Assignment 2:: Computer Network

Assignment 2

Computer Network

067/BEX/445

Tej Prasad Adhikari

Peer-to-Peer vs. Client Server Networks

In computer networking, the architecture or way the network entities are arranged, plays a key role in determining the communication and privilege flow. Two main models of computer networks, are client server and peer-to-peer.

Peer-to-Peer vs. Client Server Networks Comparison

Peer-to-Peer (P2P) Model	Client Server Model
Decentralized form of networking architecture	Centralized form of networking architecture
The network access, tasks and workload are divided and shared amongst the various members. It is an "everyone pulls their own weight" sort of relationship	Working is based on a resource provider or storehouse (server) and the entities that require the resources (clients). The clients make requests to the server to access the resources. It is a "make a request and it will be granted" sort of service
Supply and consumption of resources is carried out by the peers, there is no higher body or "boss" and no separate entity exists to dole out resources. All peers in a network can request for resources as well as grant them	Two members of such a system are servers and clients. Servers contain the resources in the form of information or data. Certain resources like say printers, can be connected to the server and the client has to request access to the server, to use the printer
Members are called peers, they have the same privileges and rights and enjoy the same access to various data sources and devices. There is no difference between them in any manner. Peers communicate with each other directly, no need for a median in the middle	Clients are the respective workstations or computers that do not share their resources but work on their own and makes requests to the server for data or resources or functions
The Peer-to-Peer network paradigm is commonly used in P2P file sharing programs like Napster and Bitorrent	Email, banking services, even the HTTP protocol are all examples of client server model
Computers A, B, C and D are connected in a P2P network. Comp A wants a file from Comp C, it sends a request to C. C decides to accept the request, finds the file and sends it to A. B and D are ignorant to what is going on but function normally. There is a network printer to which all computers are connected to. A sends a request to print and B sends one too. A's request reached first, so it is granted. Then the printer will grant B's request	Computer A is the server. Computers B, C and D are the clients. B wants to print a page. The printer is attached to Comp A. B will send a request to A, asking to print a page. A will print the page and respond to B. C wants to access a file, it will send a request to A, asking for the file. A will check C's credentials, C is not authorized to access the data, A will reject the request and respond to C by turning down its request
2 types of peer-to-peer networks exist. Structured P2P arranges peers in a order or manner based on certain rules and algorithms. There is no change in privilege, just in the way the members communicate. Unstructured P2P have no such order or manner and consists of 3 models - pure, hybrid and centralized	There is no specific model or type of client-server networks. It's more like a mixed bag of different styles. For example, having two servers, one just for data and one for devices and clients have to make requests to 2 different entities for accessing such resources
The physical structure is independent from the underlying network structure of behavior. Peers can be arranged in any network topology but in small networks, are located near to each other physically. The computers are similar in software content and protocols used for networking	Physical structure is divided. Servers are powerful machines, designed for a dedicated purpose and should be robust to handle multiple transactions. Their hardware makeup is more powerful with more storage space or RAM and powerful processors. The server machine is normally contained in a different room with increased security and better environmental conditions. Clients are ordinary workstations, accessed by different users. They have their own data
Used by small businesses and home users	Big corporations or organizations with high security data

Comparison between OSI reference model and TCP/IP reference model

OSI

1.      It has 7 layers

2.      Transport layer guarantees delivery of packets

3.      Horizontal approach

4.      Separate presentation layer

5.      Separate session layer

6.      Network layer provides both connectionless and connection oriented services

7.      It defines the services, interfaces and protocols very clearly and makes a clear distinction between them

8.      The protocol are better hidden and can be easily replaced as the technology changes

9.      OSI truly is a general model

10.  It has a problem of protocol filtering into a model

TCP/IP

1.      Has only 4 layers

2.      Transport layer does not guarantees delivery of packets

3.      Vertical approach

4.      No session layer, characteristics are provided by transport layer

5.      No presentation layer, characteristics are provided by application layer

6.      Network layer provides only connection less services

7.      It does not clearly distinguishes between service interface and protocols

8.      It is not easy to replace the protocols

9.      TCP/IP cannot be used for any other application

10.  The model does not fit any protocol stack.

Short Notes

X.25

X.25 is an ITU-T standard protocol suite for packet switched wide area network (WAN) communication. An X.25 WAN consists of packet-switching exchange (PSE) nodes as the networking hardware, and leased lines, plain old telephone service connections or ISDN connections as physical links. X.25 is a family of protocols that was popular during the 1980s with telecommunications companies and in financial transaction systems such as automated teller machines. X.25 was originally defined by the International Telegraph and Telephone Consultative Committee (CCITT, now ITU-T) in a series of drafts and finalized in a publication known as The Orange Book in 1976. The general concept of X.25 was to create a universal and global packet-switched network.

Much of the X.25 system is a description of the rigorous error correction needed to achieve this, as well as more efficient sharing of capital-intensive physical resources. The X.25 specification defines only the interface between a subscriber (DTE) and an X.25 network (DCE). X.75, a very similar protocol to X.25, defines the interface between two X.25 networks to allow connections to traverse two or more networks. X.25 does not specify how the network operates internally—many X.25 network implementations used something very similar to X.25 or X.75 internally, but others used quite different protocols internally. The ISO equivalent protocol to X.25, ISO 8208, is compatible with X.25, but additionally includes provision for two X.25 DTEs to be directly connected to each other with no network in between. By separating the Packet-Layer Protocol, ISO 8208 permits operation over additional networks such as ISO 8802 LLC2 (ISO LAN) and the OSI data link layer.

Although X.25 predates the OSI Reference Model (OSIRM), the physical Layer of the OSI model corresponds to the X.25 physical layer, the data link layer to the X.25 data link layer, and the network layer to the X.25 packet layer. The X.25 data link layer, LAPB, provides a reliable data path across a data link (or multiple parallel data links, multilink) which may not be reliable itself. The X.25 packet layer, provides the virtual call mechanisms, running over X.25 LAPB. The packet layer includes mechanisms to maintain virtual calls and to signal data errors in the event that the data link layer cannot recover from data transmission errors. All but the earliest versions of X.25 include facilities which provide for OSI network layer

X.25 was developed in the era of computer terminals connecting to host computers, although it also can be used for communications between computers. Instead of dialing directly “into” the host computer – which would require the host to have its own pool of modems and phone lines, and require non-local callers to make long-distance calls – the host could have an X.25 connection to a network service provider. Now dumb-terminal users could dial into the network's local “PAD” (Packet Assembly/Disassembly facility), a gateway device connecting modems and serial lines to the X.25 link as defined by the X.29 and X.3 standards.

Frame Relay

Frame Relay is a standardized wide area network technology that specifies the physical and logical link layers of digital telecommunications channels using a packet switching methodology. Originally designed for transport across Integrated Services Digital Network (ISDN) infrastructure, it may be used today in the context of many other network interfaces.

Network providers commonly implement Frame Relay for voice (VoFR) and data as an encapsulation technique, used between local area networks (LANs) over a wide area network (WAN). Each end-user gets a private line (or leased line) to a Frame Relay node. The Frame Relay network handles the transmission over a frequently-changing path transparent to all end-user extensively-used WAN protocols. It is less expensive than leased lines and that is one reason for its popularity. The extreme simplicity of configuring user equipment in a Frame Relay network offers another reason for Frame Relay's popularity.

With the advent of Ethernet over fiber optics, MPLS, VPN and dedicated broadband services such as cable modem and DSL, the end may loom for the Frame Relay protocol and encapsulation. However many rural areas remain lacking DSL and cable modem services. In such cases the least expensive type of non-dial-up connection remains a 64-kbit/s frame-relay line. Thus a retail chain, for instance, may use Frame Relay for connecting rural stores into their corporate WAN.

Voice over IP

Voice over IP (voice over Internet Protocol, VoIP) is a methodology and group of technologies for the delivery of voice communications and multimedia sessions over Internet Protocol (IP) networks, such as the Internet.

Early providers of voice over IP services offered business models and technical solutions that mirrored the architecture of the legacy telephone network. Second generation providers, such as Skype, have built closed networks for private user bases, offering the benefit of free calls and convenience, while potentially charging for access to other communication networks, such as the PSTN. This has limited the freedom of users to mix-and-match third-party hardware and software. Third generation providers, such as Google Talk have adopted the concept of federated VoIP – which is a departure from the architecture of the legacy networks. These solutions typically allow dynamic interconnection between users on any two domains on the Internet when a user wishes to place a call.

VoIP systems employ session control and signaling protocols to control the signaling, set-up, and tear-down of calls. They transport audio streams over IP networks using special media delivery protocols that encode voice, audio, video with audio codecs and video codecs as Digital audio by streaming media. Various codecs exist that optimize the media stream based on application requirements and network bandwidth; some implementations rely on narrowband and compressed speech, while others support high fidelity stereo codecs. Some popular codecs include μ-law and a-law versions of G.711, G.722 which is a high-fidelity codec marketed as HD Voice by Polycom, a popular open source voice codec known as iLBC, a codec that only uses 8 Kbit/s each way called G.729, and many others.

VoIP is available on many smartphones, personal computers, and on Internet access devices. Calls and SMS text messages may be sent over 3G or Wi-Fi.

Next-generation network

The next-generation network (NGN) is body of key architectural changes in telecommunication core and access networks. The general idea behind the NGN is that one network transports all information and services (voice, data, and all sorts of media such as video) by encapsulating these into packets, similar to those used on the Internet. NGNs are commonly built around the Internet Protocol, and therefore the term all IP is also sometimes used to describe the transformation toward NGN.

According to ITU-T, the definition is:

    A next-generation network (NGN) is a packet-based network which can provide services including Telecommunication Services and able to make use of multiple broadband, quality of Service-enabled transport technologies and in which service-related functions are independent from underlying transport-related technologies. It offers unrestricted access by users to different service providers. It supports generalized mobility which will allow consistent and ubiquitous provision of services to users.

From a practical perspective, NGN involves three main architectural changes that need to be looked at separately:

·         In the core network, NGN implies a consolidation of several (dedicated or overlay) transport networks each historically built for a different service into one core transport network (often based on IP and Ethernet). It implies amongst others the migration of voice from a circuit-switched architecture (PSTN) to VoIP, and also migration of legacy services such as X.25, frame relay (either commercial migration of the customer to a new service like IP VPN, or technical emigration by emulation of the "legacy service" on the NGN).

·         In the wired access network, NGN implies the migration from the dual system of legacy voice next to xDSL setup in local exchanges to a converged setup in which the DSLAMs integrate voice ports or VoIP, making it possible to remove the voice switching infrastructure from the exchange.

·         In the cable access network, NGN convergence implies migration of constant bit rate voice to Cable Labs Packet Cable standards that provide VoIP and SIP services. Both services ride over DOCSIS as the cable data layer standard.

In an NGN, there is a more defined separation between the transport (connectivity) portion of the network and the services that run on top of that transport. This means that whenever a provider wants to enable a new service, they can do so by defining it directly at the service layer without considering the transport layer – i.e. services are independent of transport details. Increasingly applications, including voice, tend to be independent of the access network (de-layering of network and applications) and will reside more on end-user devices (phone, PC, set-top box).

Multiprotocol Label Switching

Multiprotocol Label Switching (MPLS) is a mechanism in high-performance telecommunications networks that directs data from one network node to the next based on short path labels rather than long network addresses, avoiding complex lookups in a routing table. The labels identify virtual links (paths) between distant nodes rather than endpoints. MPLS can encapsulate packets of various network protocols. MPLS supports a range of access technologies, including T1/E1, ATM, Frame Relay, and DSL.

MPLS is a highly scalable, protocol agnostic, data-carrying mechanism. In an MPLS network, data packets are assigned labels. Packet-forwarding decisions are made solely on the contents of this label, without the need to examine the packet itself. This allows one to create end-to-end circuits across any type of transport medium, using any protocol. The primary benefit is to eliminate dependence on a particular OSI model data link layer technology, such as Asynchronous Transfer Mode (ATM), Frame Relay, Synchronous Optical Networking (SONET) or Ethernet, and eliminate the need for multiple layer-2 networks to satisfy different types of traffic. MPLS belongs to the family of packet-switched networks.

MPLS operates at a layer that is generally considered to lie between traditional definitions of layer 2 (data link layer) and layer 3 (network layer), and thus is often referred to as a "layer 2.5" protocol. It was designed to provide a unified data-carrying service for both circuit-based clients and packet-switching clients which provide a datagram service model. It can be used to carry many different kinds of traffic, including IP packets, as well as native ATM, SONET, and Ethernet frames.

A number of different technologies were previously deployed with essentially identical goals, such as Frame Relay and ATM. MPLS technologies have evolved with the strengths and weaknesses of ATM in mind. Many network engineers agree that ATM should be replaced with a protocol that requires less overhead, while providing connection-oriented services for variable-length frames. MPLS is currently replacing some of these technologies in the marketplace. It is highly possible that MPLS will completely replace these technologies in the future, thus aligning these technologies with current and future technology needs.

In particular, MPLS dispenses with the cell-switching and signaling-protocol baggage of ATM. MPLS recognizes that small ATM cells are not needed in the core of modern networks, since modern optical networks (as of 2008) are so fast (at 40 Gbit/s and beyond) that even full-length 1500 byte packets do not incur significant real-time queuing delays (the need to reduce such delays — e.g., to support voice traffic — was the motivation for the cell nature of ATM).

At the same time, MPLS attempts to preserve the traffic engineering and out-of-band control that made Frame Relay and ATM attractive for deploying large-scale networks.

While the traffic management benefits of migrating to MPLS are quite valuable (better reliability, increased performance), there is a significant loss of visibility and access into the MPLS cloud for IT departments

Digital subscriber line (xDSL)

Digital subscriber line (DSL, originally digital subscriber loop) is a family of technologies that provide Internet access by transmitting digital data over the wires of a local telephone network. In telecommunications marketing, the term DSL is widely understood to mean asymmetric digital subscriber line (ADSL), the most commonly installed DSL technology. DSL service is delivered simultaneously with wired telephone service on the same telephone line. This is possible because DSL uses higher frequency bands for data. On the customer premises, a DSL filter on each non-DSL outlet blocks any high frequency interference, to enable simultaneous use of the voice and DSL services.

The bit rate of consumer DSL services typically ranges from 256 Kbit/s to 40 Mbit/s in the direction to the customer (downstream), depending on DSL technology, line conditions, and service-level implementation. In ADSL, the data throughput in the upstream direction, (the direction to the service provider) is lower, hence the designation of asymmetric service. In symmetric digital subscriber line (SDSL) services, the downstream and upstream data rates are equal.