Transmission Methods

The Role of Network Transmission Methods

•	Circuit and packet switching are the two methods used to establish and manage connections across a network
•	Three broad categories:
•	Methods to establish and terminate network connections
•	Rules for the orderly transfer of data across the network
•	Procedures to manage and control transmission link operations
•	These rules and procedures: Standards and Protocols
•	Data is sent along two types of transmission paths
•	Physical connection – established for the duration of the session, used for voice traffic and some data transmission
•	Virtual path – route established through the network but not dedicated and may be shared with other terminals, used for packet switching

Three Functions of Every Telecommunications Network

•	Connection Establishment – making a connection between two terminals and releasing it upon termination
•	Data Transfer – moving info using precisely defined procedures for formatting and synchronizing
•	Data Link Control – ensure the data Is correctly sent and received and errors are recognized and resolved

Communications Protocols

•	Protocol – the set of rules or conventions by which two machines talk to each other
•	ITU-T Standard Protocols (a few of many)
•	X.25 – basic packet switching
•	X.51 – multiplexing data across international interface on synchronous networks
•	X.75 – call control and data transfer procedures on international circuits between packet-switched networks
•	X.400 – routing electronic messages across different networks
•	X.500 – network addressing across networks
•	Vendor Specific Protocols – AppleTalk, BSC, SDLC, HDLC
•	Specialized Protocols – precision in procedures and technical innovations

Connection Establishment Protocols

•	Software that:
•	interprets the destination address and locates the device
•	establishes a path through the network that links two terminals
•	signals the receiving terminal to get ready
•	releases the connection on completion of transmission
•	Connection-Oriented Service – designates a specific path
•	Connectionless Service – includes logic in each switching node, minimizes network congestion

Data Transfer Protocols

•	A protocol specifies:
•	Full duplex – two directions simultaneously
•	Half duplex – two directions, but only one way at at time
•	Synchronous – sychronized transmitters and receivers: large blocks of data
•	Asynchronous – start and stop bits with each character: bursty transmissions

Data Link Control Protocols

•	Manage the flow of information across the network
•	Detect and correct errors.
•	Like humans in conversation, they say:
•	Message garbled, please repeat
•	Speed up or slow down
•	Usually include a buffer for temporary storage of info blocks
•	Usually include some form of acknowledgement system

Error Control Protocols

•	In today's world of electronic funds transfer, errors are unacceptable
•	Good error control makes up for a multitude of sins in a network
•	Two functions of these protocols:
•	Error detection
•	Error correction
•	Parity Bits – simple method of adding the bits in the byte
•	Cyclic Redundancy Checking– complex mathematical calculations that allow only three bits in a hundred million to be incorrect

The Open Systems Interconnection Model

•	Once, the only telecomm network was the public switched phone system and the only computers were IBM mainframes
•	Protocols were engineered to the specifications of a single vendor – AT&T or IBM
•	Then came minicomputers and PCs, relaxed regulations, new kinds of traffic, and many and varied vendors
•	There was a need for interconnection, but protocols remained vendor-specific
•	Standards-setting bodies could not keep up and the market took over
•	In response to the mayhem came a new approach: the Open Systems Interconnection Model

The Layered Protocol System Concept

•	The OSI Model has seven layers
•	Many protocols are included within each layer – they are independent from other layers
•	Protocols within each layer address a specific set of interrelated transmission functions
•	For example: HDLC: layer 2, IP: layer 3, X.25: layer 4, FTP: layer 5, X.400: layer 6
•	Layered approach enables using different vendors as long as standard interfaces are supported
•	Three key elements:
•	Generic services to adjacent layers: functions that a lower layer provides to an adjacent higher layer
•	Interfaces between layers: specified links between layers – part of the telecommunications architecture
•	The protocols contained within each layer: from dozens to hundreds

Connection Establishment Methods

•	Are a layer three function
•	Routes data between sender and receiver
•	The three main alternatives:
•	Circuit switching
•	Packet switching
•	Fast packet switching

Circuit Switching

•	Most common method historically
•	Used for public and private voice and data all over the world
•	Used in IBM Systems Network Architecture (SNA)
•	Establishes a dedicated path for the duration of the session
•	Establishes a circuit
•	Transfers the information
•	Disconnects the circuit
•	Typically operates in full duplex mode
•	Information travels at the same data rate both ways
•	Sending and receiving devices must operate at the same data rate

Circuit Switching Inefficient But Widespread

•	Can be an inefficient transmission method if:
•	The circuit Is not fully utilized
•	There are delays in setting up the circuit
•	Applications are widespread
•	Public telephone service
•	Dial-up data services
•	Private Branch Exchange (PBX)
•	Wide area private voice exchange
•	Several PBXs linked by a software-defined network (SDN)
•	Data switch (data version of a PBX)

Packet Switching

•	A bit stream Is subdivided into packets
•	Typical (but not standard) size Is 128 characters (bytes)
•	Formed at terminal or switching node
•	Each packet contains
•	User information (message or data)
•	Control information
•	Destination address
•	Sequence number of the packet
•	Error detection/correction codes

Switching Through Nodes

•	In regular packet switching each node:
•	Receives the packet in a buffer, reads the address, adjusts the transmission rate
•	Queues the packet for transmission to the nearest node on the destination path
•	At the node connected to the destination terminal:
•	Control information is stripped off
•	Data is reassembled, if necessary
Packet Switching Methods

•	Datagram Routing Method
•	First switching node determines route for each packet based on current congestion
•	Packets may take many different routes, arrive at different times out of sequence
•	Final switching node buffers, then reassembles the packets in sequence before sending to destination terminal
•	Virtual Circuit Routing Method
•	Single connection is established through the network at the beginning of the session based on congestion at that moment
•	All packets travel on the same path for the duration of the transmission
•	Packets travel and arrive in sequence
•	Transmission path is shared with packets from other communications sessions

Fast Packet Switching

•	Regular packet switching
•	Speed limitations due to error and flow control checks at every node
•	An artifact of older, noisier networks
•	Fast Packet Switching
•	Today’s networks – virtually error-free
•	End-user terminals perform error and flow control on an end-to-end basis
•	Much faster than regular packet switching
•	Offers lower costs and higher performance

Fast Packet Switching Methods

•	Frame Relay
•	Intended for data but supports compressed and packetized voice and video
•	Designed mainly for LAN to LAN internetworking
•	Encapsulates LAN packets of variable size 
•	Transmits at 56/64 kbps, T1 (1.5 mbps), or T3 (45 mbps)
•	Cell Relay
•	Supports voice, video, and data at very high speeds
•	Fixed-size frame encapsulates the LAN packet without altering it 
•	53 bytes in a cell: 48 for data and 5 for addressing
•	Small cells favor isochronous service
•	Access speeds of 1.5 mbps to 622 mbps
•	Backbone speeds of 2.5 gbps – later up to 200 gbps

Switched Multimegabit Data Service (SMDS)

•	Based on cell relay technology
•	Connectionless, high-speed data transmission service
•	Offered by many service providers primarily in a Metropolitan Area Network (MAN) environment
•	Enables simple LAN/WAN interface 
•	Moves data over WANs at up to T3 speeds
•	Supports asynchronous, synchronous, and isochronous data
•	Designed as a smooth transmission path to ATM

Representative WAN Protocols

•	Transmission Control Protocol/Internet Protocol (TCP/IP)
•	Origins in the ARPANET project
•	Relatively simple protocol
•	Used in UNIX environments and multivendor networks, including IBM's Synchronous Network Architecture (SNA)
•	Advanced Peer-to-Peer Network (APPN)
•	IBM's peer-to-peer extension of SNA
•	Enables direct user-to-user communication without an intervening server
•	DECnet Phase V
•	Developed by Digital Equipment Corporation
•	Is a peer-to-peer protocol
•	Is proprietary but supports OSI protocols

Representative LAN Protocols

•	Appletalk 
•	Developed by Apple Computers based on a 60s Xerox protocol
•	Centers on LANs – simple and cheap
•	Was not designed for internetworking
•	Overhead is extensive
•	But works well in conjunction with TCP/IP
•	Internet Packet eXchange (IPX) 
•	Designed by Novell Netware based on a Xerox protocol
•	Dominated LANs in the early and mid 90s
•	Like Appletalk, overhead is extensive
•	Packets can be encapsulated and carried on a variety of networks
•	Ethernet 802.3 CSMA/CD
•	Designed at Xerox by Bob Metcalfe, who went on to found 3Com
•	It's a physical and data link protocol operating on layers one and two of the OSI model
•	Runs over 2X, coax, and fiber from 10 mbps to 100 gbps