packet networking concepts

Module: Core Network and Internet InfrastructureUnit: Packet NetworkingLesson: Packet Networking Concepts

2014 Resource Development International Ltd. All rights reserved.

Resource Development International Limited reserves all rights of copyright and all other intellectual property rights in these

learning materials. No part of any learning materials may be reproduced, stored in a retrieval system or transmitted in any form

or by any means, including without limitation electronic, mechanical, photocopying, recording or otherwise, without the prior

written consent of Resource Development International Limited.

Packet Networking Concepts Packet Networking

Packet-Switched connections in contrast to the circuit-switched connection are connectionless.

Each packet is transmitted independently of other packets and packets are routed based upon theactivity and availability of circuits connecting packet switches, which results in the selection of anoptimum route at a given time. This is analogous to sending a message by a letter - the packet issimply received and forwarded. Since packets can traverse different paths, packets can bereceived at a destination switch out of sequence. Thus, destination switches must be capable ofhaving sufficient memory to store packets until they can be sequenced into their appropriate orderprior to their delivery to their ultimate destination. Although packets from each source routed to acommon destination can take different paths, they are reassembled into their original order at theirdestination nodes. There is no call setup phase. Thus, if a communicating device wishes to sendonly one or a few packets, datagram delivery will be quicker. It is more flexible because ifcongestion develops in one part of the network, incoming datagrams can be routed away from thecongestion. Datagram delivery is also more reliable because if a node fails, subsequent packetsmay find an alternate route that bypasses that node.

In packet switching, there is no resource allocation for a packet. This means that there is noreserved bandwidth on the links, and there is no scheduled processing time for each packet.Resources are allocated on demand. The allocation is done on a first- come, first-served basis.When a switch receives a packet, no matter what is the source or destination, the packet must waitif there are other packets being processed. As with other systems in our daily life, this lack ofreservation may create delay. For example, if we do not have a reservation at a restaurant, wemight have to wait.

In a datagram network, each packet is treated independently of all others. Even if a packet is part ofa multipacket transmission, the network treats it as though it existed alone. Packets in this approachare referred to as datagrams. Datagram switching is normally done at the network layer.

Figure 4.1.1 depicts a datagram-switching network in comparison to circuit-switched network.


Figure 4.1.1 - Datagram Switching Network Vs Circuit Switched Network

Comparison Between Circuit Vs Packet Switching Networks

In Packet Switching, it is up to routers to use store-and-forward transmission to send each packeton its way to the destination on its own. This procedure is unlike circuit switching, in which the resultof the connection setup is the reservation of bandwidth all the way from the sender to the receiver.All data on the circuit follows this path. Among other properties, having all the data follow the samepath means that it cannot arrive out of order. With packet switching there is no fixed path, sodifferent packets can follow different paths, depending on network conditions at the time they aresent, and they may arrive out of order.

Packet-switching networks place a tight upper limit on the size of packets. This ensures that no usercan monopolize any transmission line for very long (e.g., many milliseconds), so that packet-switched networks can handle interactive traffic. It also reduces delay since the first packet of a longmessage can be forwarded before the second one has fully arrived. However, the store-and-forward delay of accumulating a packet in the router's memory before it is sent on to the next routerexceeds that of circuit switching. With circuit switching, the bits just flow through the wirecontinuously.

Packet and circuit switching also differ in other ways. Because no bandwidth is reserved withpacket switching, packets may have to wait to be forwarded. This introduces queuing delay andcongestion if many packets are sent at the same time. On the other hand, there is no danger ofgetting a busy signal and being unable to use the network. Thus, congestion occurs at differenttimes with circuit switching (at setup time) and packet switching (when packets are sent).

If a circuit has been reserved for a particular user and there is no traffic, its bandwidth is wasted. Itcannot be used for other traffic. Packet switching does not waste bandwidth and thus is moreefficient from a system perspective. Under- standing this trade-off is crucial for comprehending thedifference between circuit switching and packet switching. The trade-off is between guaranteedservice and wasting resources versus not guaranteeing service and not wasting resources.


Packet switching is more fault tolerant than circuit switching. In fact, that is why it was invented. If aswitch goes down, all of the circuits using it are terminated and no more traffic can be sent on any ofthem. With packet switching, packets can be routed around dead switches.

A final difference between circuit and packet switching is the charging algorithm. With circuitswitching, charging has historically been based on distance and time. For mobile phones, distanceusually does not play a role, except for international calls, and time plays only a coarse role (e.g., acalling plan with 2000 free minutes costs more than one with 1000 free minutes and sometimesnights or weekends are cheap). With packet switching, connect time is not an issue, but the volumeof traffic is. For home users, ISPs usually charge a flat monthly rate because it is less work forthem and their customers can understand this model, but backbone carriers charge regionalnetworks based on the volume of their traffic.

Summarize the differences between circuit and packet switching based on followingcharacteristics.

Figure Table1 - Table 1

Additional Resource:

Packet Switching Animation:

http://www.pbs.org/opb/nerds2. 0.1/geek_glossary/packet_switc hing_flash.html

Virtual Circuits

A virtual-circuit network is a cross between a circuit-switched network and a datagram

network. It has some characteristics of both.

As in a circuit-switched network, there are setup and teardown phases in addition to the datatransfer phase.

1.

Resources can be allocated during the setup phase, as in a circuit-switched network,2.

or on demand, as in a datagram network.3.

As in a datagram network, data are packetized and each packet carries an address in theheader. However, the address in the header has local jurisdiction not end-to-end jurisdiction.

4. 2014 Resource Development International Ltd. All rights reserved.

The reader may ask how the intermediate switches know where to send the packet if there isno final destination address carried by a packet. The answer will be clear when we discussvirtual-circuit identifiers in the next section.

4.

As in a circuit-switched network, all packets follow the same path established during theconnection.

5.

A virtual-circuit network is normally implemented in the data link layer; while a circuit-switchednetwork is implemented in the physical layer and a datagram network in the network layer.But this may change in the future.

6.

Figure 4.1.2 is an example of a virtual-circuit network. The network has switches that allow trafficfrom sources to destinations. A source or destination can be a computer, packet switch, bridge, orany other device that connects other networks.

Figure 4.1.2 - Virtual Circuit Switching

In a virtual-circuit network, two types of addressing are involved: global and local (virtual-circuitidentifier).

Global Addressing: A source or a destination needs to have a global address-an address thatcan be unique in the scope of the network or internationally if the network is part of aninternational network. However, we will see that a global address in virtual-circuit networks isused only to create a virtual-circuit identifier, as discussed next.

Virtual-Circuit Identifier: The identifier that is actually used for data transfer is called the virtual-circuit identifier (Vel). A vel, unlike a global address, is a small number that has only switchscope; a frame between two switches uses it. When a frame arrives at a switch, it has a VCI;when it leaves, it has a different VCl.

As in a circuit-switched network, a source and destination need to go through three phases in avirtual-circuit network: setup, data transfer, and teardown. In the setup phase, the source anddestination use their global addresses to help switches make table entries for the connection. In theteardown phase, the source and destination inform the switches to delete the corresponding entry.Data transfer occurs between these two phases. We first discuss the data transfer phase, which ismore straightforward; we then talk about the setup and teardown phases.

Virtual-circuit networks are used in switched WANs such as Frame Relay and ATM networks. Thedata link layer of these technologies is well suited to the virtual-circuit technology.uaranteed servicelevels

1. 2014 Resource Development International Ltd. All rights reserved.

Point-to-point and point-to-multipoint connections1.

Constant as well as variable bit rate services2.

Connection-oriented or connectionless application services3.

Scalability, both in terms of:

Distance - A single technology in the local area, campus, and wide area1.

Speed - Currently defined physical layer interfaces vary from 1.5 Mbps up to2.

622 Mbps

OSI Reference Model

Packet-Switching as discussed in the previous section finds its largest application in datanetworking.

A layered-approach was adopted for effective communication between computers by the early datanetworking architects and the International standards organization. The model developed is calledthe ISO OSI (Open Systems Interconnection) Reference Model because it deals with connectingopen systemsthat is, systems that are open for communication with other systems. We will just call itthe OSI model for short.

The OSI model has seven layers. The principles that were applied to arrive at the seven layers canbe briefly summarized as follows:

Each layer should perform a well-defined function.

The function of each layer should be chosen with an eye toward defining internationallystandardized protocols.

The layer boundaries should be chosen to minimize the information flow across the interfaces.

The number of layers should be large enough that distinct functions need not be throwntogether in the same layer out of necessity and small enough that the architecture does notbecome unwieldy.

OSI model makes it possible to determine the specifications of each layer of the technology inquestion and render each layer compatible with higher and lower layers. This also helps theinteroperability of equipment from various manufacturers. The seven layers proposed in the OSImodel are the following:

7. Application: The application layer contains a variety of protocols that are commonly needed byusers.

6. Presentation: Unlike the lower layers, which are mostly concerned with moving bits around, thepresentation layer is concerned with the syntax and semantics of the information transmitted.

5. Session: The session layer allows users on different machines to establish sessions be-tween them.

4. Transport: The basic function of the transport layer is to accept data from above it, split it upinto smaller units if need be, pass these to the network layer, and ensure that the pieces all arrivecorrectly at the other end.


3. Network: The network layer controls the operation of the subnet. A key design issue isdetermining how packets are routed from source to destination.

2. Data Link: The main task of the data link layer is to transform a raw transmission facility into aline that appears free of undetected transmission errors.

1. Physical: is concerned with transmitting raw bits over a communication channel.

While the ISO model was being ratified, the Department of Defence (DoD) of the United Statesbegan to develop its own model. The project, funded by the Defence Advanced Research ProjectsAgency (DARPA), involved in the development of an experimental packet-switched network(ARPANET), and the development of all the necessary protocols to deal with its functionality. Theresult was an internetworking protocol suite known as The TCP/IP Internet Protocols; however, mostpeople nowadays simply refer to them as TCP/IP.

TCP/IP was the first protocol suite developed for use in an internet. The OSI seven-layer modeldescribed in the previous section was defined before the idea of internetworking take up. The resultwas that there was no provision for a layer for internet protocols.

Figure 4.1.3 shows the difference between the OSI and TCP/IP reference model.

Figure 4.1.3 - OSI Vs TCP/IP Reference Model

The OSI and TCP/IP reference models have much in common. Both are based on the concept of astack of independent protocols. Also, the functionality of the layers is roughly similar. For example,in both models the layers up through and including the transport layer are there to provide an end-to-end, network-independent transport service to processes wishing to communicate. These layersform the transport provider. Again in both models, the layers above transport are application-oriented users of the transport service.

Despite these fundamental similarities, the two models also have many differences. In this sectionwe will focus on the key differences between the two reference models. It is important to note thatwe are comparing the reference models here, not the corresponding protocol stacks. Threeconcepts are central to the OSI model:


1. Services.

2. Interfaces.

3. Protocols.

Probably the biggest contribution of the OSI model is that it makes the distinction between thesethree concepts explicit. Each layer performs some services for the layer above it. The servicedefinition tells what the layer does, not how entities above it access it or how the layer works. Itdefines the layer's semantics.

A layer's interface tells the processes above it how to access it. It specifies what the parameters areand what results to expect. It, too, says nothing about how the layer works inside.

Finally, the peer protocols used in a layer are the layer's own business. It can use any protocols itwants to, as long as it gets the job done (i.e., provides the offered services). It can also change themat will without affecting software in higher layers.

Critique OSI Reference model and protocols1.

Critique TCP/IP reference model2.

Why is there a need for protocols? What was the main driving force behind designingprotocols?

3.

How does OSI model fit into telephony?4.

Use the following references for the above research:

Tanenbaum, A. S. (2007). Computer Networks, 5-th Edition.1.

Roger.L.Freeman(1999), Fundamentals of Telecommunications, Wiley Publication2.

Roger.L.Freeman(1999), Fundamentals of Telecommunications, Wiley Publication3.

View a movie on TCP/IP Networks here

https://www.youtube.com/watch? v=PBWhzz_Gn10

TCP Animations

http://www.billingcollege.com/ animation/protocols.swf

WAN Technologies

Packet Switched technologies enables data networking. Networks of all size and scale exist todaystarting from:

Personal Area Networks (PAN): They let devices communicate over the range of a person. A 2014 Resource Development International Ltd. All rights reserved.

common example is a wireless network that connects a computer with its peripherals.

Local Area Networks (LAN): A LAN is a privately owned network that operates within andnearby a single building like a home, office or company.

Metropolitan Area Networks (MAN): covers a city. The best-known examples of MANs are thecable television networks available in many cities.

Wide Area Networks (WAN) spans a large geographical area, often a country or continent.

Wide area network (WAN) can be defined as a collection of interconnected LANs. WANs allownetworks that are spread over a large geographical space to share data and network resources.The sub-networks that comprise a WAN use routers to route their packets to the proper destination.Routers are hardware devices that link different networks to provide the most efficient path for thetransmission of data. These routers are connected by high-speed telecommunications links. Thecharacteristics of some of these links are shown in figure 4.1.4

Figure 4.1.4 - List of WAN Technologies and their operating Speeds

T1 - comprised of 24 channels, each using 8 bits per channel. So, one T1 lines uses a twisted pairfor 24 voice signals. It uses baseband transmission. T1 lines can carry both voice and data. Voiceand data multiplexors are required if you plan to mix both voice and data channels. The lines arefractional, meaning that they can be divided into channels for voice or data.

T2 - four T1 lines can be combined to form a T2 line.

T3 - 28 T1 lines or 7 T2 lines form the T3 line for users who need a higher bandwidth.

T4 - 3 lines are multiplexed into a T4(M) line, equivalent to 178 T1 circuits.

E1, E2, etc are used for European standard [5].

ISDN (Integrated Services Digital Network) - supports voice, data, video, electronic mail, andnumerous other services integrated together. It consists of D channel, B channel, and H channel.


B channel - voice, videos, and data (64 Kbps), can be used for packet and circuit switchingapplications.

D channel - user signalling messages, uses out-of-band signalling (16 or 64 Kbps). The D channelis used to transfer requests for services that are delivered on a B channel (384 Kbps).

H channel - higher transfer rate, real time video conferencing, etc. (1536 or 1920 Mbps).

ATM - cell-based (53-byte cell), fast-packet switching technology. ATM layer functions - switching,multiplexing, routing, and congestion management.

Asynchronous Transfer Mode

ATM is a connection-oriented packet-switching technology that uses fixed-size packets, referred toas cells, to carry the traffic in the network. ATM embodies various design objectives that include:

Integration of voice, video, image and data services into a single framework

Characterizing ATM as asynchronous indicates that cells may occur at irregular timesdetermined by the nature of the application rather than the framing structure of the transmissionsystem. In effect, ATM has isochronous support built in; consequently, ATM can transportvoice, data, and video, all on the same circuit.

Scalability, both in terms of: bullet style?

.-Distance - A single technology in the local area, campus, and wide area

.-Speed - Currently defined physical layer interfaces vary from 1.5 Mbps up to 622 Mbps

Because of its adaptability, ATM was viewed as the key technology for the future , However in therecent times other WAN technologies such as SONET/SDH have become more prevalent in privatewide area networks, and on the campus as the multimedia LAN. One common misconception,though, is to confuse the technology with the services that will be offered in the broadband networkof the future. The types of services available in ATM networks include:

Bandwidth on demand1.

Guaranteed service levels2.

Point-to-point and point-to-multipoint connections3.

Constant as well as variable bit rate services4.

Connection-oriented or connectionless application services5.

ATM is a cell-switched network. The user access devices, called the endpoints, are connectedthrough a user-to-network interface (UNI) to the switches inside the network. The switches areconnected through network-to-network interfaces (NNIs). Figure 4.1.5 shows an example of an ATMnetwork.


Figure 4.1.5 - ATM Network Architecture

SONET/SDH

The advances in optical transmission technology have occurred at a rapid rate, and the backboneof telephone networks has become dominated by fibre optic digital transmission systems.

The deregulation of telecommunications in the United States led to a situation in which the long-distance carriers were expected to provide the interconnection between local telephone serviceproviders. To meet the urgent need for standards to interconnect optical transmission systems, theSynchronous Optical Network (SONET) standard was developed in North America. The CCITT laterdeveloped a corresponding set of standards called Synchronous Digital Hierarchy (SDH).

SONET and SDH form the basis for current high- speed backbone networks.

The SONET standard uses a 51.85 Mbps signal as a building block to extend the digitaltransmission hierarchy into the Multi-gigabit range. SONET incorporates extensive capabilities forthe operations, administration, and maintenance (OAM) functions that are required to operatedigital transmission facilities. It also introduces a synchronous format that greatly simplifies thehandling of the lower-level digital signals and that enables network topologies that are self-healing inthe presence of faults.

Figure 4.1.6 shows the SONET and SDH digital hierarchy. The synchronous transport signal level-1(STS-1) is the basic building block of the SONET hierarchy. A higher-level signal in the hierarchy isobtained through the interleaving of bytes from the lower-level component signals. Each STS-nelectrical signal has corresponding optical carrier level-n (OC-n) signal. The bit format of STS-n andOC-n signals is the same except for the use of scrambling in the optical signal.3 The SDH standardrefers to synchronous transfer modules-n (STM-n) signals and begins at a bit rate of 155.52 Mbps.The SDH STM-1 signal is equivalent to the SONET STS-3 signal. The STS-1 signal accommodatesthe DS3 signal from the existing digital transmission hierarchy in North America. The STM-1 signalaccommodates the CEPT-4 signal in the CCITT digital hierarchy. The STS-48 signal is widelydeployed in the backbone of modern communication networks.


Figure 4.1.6 - Synchronous Digital Hierarchy

Use appropriate internet resources to investigate the pre-SONET/SDH switching and the currentdeployment status of SDH technology.

Reading:

a) Introduction to ATM - http://technet.microsoft.com/e n-us/library/cc976964.aspx

b) Introduction to ATM Technology and its Applications http://ieeexplore.ieee.org/.../abs_all.jsp%3Farnumber%3D5807 58

c) An introduction to ATM Networks - http://freecomputerbooks.com/A n-Introduction-to-ATM-Networks .html

d) Internetworking over ATM: An Introduction - http://www.redbooks.ibm.com/abstracts/sg244699.html

Video:

ATM: Asynchronous Transfer Mode - https://www.youtube.com/watch? v=IPuLZSOye4c

(SONET/SDH) James F Kurose, Keith W Ross, Computer Networking, A Top-Down ApproachFeaturing the Internet, Third Edition, Addison Wesley

(SONET/SDH) Roger.L.Freeman(1999), Fundamentals of Telecommunications, Wiley Publication


packet networking concepts

Documents

packet switches

packet networkinglesson

networksin packet

circuit vs packet

datagram network

datagram switching

network vs circuit

core network