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CS457/557 - Computer Networks: Chapter 8 - Wide Area Networking Concepts, Architectures, and Services

Objectives

  1. Understand the concept of multiplexing in general as well as several multiplexing techniques and related technology and applications in particular.
  2. Understand the relationship between business motivation, available technology, and carrier services in creating wide area networking solutions.
  3. Understand the advantages, limitations, and technology of current and forthcoming packet-switching networks.
  4. Understand the importance of standards as applied to wide area networking.
  5. Understand the interrelationships and dependencies between the components of any wide area network architecture.
  6. Understand the impact of the evolution in switching methodologies as it applies specifically to frame relay and cell relay.

Introduction

Basic Principles of Wide Area Networking

CHOICE: dedicated system-to-system connection vs. single wide area link

Packetizing and Multiplexing

  • packetizing segmenting of data transmissions between devices into structured blocks or packets of data which contain enough management information to assure delivery of the packet of data
  • multiplexing takes packet of data and sends it over a shared wide area connection along with other packetized data from other sources
    Raw Data Packetizing: packetized data Multiplexing: data packets from multiple different sources are loaded onto a single wide area link for transmission Transmission of multiplexed data over a single wide area connection -- may be packet-switched or circuit-switched Demultiplexing: data packets from multiple different sources are received off of the single shared wide area connection and forwarded to their proper destination addresses Depacketizing: overhead or header information is removed and initial raw data is forwarded to its designeated destination for processing

    Starting Flag Source Address Destination Address Type Length Control Data Error Control Closing Flag

    Frequency Division Multiplexing (FDM)

    requires guardbands between separate frequency channels to prevent crosstalk
    data over voice units (DOV) allows simultaneous transmission over phone wires

    Time Division Multiplexing (TDM)

    Uses polling

    Statistical Time Division Multiplexing (STDM)

    1. eliminates "idle time" allocations to inactive terminals
    2. eliminates padded blanks or null characters in the composite message blocks

    Wide Area Network Switching

    Circuit-Switching vs. Packet Switching

    Switching allows temporary connections to be established, maintained, and terminated between message sources and message destinations

    Circuit Switching

    - a switched dedicated circuit is created to connect two or more devices

    Packet Switching

    - packets travel one at a time from the message source through a packet switched network (PSN), or public data network (PDN) to the message destination
    requires a PAD - packet assembler / disassembler on each end of the PSN
    datagram - packet containing full source and destination address plus message segment

    Connection-Oriented vs. Connectionless Packet-Switched Networks

    connectionless packet networks have no error-detection or flow control techniques but rather uses end-user devices

    Connection-oriented packet networks establish virtual circuits enabling message packets to follow one another

    Connectionless Connection-oriented
    Overhead Less More
    Greatest Strength Ability to dynamically reroute data Reliability
    Call Set-up None Yes
    Addressing Global Local logical channel number
    Also Known As... Datagram, unreliable Reliable, Virtual Circuit
    Virtual Circuit None Created for each call, virtual circuit table established
    Error Correction Left to end-user devices By virtual circuit
    Flow control Left to end-user devices By virtual circuit

    Wide Area Networking Transmission

    X.25 Defines Standard Interface to Packet-Switched Networks

    International CCITT standard that defines the interface between DTEs and a packet-switched network
    7. Application X.25 Provides transparency to upper layers;
    the top 4 layers need not worry about
    delivery of data via a packet switched network
    6. Presentation
    5. Session
    4. Transport
    3. Network Packet Layer Protocol (PLP)
    2. Datalink High-Level Data Link Control (HDLC) Link Access Procedure-Balanced (LAP-B)
    1. Physical RS-232
    Functions of HDLC
    1. Organize data into structured frames that may contain more than one packet
    2. Assure reliable delivery of data via error checking
    3. Provides point-to-point data delivery between adjacent nodes
    8-bit Flag 8-bit Address Field 8-bit Control Field Variable length Information Field 16 bit Frame check sequence 8-bit Flag
    LAP-B is a subset of HDLC

    PLP - establishes, maintains and terminates virtual circuits within a connection-oriented packet switched network

    Important Standards Related to X.25

    A Business Perspective on circuit-Switching vs. Packet Switching

    Circuit Switched Perferred Packet Switched Perferred
    Data Characteristic Chacteristic Value
    Application Source Overnight batch jobs
    File transfers    		 Transaction oriented
    Interactive data
    Data flow Large and steady Large and bursty
    Delivery pressure No great hurry Seconds count
    Reliability Communication software for error detection Error detection and flow control built into network itself
    Business Characteristic Characteristic Value
    Usage charge basis Per TIME connected Per PACKET connected

    Wide Area Network Switching and Transmission Architectures

    Local Loop Transmission Alternatives

    POTS

    using modems

    ISDN: Integrated Services Digital Network

    Narrowband ISDN ISDN provides an all-digital circuit-switched network that could deliver leased-line transmission quality and bandwidth with usage-based rates

    Services Video freeze-frame
    Voice
    Data    		 LAN Interconnect
    Full motion video
    Voice
    Data
    Transport category BRI (Basic Rate Interface) PRI (Primary Rate Interface)
    Transport capacity 2B+D
    2 x 64KBps + 16 KBps = 144 KBps    		 23B+D
    23x64K + 64K = 1.536MBps
    Transport architecture 2-wire dial-up T-1
    Inter-switch protocol and switching architecture Signalling System 7 (SS7)

    ADSL

    works over POTS, at higher frequencies (can use simultaneously with voice communications)
    ADSL Upstream ADSL Downstream Distance Limitation
    150 Kbps 1.5 Mbps 18,000 ft.
    640 Kbps 6.0 Mbps 12,000 ft.
    Other DSL Solutions

    Cable TV

    Two options:
    1. requires POTS for upstream
    2. modify cable architecture to support simultaneous upstream/downstream transmission (up to 30 Mbps downstream & 768 Kbps upstream)

    A Look at Transmission Services

    Circuit Switched Leased/Private Lines Packet Switched
    POTS
    Switched 56K, 64K, 384K, T-1
    ISDN-BRI, ISDN-PRI, ISDN-multirate    		 Analog leased
    DDS (digital 1200bps-56K)
    DS-0 64K
    Fractional T-1
    T-1, T-2, T-3
    SONET    		 X.25
    Frame Relay
    SMDS
    B-ISDN

    Digital Transmission Standards

    uses periodic framing or synchronous TDM(time division multiplexing)
    [(24 voice channels x 8 bits/channel ) + 1 framing bit] x 8000 frames / sec = 1.544 MBps

    Digital Service Hierarchy

    Digital Service Level Number of Voice Channels Transmission Rate Corresponding Transmission Service
    DS-0 1 64K DS-0 or switched 64K
    DS-1 24 1.544M T-1 or switched T-1
    DS-1C 48 3.152M T-1C
    DS-2 96 6.312M T-2
    DS-3 672 44.736M T-3
    DS-4 4032 274.176M T-4

    Digital Service Level Number of Voice Channels Transmission Rate Corresponding Transmission Service
    1 30 2.048M E-1
    2 120 8.448 M E-2
    3 480 34.368M E-3
    4 1920 139.264M E-4
    5 7680 565.148M E-5
    T-1: requires a dedicated / leased line terminated with a CSU/DSU connected to the local router

    SONET: Synchronous Optical Network

    optic transmission service delivering multiple channels using TDM
    Optical Carrier Level Transmission Rate
    OC-1 51.84M
    OC-3 155.52M
    OC-9 466.56M
    OC-12 622.08M
    OC-18 933.12M
    OC-24 1.244G
    OC-36 1.866G
    OC-48 2.488G
    SONET Framing
    [(3 octets overhead + 87 octets data) x 8 bits /octet] x 9 rows / frame x 8,000 frames /sec = 51.84Mbits / sec
    superframe -- 9 rows of 90 octets
    synchronous payload envelope (SPE) -- 87 octets of data

    Major Components of a Wide Area Network Architecture

    User Demands (voice, data, video, imaging, fax)
    Interface Specification
    Network services
    Network architecture
    Switching architecture Transmission architecture
    Switching architectures - assures proper routing of information from source to destination
    Transmission architectures - circuits over which the information is actually delivered

    maximum bandwidth for X.25 is 64Kbps

    X.25 standard consists of the lower three level protocol
    Packet Layer Protocol (PLP)
    High-Level Data Link Control (HDLC) Link Access Procedure-Balanced (LAP-B)
    RS-232

    Fast Packet Switching: Frame Relay and Cell Relay

    X.25's Hop-by-Hop Error Checking

    Frame Relay

    uses higher level protocols to perform end-to-end error checking (sliding window file transfer protocol)
    X.25 uses ACK/NAK (store and forward)
    Frame Relay - point to point error detection, end-to-end error correction
    Frame Relay - 2-layer protocol (physical & data layers)
    8-bit FLAG 1 bit Extended Address 1-bit Command/Response 6-bit Data-Link Connection Identifier 1-bit Extended Address 1-bit Discard Eligilibity 1-bit Backward Explicit Congestion Notification 1-bit Forward Explicit Congestion Notification 4-bit Data Link Connection Identifier Information 16-bit Frame Check Sequence (CRC-16) 8-bit FLAG

    Frame Relay Switching Architecture Advantages

    uses PVC (permanent virtual circuits) to forward frames from source to destination through the frame relay cloud
    transmission rates usually T-1, but can be up to T-3
    protocol independent / transparent

    Cell Relay: A Switching Alternative to Frame Relay

    all cells are of a fixed length - 53 octets
    can provide switching capability upto T-3s (45Mbps) and fiber optic transmission upto 2.4 Gbps
    key standard is ATM (asynchronous transfer mode)
    ATM is defined by two different cell formats

    ATM CELL Structures

    UNI: User- Network Interface
    4-bit Generic Flow Control 8-bit Virtual Path Identifier 16-bit Virtual Circuit Identifier 2-bit Payload Type 1-bit Reserved 1-bit Cell Loss Priority 8-bit Header Error Control 48-octets of Information
    NNI: Network- Network Interface
    12-bit Virtual Path Identifier 16-bit Virtual Circuit Identifier 2-bit Payload Type 1-bit Reserved 1-bit Cell Loss Priority 8-bit Header Error Control 48-octets of Information

    Where Does ATM Fit?

    User Demands High-speed multimedia intra-LAN connectivity LAN internetworking connectivity Switched LAN Internetworking data dial tone Wide area simultaneous Voice/data interactive video conferencing real-time imaging
    Network Services ATM LAN Hubs Frame relay SDMS-Switched Multimegabit Data Service B-ISDN-Broadband Integrated Services Digital Network
    Networking categories LANs Inter LANs MANs WANs
    Cell relay standard ATM ATM/SMDS interface IEEE 802.6 DQDB - Distributed Queue Dual Bus ATM
    Connectionless or connection oriented Connection oriented Connection oriented Connectionless Connection oriented
    Switching architecture Cell relay
    Transmission architecture SONET-Synchronous Optical Network

    Introduction

    Data Dial-Tone: SDMS - Switched Multimegabit Data Service

    SDMS - connectionless network service delivering switched LAN internetworking and data dial tone in a MAN deployment while adhering to IEEE 802.6 & DQDB protocols by delivering fixed length cells of data to their destination via a SONET transmission system

    MAN - metropolitan area network ( less than 50 Km)
    MAN Services Switched multimegabit data service (SMDS) Fiber ditsributed data interface (FDDI) Deutsche Bundepost Telekom (DBP Telecom) Metrofit multimegabit data service (MMDS)
    Current speed 44.736 MBps (T-3) 100 MBps 2.048 MBps (E-1) 100 MBps
    Currently transporting Data only Data only Data, voice & video Data only
    Underlying Architecture DQDB FDDI DQDB Fiber backbone
    Architecture type Switched access Shared media Switched access Circuit switched

    Distributed Queue Dual Bus: IEEE 802.6

    comprised of dual buses (actually dual rings)
    each bus carries traffic (data & reservations) in one direction only (downstream, upstream)
    Access units can read and write from each bus
    Slots on each bus are preallocated 53-octet cells
    Two counters (request counter and countdown counter are used to keep track of the number of downstream access units have made reservations on future empty cells

    Low Traffic Functionality

    1. reads cell in front of it on bus to determine if it's empty
    2. checks the request counter to see how many downstream reservations have been made
    3. if cell empty & counter=0, then loads 53-octet into empty cell

    High Traffic Functionality

    1. reads cell in front of it on bus to determine if it's empty
    2. checks the request counter to see how many downstream reservations have been made and copies the value of the request counter to the countdown counter
    3. makes a reservation on the upstream bus which increases upstream access units' downstream request counter by one

    Effects of Variable Traffic Conditions

    for timing-sensitive traffic (voice, video), variable waiting periods can be a problem
    prearbitrated access prereserving or prearbitrating empty cells on a timed basis (every 125 microseconds); used for isochronous traffic

    SMDS Interfaces User Data with the MAN

    dual fiber optic bus often replaced by a SMDS switch which works at the MAC sublayer
    uses phones at T-1, T-3 or SONET (155 MBps) speeds

    Security

    allows group tables to specify which users can access the MAN (virtual private network)

    Frame Relay is an ISDN Service

    ISDN (circuit-switched) acts as a transport mechanism to deliver frame relay service (packet-switched)

    ISDN Incompatibilities

    1. two competing switches: AT&T 5ESS & Northern Telecom's DMS100 have different interface specs with customer premises equipment (CPE)
    2. requires special phones and modems (terminal adapter)
    3. SS7 (signalling system 7) defines specs for network interoperabililty
    4. LPN-ISDN (local packet network-ISDN) provides interface to X.25

    ISDN/LAN Interconnections

    1. ISDN/LAN gateway -- LAN attached PC with an NIC and ISDN card
    2. ISDN/LAN bridge/router -- ISDN to MAC sublayer bridge looks at destination address and dials remote ISDN node
    3. ISDN/LAN server -- multiple ISDN cards
    Features of an ISDN PC adapter card

    SONET + ATM = B-ISDN

    SONET is the optical transmission media
    ATM is the switching architecture
    together they form the underlying network architecture of the B-ISDN of the future which will provide bandwidth on demand

    Slides

    Review Questions: 7, 29 (due Oct. 29, 1998)

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