Chapter 11 Approaches to Networking Business Data Communications

Chapter 11: Approaches to Networking Business Data Communications, 4 e

LANs, WANs, and MANs Ownership n n WANs can be either public or private LANs are usually privately owned Capacity n LANs are usually higher capacity, to carry greater internal communications load Coverage n n n LANs are typically limited to a single location WANs interconnect locations MANs occupy a middle ground

Comparison of Networking Options

Types of WANs Circuit-switched Packet-switched

Circuit-Switching Definition: Communication in which a dedicated communications path is established between two devices through one or more intermediate switching nodes Dominant in both voice and data communications today n e. g. PSTN is a circuit-switched network Relatively inefficient (100% dedication even without 100% utilization)

Circuit-Switching Stages Circuit establishment Transfer of information n n point-to-point from endpoints to node internal switching/multiplexing among nodes Circuit disconnect

Circuit Establishment Station requests connection from node Node determines best route, sends message to next link Each subsequent node continues the establishment of a path Once nodes have established connection, test message is sent to determine if receiver is ready/able to accept message

Information Transfer Point-to-point transfer from source to node Internal switching and multiplexed transfer from node to node Point-to-point transfer from node to receiver Usually a full-duplex connection throughout

Circuit Disconnect When transfer is complete, one station initiates termination Signals must be propagated to all nodes used in transit in order to free up resources

Public Switched Telephone Network (PSTN) Subscribers Local loop (Subscriber Line) n Connects subscriber to local telco exchange Trunks n n Exchanges n n n Telco switching centers Also known as end office >19, 000 in US n Connections between exchanges Carry multiple voice circuits using FDM or synchronous TDM Managed by IXCs (inter-exchange carriers) First Mile Last Mile

Routing in Circuit-Switched Networks Requires balancing efficiency and resiliency Traditional circuit-switched model is hierarchical, sometimes supplemented with peer-to-peer trunks Newer circuit-switched networks are dynamically routed: all nodes are peer-topeer, making routing more complex

Alternate Routing Possible routes between two end offices are predefined Originating switch selects the best route for each call Routing paths can be fixed (1 route) or dynamic (multiple routes, selected based on current and historical traffic)

Control Signaling Manage the establishment, maintenance, and termination of signal paths Includes signaling from subscriber to network, and signals within network In-channel signaling uses the same channel for control signals and calls Common-channel signaling uses independent channels for controls (SS 7) Signaling System Number 7

ISDN - Integrated Service Digital Network 1 st generation: narrowband ISDN n n n Basic Rate Interface (BRI) two 64 Kbps bearer channels + 16 Kbps data channel (2 B+D) = 144 Kbps circuit-switched 2 nd generation: broadband ISDN (B-ISDN) n n Primary Rate Interface (PRI) twenty-three 64 Kbps bearer channels + 64 data channel (23 B+D) = 1. 536 Mbps packet-switched network development effort led to ATM/cell relay

Past Criticism of ISDN “Innovations Subscribers Don’t Need” , “It Still Doesn’t Network” , “It Still Does Nothing” Why so much criticism? n n overhyping of services before delivery high price of equipment delay in implementing infrastructure incompatibility between providers' equipment. Didn’t live up to early promises

ISDN Principles Support of voice and nonvoice using limited set of standard facilities Support for switched and nonswitched applications Reliance on 64 kbps connections Intelligence in the networks Layered protocol architecture (can be mapped onto OSI model) Variety of configurations

ISDN User Interface “Pipe” to user’s premises has fixed capacity Standard physical interface can be used for voice, data, etc Use of the pipe can be a variable mix of voice and data, up to the capacity User can be charged based on use rather than time

ISDN Network Architecture Physical path from user to office n n n subscriber loop, aka local loop full-duplex primarily twisted pair, but fiber use growing Central office connecting subscriber loops n n n B channels: 64 kbps D channels: 16 or 64 kbps H channels: 384, 1536, or 1920 kbps

ISDN B Channel Basic user channel (aka “bearer channel”) Can carry digital voice, data, or mixture n Mixed data must have same destination Four kinds of connections possible n n Circuit-switched Packet-switched Frame mode Semipermanent

ISDN D Channel Carries signaling information using common-channel signaling n n call management billing data Allows B channels to be used more efficiently Can be used for packet switching

ISDN H Channel Only available over primary interface High speed rates Used in ATM

ISDN Basic Access Basic Rate Interface (BRI) Two full-duplex 64 kbps B channels One full-duplex 16 kbps D channel Framing, synchronization, and overhead bring total data rate to 192 kbps Can be supported by existing twisted pair local loops 2 B+D most common, but 1 B+D available

ISDN Primary Access Primary Rate Interface (PRI) Used when greater capacity required No international agreement on rates n n US, Canada, Japan: 1. 544 mbps (= to T 1) Europe: 2. 048 mbps Typically 23 64 kbps B + 1 64 kbps D Fractional use of n. B+D possible Can be used to support H channels

Packet-Switching Networks Includes X. 25, ISDN, ATM and frame-relay technologies Data is broken into packets, each of which can be routed separately Advantages: better line efficiency, signals can always be routed, prioritization option Disadvantages: transmission delay in nodes, variable delays can cause jitter, extra overhead for packet addresses

Packet-Switching Techniques Datagram n n each packet treated independently and referred to as a datagram packets may take different routes, arrive out of sequence Virtual Circuit n n preplanned route established for all packets similar to circuit switching, but the circuit is not dedicated

Virtual Circuit

Packet-Switched Routing Adaptive routing changes based on network conditions Factors influencing routing are failure and congestion Nodes must exchange information on network status Tradeoff between quality and amount of overhead

Packet-Switched Congestion Control When line utilization is >80%, queue length grows too quickly Congestion control limits queue length to avoid throughput problems Status information exchanged among nodes Control signals regulate data flow using interface protocols (usually X. 25)

X. 25 Interface Standard ITU-T standard for interface between host and packet-switched network Physical level handles physical connection between host and link to the node n Technically X. 21, but other standards can be substituted, including RS-232 Link level provides for reliable data transfer n Uses LAPB, which is a subset of HDLC Packet level provides virtual circuits between subscribers LAPB: Link Access Protocol-Balanced

Virtual-Circuit Service External virtual circuit: logical connection between two stations on the network Internal virtual circuit: specific preplanned route through the network X. 25 usually has a 1: 1 relationship between external and internal circuits In some cases, X. 25 can be implemented as a packet-switched network

WANs for Voice Requires very small and nonvariable delays for natural conversation--difficult to provide this with packet-switching As a result, the preferred method for voice transmission is circuit-switching Most businesses use public telephone networks, but a few organizations have implemented private voice networks

WANs for Data Public packet-switched networks (X. 25) Private packet-switched networks Leased lines between sites (non-switched) Public circuit-switched networks Private circuit-switched networks (interconnected digital PBXs) ISDN (integrated X. 25 and traditional circuit-switching)

WAN Considerations Nature of traffic n n stream generally works best with dedicated circuits bursty better suited to packet-switching Strategic and growth control--limited with public networks Reliability--greater with packet-switching Security--greater with private networks