[Network Week1] Switched networks, Packet switching, Datagram, Virtual Circuit

Contents
1. Network model
2. Switched Networks (Circuit Switching, Packet Switching)
3. Packet Switched Networks
4. Datagram vs Virtual Circuit

1. Network model

1-1. Data communications model

• Telecommunication
  : Communication at a distance
• Data
  : Information presented in whatever form is agreed upon by the parties
• Data communications are the exchange of data between two devices via some form of trasmission medium such as a wire cable and wireless

1-2. Networks

• A set of devices (often referred to as nodes) connected by communication links
  (즉 커뮤니케이션 링크로 연결된 노드)
  Node can be a computer, printer, or any other device capable of sending and/or receiving data generated by other nodes on the network
  (그럼 노드란..? 노드는 네트워크에서 다른 노드에 의해 생성된 데이터를 주고 받을 수 있는 컴퓨터, 프린터 등의 장치)
  

1-3. Simplified Network Model

2. Switched Networks

2-1. Switched Communication Network

• Transmitting data from SRC to DST thru a network of intermediate network nodes (routers)
• Long distance transmission is typically done over a network of switched nodes
  (Nodes are 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 (forwarded) from node to node

2-2. Simple Switched Network
(최적의 경로를 찾으며 경로를 바꾸어 주는 것)
ex. 6 devices, 7 routers

2-3. Two switching Technologies

• Circuit Switching
  
• Packet Switching
  

2-4. Circuit Switching

• Dedicated communication path between two stations physically and logically
• Path is established before data transmission
• Three phases
  (Establish -> Transfer -> Disconnect)
• Must have switching capacity and channel capacity to establish connection
• Must have intelligence to work out routing (Centralized)
  

2-4. Circuit switching applications

• Inefficient
• Channel capacity dedicated for duration of connection
• If no data, capacity wasted
• Setup (connection) takes time
• Delay prior to data transfer
• Once connected, transfer is transparent
  

2-5. Packet switching Principles

• Circuit Switching designed for voice
  -- Resources dedicated to a particular call
  -- Much of the time a data connection is idel
  -- Data rate is fixed(CBR : Constant Bit Rate)
  -- Both ends must operate at the same time)
• Packet swtiching are shared, channelized
  -- A link between switch may carry several packets at the same time, each sent by a different source and going to different destination

2-6. Basic operation of Packet Switching

• Data transmitted in small packets
  -- Typically 1000 octets
  -- Longer messages split into series of packets
  -- Each packet contains a portion of user data plus some control information
• Control info
  -- Routing (addressing) info
• Packets are received, stored briefly (buffered) and past on to the next node
  -- Store and forward

2-7. Use of Packets

2-8. Packet Switching Advantages

• Line efficiency
  -- Single node to node link can be shared by many packets over time
  -- Packets queued and transmitted as fast as possible

• Data rate conversion (VBR : Variable Bit Rate)
  -- Each station connects to the local node at its own speed
  -- Nodes buffer data if required to equalize rates

• Packets are accepted even when network is busy
  -- Delivery may slow down

• Priorities can be used
  

3. Packet Switched Networks

3-1. 5 Layers in Packet Switched Networks

3-2. Network Layer (3rd Layer)

• Goal
  -- Allow packets to be forwarded from any source to any destination through heterogeneous networks and routers
• Services
  -- Unreliable connectionless service
  -- Reliable connection-oriented service

3-3. Transport Layer

• Goals
  --Imporves the service by the network
  (Reliability, Multiplexing)
• Transport layer services
  -- Unreliable connectionless service
  -- Reliable connection-oriented service

3-4. Routers in Routing

3-5. Network Layer Basic Principles

• Each host/router must be identified by a network layer address
  -- Independent from its datalink Layer address (MAC)
• Network layer forwards packets from source to destination through multiple routers
• Network layer service must be completely independent from the service provided by the datalink layer
  (Hop by hop)
• Network layer user should not need to know anything about the internal structure of the network layer to be able to send packets

4. Datagram vs Virtual Circuit

4-1. Two services in Packet Switching

• Two possible nodes
  -- Datagrams
  -- Virtual Circuits
• Datagram mode is used to provide a connectionless ervice
  -- There is no "network-layer-connection" between two hosts
• Virtual circuits are used to provide a connection-oriented service

4-2. Datagram

• Each packet treated independently
• Packets can take any practical route
• Packets may arrive out of order
• Packets may go missing
• Up to receiver to re-order packets and recover from missing packets
• Used in the network layer

4-3. Virtual Circuit

• Preplanned route established before any packets sent
• Call request and call accept packets establish connection (handshake)
• Each packet contains a virtual circuit identifier instead of destination address
• No routing decisions required for each packet
• Clear request to drop circuit
• Not a dedicated path
  

4-4. Virtual Circuits vs. Datagram

• Virtual circuits
  -- Network can provide sequencing and error control
  -- Packets are forwarded more quickly
  : No routing decisions to make
  -- Less reliable
  : Loss of a node loses all circuits through that node
• Datagram
  -- No call setup phase
  (Better if few packets)
  -- More flexible
  (Routing can be used to avoid congested parts of the network)

5. Key Points

• Circuit switching is used in public telephone networks
  (Designed to handle voice traffic)
• With circuit switching, a dedicated path is established between two stations
  -- Switching and transmission resources within network are reserved for exclusive use of circuit for duration of connection
  -- Connection is transparent
  (Once established it appears as if direct connection)
• Packet switching was designed for more efficient facility than circuit switching for bursty data traffic
• With Packet switching, a station transmit data in small blocks, called packet
• Virtual circuit and datagram
  -- A route is defined between two endpoints and all packets follow the same route
  -- Each packet is treated independently and packet for the same destination may follow different path