Chapter 10 Switching in Data Networks School of Information Science and Engineering, Shandong University Associate Prof., Deqiang Wang
Outline Data transmission in PSTNs Switching Techniques for Data Transmission Data Communication Architecture Link-to-link Layers End-to-end Layers Internetworking
Introduction Convergence of computer and communication technologies Data transmission Remote computing, 1950s and 1960s Data transmission: telegraph networks & telex networks, 50~110 bauds Data over PSTNs: Modem, 1.2~56 kbps Public data networks: PDNs Internet: TCP/IP
Introduction Data networks Wide area networks (WANs) Metropolitan area networks (MANs) Local area networks (LANs) Personal area networks (PANs) Body area networks (BANs) Switching technologies Circuit switching Store-and-forward based switching
Data network hierarchy Wide area network Metropolitan area network Local area network
Coverage and Speeds 10 4 SONET 10 3 10 2 10 1 10-1 10-2 10-3 LAN EPABX LAN FDDI MAN PSTN MAN TDN SBDN 0.1 1 10 100 1000 10,000
10.1 Data Transmission in PSTNs Modem Combined modulator/demodulator Transmission of data over PSTNs Modulation: ASK/PSK/FSK/QAM Data rates: 1200bps~56kbps
Data Transmission in PSTNs Computer M M M M T T M M PSTN M M T T M M M M Computer T: terminal
10.2 Switching Techniques Differences between voice and data traffic Switching techniques: Circuit switching, S&F Voice traffic Data traffic Continuous Bursty Low bandwidth for long duration Typical utilization 85%~95% Half duplex Real time Loss acceptable Error tolerable High bandwidth for short duration Typical utilization 5%~15% Half of full duplex Nonreal time Loss unacceptable Error unacceptable
Circuit Switching Features of Circuit Switching An electrical path is established between the source and the destination before any data transfer takes place. The path is dedicated to the communicating pair for the entire duration of the transmission. No other potential user can use the path even if it is idle. The connection is released only when either of the communicating entities sends a request signal.
Principle of Circuit Switching Phases involved Connection establishment Data transmission Connection release Routing algorithm/criteria Path length Traffic condition etc.
Principle of Circuit Switching T H 2 H 3 N 2 N 4 T N 1 N 6 N 5 T N 3 H 1 H 5 H4 H: Host N: Node T: Terminal
Principle of Circuit Switching Time taken T cs = T e + T t +T r = (N-1)T m + M/R + NT h time for path setup: T e =(N-1)T m T m =average route selection time; N=number of nodes in the path time for data transmission: T t =M/R M=message length; R=data rate time for path release: T r =NT h T h =time taken by a node to make housekeeping
Principle of Circuit Switching Disadvantages of CS The path setup time turns out to be an excessive overhead for bursty data traffic. The entire line quality is affected if there is one bad link in the circuit. The required bandwidth is statically allocated and the unused bandwidth is wasted. The network provides no error control facilities. The end systems should have the capability to handle errors.
Store and Forward Switching Features of S&F Switching No end-to-end link is setup prior to data transmission. No dedicated links for a communicating pair. Links are allocated on demand. Users deposit their messages to the nearest switching node, and then on the network takes the responsibility for delivering the message to the destination.
Principle of S&F Switching Configuration of a network Node Processor Storage Communication subsystem Data transmission procedure Deposit Transfer hop-by-hop Message switching vs. Packet switching
S&F Network Configuration N N N N Communication subsystem Processor Storage T T T
Message Switching Concept Once the transmission is initiated, a message is transmitted in its entirety without a break from one node to another. Functions of node processor Receive and store the full user message Check the message for data transmission errors and perform error recovery Determine the destination address from the user message Choose an appropriate link towards destination based on certain routing criterion Forward the message to the next node on the chosen link
Message Switching Drawbacks Large buffer size is required for long messages. If an error occurs, it costs a long time to retransmit the entire message. Short message with higher priority may be blocked by long messages with lower priority. Solution: packet switching
Packet switching Concept A message is split into a number of packets of fixed size at the source host. The packets are transmitted in a S&F fashion. Each packet is routed and transmitted independently. The message is reassembled at the destination host.
Packet switching Packet format Destination ID Source ID Header Message ID Packet switching network (PSN) Topology Packet delivery Routing algorithm Packet ID Control User data
PSN H 1 H 6 H 2 P 1511 P 1513 N 5 N 4 P 1512 H 5 H 3 N 1 N 3 H 7 N 2 H 4 H 8
Packet switching Problems in packet switching Packets of a single message may travel via different routes and arrive at the destination with different delays. Packets of a single message may be out of sequence at the destination. The network is responsible for resequencing the packets before delivery to the destination host.
Packet switching Services in PSN Datagram service/connectionless service Virtual circuit service/connection-oriented service The route from the source to the destination is fixed for all packets of a message. Packets arrive at the destination in sequence. No resequencing is needed.
Virtual circuit service Principle Before packets transmission, a route is chosen for the source and destination pair. It seems like a (virtual) circuit. But the links associated to the route are not dedicated to the connection. Instead, they are shared by a great number of possible connections. An identifier is assigned to the source and destination pair. All packets carry the assigned identifier as address identification and travel the same route.
Delays in S&F networks S&F delay: T sf = T s + T f T s : Storage delay T f : Forwarding delay Forwarding delay: T f =(N-1)(T q +T m )+T t T q : average queuing delay in each node T m : processing delay in each node (routing) T t : message or packet transmission delay N: number of nodes involved in the transfer
Delays in S&F networks Transmission delay T t If the transfer rate R is uniform on all links T t =(N+1)M/R+T p If the data transfer rates are nonuniform T t =M (1/R 1 +1/R 2 + + 1/R N+1 ) +T p M: message length or packet size T p : total propagation delay from source to destination
10.3 Data Communication Architecture Concepts Closed systems/networks: not open to other vendor systems for interworking. Open systems/networks: capable of organizing data transfer with any other vendor s system. Question How to realize internetworking among systems from different vendors?
Definitions in OSI model Definitions System: one or more autonomous computers and their associated software, peripherals and users, which are capable of information processing and/or transfer. Subsystem: A logical independent smaller unit of a system. Layer: a collection of subsystems of the same rank of all the interconnected systems. Entity: The hardware subsystems and/or software packages performing some kind of functions in a layer.
Definitions in OSI model SS5 SS4 SS3 SS2 SS1 L5 L4 L3L2 SS#: Subsystem # L#: Layer # L1 SS5 SS4 SS3 SS2 SS1 SS5 SS4 SS3 SS2 SS1 System SS5 SS4 SS3 SS2 SS1
Conventions in ISO-OSI model A layer is referred to as (N) layer, N being the layer number. The layer immediately below (N) layer is referred to as (N-1) layer. The layer immediately above (N) layer is referred to as (N+1) layer. Services offered by the (N) layer are termed (N) services. The entities of (N) layer are referred to as (N) entities. Entities in the same layer, but not in the same subsystem are known as peer entities. Peer entities communicate using peer protocols. Data exchange between peer entities is in the form of protocol data units (PDUs). Data exchange between entities of adjacent layers is in the form of interface data units (IDUs). IDUs exchanged between (N) and (N+1) layers are referred to as (N) IDUs.
7-layer ISO-OSI model Application Application Protocol Application Presentation Session Presentation Protocol Session Protocol Presentation Session End-to-End Layers Transport Transport Protocol Transport Network Network Network Data Link Link Data Link Link-to-Link Layers Physical Physical Physical Communication Subnet
Link-to-link vs. End-to-end Link-to-link layers Layers 1-3 The communication proceeds on a linkby-link basis. End-to-end layers Layers 4-7 Entities in the end systems communicate with their corresponding counterparts. There is no communication with entities in the intermediate systems.
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