[정보통신공학] 9. WAN Technology and Protocols

Switched Communication Networks

• Switched Communication Networks
  - long distance transmission: intermediate switching nodes의 네트워크에서 이루어짐
  - nodes: content of data가 아니라 switching / routing과 관련
  - end devices: stations (computers, terminals, phones)
  - communications network: collection of nodes and connections (link)
  - data: node와 node 사이의 swtiching으로 routing 됨
• Nodes: 다른 node들에만 연결 or station과 다른 node들에 연결
  - node-to-station links: dedicated to point-to-point links
  - node-to-node links: multiplexed
  - network: partially connected (mesh: fully connected)
  몇몇 중복 path => reliability 제공
  - switching technologies: circuit switching, packet switching
  
• Simple Switching Networks
  
  5,7: node-to-node links
  1,4,3,6: node-to-station links

Circuit Switching Networks

Dedicated Communication Path between two stations
- path: connected sequence of links between nodes
Phases: circuit establishment, data transfer, circuit disconnect
Swtiching capacity(connection 요구 처리 위해)와 Reserve channel capacity(용량 부여, 자원 할당) 필요 => establish connection

• Applications
  - channecl capacity는 connection 유지 기간 동안 계속 헌신. 데이터가 없으면 capacity 낭비됨
  => Inefficient
  - set up (connection) takes time
  - 한 번 connect되면, transfer는 transparent
  - voice traffic (phone)을 위해 개발됨: 자원들은 particular call에 헌신. data rate는 고정.
  
• Telecommunications Components
  - Subscribers: 네트워크 연결 장비 ex) telephone
  - Subscriber line: local loop / subscriber loop. subscriber와 network 사이의 link. typical media (twisted-pair wire. FTTH). few km up to few tens of km
  - Exchanges: switching centers. end office-supports subscribers
  - Trunks: carrier systems. branches between exchanges. multiplexed (하나의 라인이 수천/수만개 운반)

Softswitch Architecture

• Traditional Circuit Switching
  

• Softswitch
  
  - circuit-switching technology의 최신 트렌드
  - 전화 switch로 변환해주는 special sw
  - 비용 저렴. 기능 많음
  - digitized voice bits을 packet 단위로 전송
  - 하나의 장치가 아니라 두 가지 function 분리해서 처리

  • Media Gateway: physical switching function (routing)
  • Media Gateway Controller: call processing logic (복잡)

Packet Switching Networks

Designed for Data

• Operation
  - data: small packets으로 전송. (1000 octets). 긴 메시지는 패킷으로 분리됨. 각 패킷은 user data(part of a larger message), control info(routing, addressing) 포함
  - Store-and-forward: packet을 받으면 buffered(stored briefly)되어 next node에 전달

• Advantages

  • line efficiency ⬆️
    - single node to node link는 packet들에 공유 가능
    - packet들은 queue를 사용하여 빠르게 전송
  • data rate conversion
    - each station connects to the local node at its own speed
    - nodes buffer data if data rate 달라도 해결 가능
    - two stations with different data rates can exchange packets
  • packets: network가 바빠도 accept됨. delivery delay는 증가
    - priorities 사용 가능: high-priority packet 먼저 전송

• Switching Technique
  : message length > maximum packet size => station은 message를 packet 단위로 분리
  - packets는 네트워크에 한 번에 전송
  - packet 처리 방식: datagram, virtual circuit
  

Datagram Packet Switching

• each packet: treated independently
• packets: 어느 route를 타도 됨. 순서 다르게 도착. missing 있음
• up to either exit node or receiver to re-order packets and recover from missing packets

Virtual Circuit Packet Switching

• packet 전송 시작 전에 preplanned route 설정

• dedicated path X, logical connection O

• handshake: call request and call accept packets establish connection
  - 각각의 packet이 자신의 길 찾아감 => destination address 필요
  - virtual circuit: preplanned route 설정. 길 공유. identifier 필요. (destination address는 필요 X)

• packet별로 routing decisions 필요 X. virtual circuit establish 전에 only one decision 필요 => Preplanned route

  E\I	Datagram(IP)	Virtual Circuit
  Datagram(UDP)	O (destination station)	X
  Virtual Circuit(TCP)	O (exit node)	O(ATM)

• Effect of packet size
  tproc, tprop = 0
  

Communication Switching Techniques 비교

Example
N: # of hops between two given stations
L: message length in bits
B: data rate, in bits per second (bps), on all links
P: fixed packet size in bits
H: overhead (header) bits per packet
S: call setup time (circuit switching or virtual circuit) in sec
D: propagation delaty per hop in sec

Asynchronous Transfer Mode(ATM)

• cell: ATM에서 사용하는 packet. 53 octets. small and 고정 길이
• Connection-Oriented
• performance of a circuit-switching network and the flexibility and efficiency of a packet-switching network
• data, voice, video support
• transmission based on priority and QoS

Protocol Architecture

ATM과 Packet Switching 유사점
- transfer of data in discrete chunks
- muptiple logical connections multiplexed over single physical interface

streamlined protocol with minimal error and flow control
- reduce overhead of processing cells
- reduce # of overhead bits in each cell
- enable ATM to operate at high data rates
use of fixed-size cells -> simplifies processing at each node
data rates (phywical layer): 25.6Mbps to 622.08Mbps

- **Physical layer**: involves specification of transmission medium and signal encoding scheme - **ATM layer** - common to all services - provides packet transfer capabilities - defines the use of logical connections - **ATM Adaptation layer (AAL)** - service dependant - maps higher-layer info into ATM cells - collects info from ATM cells for delivery to higher-layer => **Convergence Sublayer (CS), Segmentation of Reassembly Sublayer (SAR)로 구성**

Reference Model Planes

• User plane: provides for user info transfer
• Control plane: performs call and connection control functions
• Management plane
  - plane management: performs management functions related to a system. provides coordination between all the planes
  - layer management: performs management functions relating to resources and parameteres in protocol entities

Logical Connections

• Virtual Channel Connection (VCC): basic unit of switching
  - analogous to virtual circuit in X.25
  - set up between two end users and a variable-rate, full-duplex flow of fixed-size cells is exchanged

  • user-network exchange (control signaling): user-network 사이의 interface
  • network-network exchange (network management, routing): network 사이의 interface

• Virtual Channel Connection Uses
  : cell sequence integrity preserved within a VCC: cell 순서 무결성 VCC 내에서 유지

  • between end users
    - carry end-to-end user data/control signaling between end users
    - VPC provides overall capacity -> VCC organization is up to two end users, Set of VCCs does not exceed VPC capacity
  • between an end user and a network entity
    - used for user-to-network control signaling
    - user-to-network VPC can be used to aggregate traffic from end user to network exchange or server (VPC 레벨에서 말단 network로 들어오는 traffic 묶음)
  • between two network entities
    - used for network traffic management and routing functions
    - net-to-net VPC can be used to define a common route for exchange of network management information

• Virtual Path Connection (VPC): bundle of VCC with same end points
  (advantages)
  - simplified network architecture: network transport functions can be separated into those related to VCC and VPC
  - increased network perfo and reliability: network deals with fewer aggregated entities
  - reduced processing and short connection setup time: much of the work is done when VP is set up
  - enhanced network services: user may define closed user gropus or closed networks of VC bundles

Cells

• ATM cells: fixed-size cells, 총 53 octets (5 header + 48 info field)

• Advantages
  - reduce queuing delay for high-priority cells
  - can be switched more efficiently
  - easier to implement the switching mechanism in hardware
  

• Header format
  - generic flow control (GFC): only at user-network interface (UNI)
  : used to control cell flow for different QoS only at UNI
  : used to alleviate short-term overload conditions in network
  - virtual path identifier (VPI): constitutes routing field for network
  - virtual channel identifier (VCI): used for routing to and from end user
  - payload type (PT): 3 bits only
  : indicates type of info in info field ex) user info or network management/maintenance info
  : provide inband control info

  Payload Type Field Coding

• Cell loss priority (CLP): 1 bit
  : used to provide guidance to network in event of congestion (congestion의 발생을 network에 알려줌)
  - 0: cell of relatively higher priority. alternative 없으면 discard 안 됨
  - 1: cell is subject to discard
  - cell with CLP = 1 is delivered == network is not congested
  - the network sets CLP to 1 for any data cell in violation of agreement between user and network

• Header Error Contro (HEC)
  - 8-bit error control field
  - calculated on remaining 32 bits of header

  • polynomial: $x^8 + x^2 + x + 1$
  • allows not only error detection but single-bit error correction because of sufficient redundancy in the code
    * FCS: error detection을 위해 사용
    오류 생기지 않게 정규하게 cell switch 설계