Tuesday, April 23, 2013

Sistem Terdistribusi


3 model utama yang menjadi acuan dalam mendesain Sistem Terdistribusi 

Physical models are the most explicit way in which to describe a system; they capture the hardware composition of a system in terms of the computers (and other devices, such as mobile phones) and their interconnecting networks.

Architectural models describe a system in terms of the computational and communication tasks performed by its computational elements; the computational elements being individual computers or aggregates of them supported by appropriate network interconnections.

Fundamental models take an abstract perspective in order to examine individual aspects of a distributed system. Fundamental models examine three important aspects of distributed systems: interaction models; failure models; security models.

tiga generasi perkembangan dari Model Fisik Sistem Terdistribusi 

Early distributed systems: In the late 1970s - 1980s in response to the emergence of local area networking technology, usually Ethernet. These systems typically consisted of between 10 and 100 nodes interconnected by a local area network, with limited Internet connectivity and supported a small range of services such as shared local printers and file servers as well as email and file transfer across the Internet.

Internet-scale distributed systems: Building on this foundation, larger-scale distributed systems started to emerge in the 1990s in response to the dramatic growth of the Internet. They incorporate large numbers of nodes and provide distributed system services for global organizations and across organizational boundaries (Enterprise Level). The level of heterogeneity in such systems is significant in terms of networks, computer architecture, operating systems, languages employed and the
development teams involved. This has led to an increasing emphasis on open standards
and associated middleware technologies

Contemporary distributed systems: In the above systems, nodes were typically desktop computers and therefore relatively static (that is, remaining in one physical location for extended periods), discrete (not embedded within other physical entities) and autonomous (to a large extent independent of other computers in terms of their physical infrastructure). The tiniest embedded devices utilized in ubiquitous computing through to complex computational elements found in Grid 

Model Arsitektur pada Sistem Terdistribusi adalah The architecture of a system is its structure in terms of separately specified components and their interrelationships


Perbedaan antara network dan internetwork

Network, identik dengan jaringan Local Area Network (LAN). LAN merupakan hubungan komunikasi data antara berbagai device maupun perangkat komputer dalam suatu jaringan lokal. Disebut jaringan lokal, karena hubungan tersebut berada dalam satu Subnet yang sama. Bukan antar Subnet. Dalam hubungan semacam ini tidak diperlukan mekanisme Routing Protocol yang terdefinisi. Perangkat jaringan yang digunakan umumnya merupakan perangkat jaringan lokal, yang bekerja pada layer 1 dan layer 2 dari protokol jaringan. Contohnya dalam hal ini adalah Hub, Switch, Bridge maupun Repeater.

Internetwork, identik dengan jaringan Wide Area Network (WAN). WAN merupakan hubungan komunikasi data antara berbagai device maupun perangkat komputer antar Subnet. Dalam hubungan ini diperlukan mekanisme Routing Protocol yang terdefinisi. Perangkat jaringan yang digunakan umumnya merupakan perangkat jaringan yang bekerja pada layer 3 dari protokol jaringan. Contohnya dalam hal ini adalah Router.

Perbedaan antara Personal Area Network (PAN), Local Area Network (LAN), Wide Area Network (WAN) dan Metropolitan Area Network (MAN)

Personal Area Networks (PANs), are a subcategory of local networks in which the various digital devices carried by a user are connected by a low-cost, low-energy network. Wireless personal area networks (WPANs) are of increasing importance due to the number of personal devices such as mobile phones, tablets, digital cameras, music players and so on that are now carried by many people. Usually technology used is Bluetooth WPAN

Local area networks (LANs) carry messages at relatively high speeds between computers connected transmission media, such as twisted copper wire, coaxial cable or optical fibre including wireless channel. No routing of messages is required within a segment. Larger local networks, such as those that serve a campus or an office building, are composed of many segments interconnected by switches or hubs. The total system bandwidth is high and latency is low, except when message traffic is very high. Ethernet emerged as the dominant technology for wired local area networks, with a bandwidth of 10 Mbps (million bits per second) and extended to 100 Mbps, 1000 Mbps (1 gigabit per second) and 10 Gbps. Ethernet technology lacks the latency and bandwidth guarantees needed by many multimedia applications. ATM networks were developed to fill this gap, but their cost has inhibited their adoption in local area applications. Instead, high-speed Ethernets have been deployed in a switched mode that overcomes these drawbacks to a significant degree, though not as effectively as ATM.

Wide Area Networks (WANs) carry at lower speeds between nodes that are often in different organizations and may be separated by large distances. The communication medium is a set of communication circuits linking a set of dedicated computers called routers. They manage the communication network and route messages or packets. Total latency for the transmission of a message depends on the route that it follows and the traffic loads in the various network segments. In current networks these latencies can be as high as 0.1 to 0.5 seconds. Speeds of 1–10 Mbps are more typically experienced for bulk transfers of data.

Metropolitan Area Networks (MANs), is based on the highbandwidth copper and fibre optic cabling recently installed in some towns and cities for the transmission of video, voice and other data over distances of up to 50 kilometres. A variety of technologies have been used to implement the routing of data in MANs, ranging from Ethernet to ATM.


Layer-layer yang terdapat pada model OSI beserta fungsinya

Application

Designed to interface to a service.

Presentation

Handles encryption & changes to syntax

Session

Reliability and adaptation measures, such as detection of failures and automatic recovery.

Transport

Provide reliable or best-effort delivery of messages.


Data link
Responsible for transmission of packets between nodes that are directly connected by a physical link.

Physical

It transmits sequences of binary data by analogue signalling


The term protocol is used to refer to a well-known set of rules and formats to be used (for communication between processes).

Contoh: Penggunaan bahasa yang dapat dimengerti bersama.

Perbedaan Header dengan Protocol Data Unit (PDU)


PDU (Protocol Data Unit) merupakan unit data yang berada pada layer protokol tertentu (layer ke-N). Di dalamnya sudah termasuk Protocol Header (PCI) dan SDU (Service Data Unit). Sedangkan Protocol Header atau bisa dikatakan dengan sebutan Header saja, merupakan informasi tambahan yang diperlukan saat dilakukan enkapsulasi terhadap PDU layer ke-N ke layer bawahnya (N-1) agar diperoleh kontruksi PDU yang baru (PDU ke N-1).

Enkapsulasi adalah proses kontruksi PDU ke N-1 dari PDU ke N dengan cara menambahkan header pada PDU ke N

Deenkapsulasi adalah proses pemecahan kontruksi PDU ke N dari PDU ke N-1 dengan cara memisahkan SDU dengan header-nya

Circuit switching

Suatu mekanisme switching (penyambungan) di mana jaringan diduduki terus-menerus selama service diperoleh melalui mekanisme set up (dialing). Susunan jaringan yang terbentuk saat penyambungan terbentuk adalah tetap.

Packet switching

Disebut juga mekanisme store-and-forward. Di mana message diubah dalam bentuk paket-paket, dan dikirim melalui jaringan. Susunan jaringan yang terbentuk dapat berubah-ubah sesuai kondisi di mana paket di-deliver. Dan switching (penyambungan) tidak perlu secara terus-menerus diduduki manakala service diperoleh. Service untuk mengirimkan paket, diperoleh tanpa perlu melakukan mekanisme set up terlebih dahulu.

There are two approaches to the delivery of packets by the network layer:

Datagram Packet Delivery

The essential feature of datagram networks is that the delivery of each packet is a ‘one-shot’ process; no setup is required, and once the packet is delivered the network retains no information about it. Packets transmitted by a single host to a single destination may follow different routes. Every datagram packet contains the full network address of the source and destination hosts. Datagram delivery is the concept on which packet networks were originally based, and it can be found in most of the computer networks in use today.

Virtual Circuit Packet Delivery

A virtual circuit must be set up before packets can pass from a source host A to destination host B. The establishment of a virtual circuit involves the identification of a route from the source to the destination, possibly passing through several intermediate nodes. Once a virtual circuit has been set up, it can be used to transmit any number of packets. Each network-layer packet contains only a virtual circuit number in place of the source and destination addresses. The addresses are not needed, because packets are routed at intermediate nodes by reference to the virtual circuit number. When a packet reaches its destination the source can be determined from the virtual circuit number.


Apakah fungsi pengalamatan layer 4 yang berupa port ? Mengapa pengalamatan layer 3 yang berupa IP  dianggap tidak mencukupi ?

Ports

IP supports communication between pairs of computers must provide process-to-process communication. This is accomplished by the use of ports. (proses yang membutuhkan layanan (service) yang spesifik).

IP hanya digunakan sebagai pengalamatan saat pengiriman paket. Belum sampai pada inter-process communication dari message-nya sendiri yang membutuhkan layanan (service) yang bersifat spesifik.

z
  MTU yang besar membuat paket yang dapat ditampung dalam pengiriman semakin banyak. Namun MTU yang melebihi batas maksimum MTU yang seharusnya, dapat menimbulkan problem manakala terjadi kasus di mana terjadi error pada pengiriman bit. Proses retransmit bit akan membutuhkan waktu yang jauh lebih lama daripada yang seharusnya
Media
MTU (bytes)
Internet IPv4 Path MTU
At least 68[4]
Internet IPv6 Path MTU
At least 1280[7]
1500[9]
Ethernet with LLC and SNAP, PPPoE
1492[10]
Ethernet Jumbo Frames
1500-9000
7981[11]
4464
4352[5]










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