to milliseconds in practice

TYPES OF DELAY

Transmission Delay Let us denote the length of the packet by L bits. Denote the transmission rate of the link from

Router A to B by R bits/sec Transmission Delay (L/R) = Packet Length (L)

Transmission

Rate (R) Example:

It takes 1 sec to transmit a 10,000 bits packet onto a 10Kbps line. (10,000 / 10 x 1000 = 1)

R R R

L

A B

TYPES OF DELAY

Propagation Delay Time it takes a bit to propagate from one node to

the next. The time required by a bit to propagate from the

beginning of the link to the next router is called propagation delay

The bit propagates at the propagation speed of the link which depends on the physical medium being used.

It is typically in the range of: 2 x 108 meters/sec to 3 x 108 meters/second

In wide area networks, propagation delays are on the order of milliseconds

TYPES OF DELAY

Propagation Delay Propagation delay depends on the distance (d)

between the two routers/nodes and the propagation speed (s) of the link.

Propagation Delay (d/s) = Distance b/w 2 Routers (d)

Propagation Speed (s)

TYPES OF DELAY

Total Nodal Delay (the delay at a single router) If we let dproc, dqueue, dtrans and dprop denote the

processing, queuing, transmission and propagation delays respectively, then the total nodal delay is given by:

dnodal = dproc + dqueue + dtrans + dprop

QUEUING DELAY

Queuing delay is most complicated and interested delay as compared to other components of nodal delay (processing, transmission, propagation)

Queuing delay can vary from packet to packet Example: if ten packets arrive at an empty

queue, the first packet will suffer no queuing delay while the last packet will suffer large queuing delay

QUEUING DELAY

Queuing delay depends on: Average Rate at which the packets arrives at a

queue (a = packets/sec) Transmission Rate of the link (R = bits/sec) Nature of the incoming traffic (bursty/periodic) Assume that all the packets are of equal length

say L bits Then the average rate at which the bits arrive at

the queue will be La bits/sec Traffic Intensity = La/R

This ratio helps in estimating the extent of queuing delay

TRAFFIC INTENSITY

Traffic Intensity If La/R is > 1

It means that the average rate at which the bits arrive at the queue exceeds the rate at which the bits can be transmitted from the queue.

In this undesirable situation, the queue will tend to increase without bound and the queuing delay will reach to infinity!

A golden rule in traffic engineering “Design your systems so that the traffic intensity is no

greater than 1s”

TRAFFIC INTENSITY Traffic Intensity

If La/R is > 1 If the traffic intensity is close to one, there will be

intervals of time when the arrival rate exceeds the transmission capacity and a queue will form

As the traffic intensity approaches 1, the average queue length gets larger and larger

If La/R is < 1 If the traffic intensity is close to zero, then the packets

arrivals are few and far between, and it is unlikely that an arriving packet will find another packet in the queue

Average queuing delay will be close to zero

TRAFFIC INTENSITY

Traffic Intensity (La/R)

Average Queuing Delay

0 1

PACKET LOSS

In reality a queue has a finite capacity As the traffic intensity approaches 1, a

packet can arrive to find a full queue. With no place to store such a packet, a router

will drop that packet; that is the packet will be lost

The fraction of lost packets increases as the traffic intensity increases

Thus, a node performance also includes the probability of packet loss

A lost packet may be retransmitted on an end-to-end basis, either the application or transport layer protocol.

END-TO-END DELAY

The total delay from source to destination is referred to as end-to-end delay Example:

Suppose that the queuing delay is negligible as the network is uncongested, then the end-to-end delay between the source and destination having N-1 routers in between will be:

dend-end = N (dproc + dtrans + dprop )

R R R

L

DELAYS AND ROUTES IN THE INTERNET Traceroute

A program that sends multiple special packets towards the destination

As these packets work their way towards the destination, they pass through a series of routers.

When a router receives one of these special packets, it sends a short message back to the source.

This message contains the name and address of the router

http://www.traceroute.org For Details: Consult Traceroute: RFC 1393 To Do: Explore the Netstat tracert

commands

LAYERED ARCHITECTURE

Design Philosophy of Layered Architecture The complex task of communication is broken

into simpler sub-tasks or modules Each layer performs a subset of the required

communication functions Each layer relies on the next lower layer to

perform more primitive functions Each layer provides services to the next higher

layer Changes in one layer should not require changes

in other layers Helps in troubleshooting and identifying the

problem

INTERNET PROTOCOL STACK

Application

Transport

Network

Data Link

Physical

TCP/IP PROTOCOL SUITE Application Layer

Responsible for supporting network applications Protocols include: HTTP. SMTP, FTP etc.

Transport layer (End-to-end Communication) Two transport layer protocols (TCP and UDP) Transports messages between client and server

applications Network Layer (Host-to-host Communication)

Routing of datagrams from one host to another IP works on this layers

Data link Layer (Node-to-node Communication) Logical interface between end system and network Examples: Ethernet, PPP, ATM and Frame Relay

technologies Physical Layer

Transmission medium Signal rate and encoding

PDUS IN TCP/IP

SOME PROTOCOLS IN TCP/IP SUITE