Question

The three major types of Backbone Networks are
based on the devices used. In practice, it is most common to
use a combination of these architectures. In your opinion what
best practices do you recommend in the following?
(a) Architectures.
(b) Technologies.
(c) Implications for management

Answer 1

Backbone Networks

•High speed networks linking an organization’s LANs

• Making information transfer possible between departments
• Use high speed circuits to connect LANs
• Provide connections to other backbones, MANs, and WANs

•Sometimes referred to as

• An enterprise network
• A campus-wide network

Backbone Network Components

•Network cable

• Functions in the same way as in LANs
• Optical fiber - more commonly chosen (provides higher data rates)

•Hardware devices

Computers or special purpose devices used for interconnecting networks

• Bridges
• Routers
• Gateways

Backbone Network Devices

(a)Architectures

• Identifies the way backbone interconnects LANs
• Defines how it manages packets moving through BB
• Fundamental architectures
  • Bridged Backbones
  • Routed Backbones
  • Collapsed Backbones
    • Rack-based
    • Chassis-based
  • Virtual LANs
    • Single-switch VLAN
    • Multiswitch VLAN
• Access Layer (not part of BB)
  • Closest to the users;
• Backbone Design Layers
• –Distribution Layer
  • Connects the LANs together (often in one building
• –Core Layer (for large campus/enterprise networks)
  • Connects different BNs together (building to building)

Bridged Backbone

• Move packets between networks based on their data link layer addresses
• Cheaper (since bridges are cheaper than routers) and easier to install (configure)
  • Just one subnet to worry
  • Change in one part may effect the whole network
• Performs well for small networks
  • For large networks broadcast messages (e.g., address request, printer shutting down) can lower performance
• Formerly common in the distribution layer
  • Declining due to performance problems

Routed Backbone

• Move packets using network layer addresses
• Commonly used at the core layer
  • Connecting LANs in different buildings in the campus
  • Can be used at the distribution layer as well
• LANs can use different data link layer protocols
• Main advantage: LAN segmentation
  • Each message stays in one LAN; unless addressed outside the LAN
  • Easier to manage
• Main disadvantages
  • Tend to impose time delays compared to bridging
  • Require more management than bridges & switches

Collapsed Backbone

• Replaces the many routers or bridges of the previous designs
  • Backbone has more cables, but fewer devices
  • No backbone cable used; switch is the backbone.
• Advantages:
  • Improved performance (200-600% higher)
  • Simultaneous access; :switched” operations
• A simpler more easily managed network – less devices
  • Two minor disadvantages
  • Use more and longer cables
• Reliability:
  • If the central switch fails, the network goes down.

Rack-Based Collapsed Backbones

• Places all network equipment (hubs and switch) in one room (rack room)
  • Easy maintenance and upgrade
  • Requires more cable (but cables are cheap)
• Main Distribution Facility (MDF) or Central Distribution Facility
  • Another name for the rack room
  • Place where many cables come together
    • Patch cables used to connect devices on the rack
• Easier to move computers among LANs
  • Useful when a busy hub requires offloading

Chassis-Based Collapsed Backbones

• Use a “chassis” switch instead of a rack
• A collection of modules
  • Number of hubs with different speeds
  • L2 switches
  • Example of a chassis switch with 710 Mbps capacity
    • 5 10Base-T hubs, 2 10Base-T switches (8 ports each)
    • 1 100Base-T switch (4 ports), 100Base-T router
    • ( 5 x 10) + (2 x 10 x 8) + (4 x 100) + 100 = 710 Mbps
• Flexible
  • Enables users to plug modules directly into the switch
  • Simple to add new modules

(b) Technologies

•Gigabit Ethernet

•Fiber Distributed Data Interface (FDDI)

•Asynchronous Transfer Mode (ATM)

Fiber Distributed Data Interface (FDDI)

• A set of standards designed in 80’s for MANs (ANSI X3T9.5)
  • Also used as BB and LAN technologies
• Limited future
  • Gigabit Ethernet’s strong presence
• A ring network operating at 100 Mbps over fiber cables
  • Assumes a mix of 1,000 stations and 200 Km path
    • With repeaters at every 2 Km
  • Uses 2 counter rotating rings: primary and secondary
    • Data on the primary; secondary used as backup

ATM

• Originally designed for use in WAN
  • Often used now in BNs
• Standardized; simple to connect BNs and WANs
• Also called cell relay
• Includes Layer 3, Layer 2 and Layer 1 technologies in the specifications
  • Compatible with TCP/IP and Ethernet as if ATM was Layer 2 technology
• A connection oriented technology
• ATM switches
  • Provide point-to-point full duplex circuits at 155 Mbps (622 Mbps for switch-to-switch)

ATM vs. Ethernet

• Packet format:
  • Uses fixed-length packets (cells) of 53 bytes: 5-byte header, 48 byte data
  • Designed to make switching faster (in hardware)
• Error Checking
  • Error checking done for header only (not on data)
  • If error detected, cell is discarded
• Addressing
  • Uses a virtual channel(VC) between sender and receiver
  • All cells use VC Identifier as addresses
• QoS (prioritized transmissions)
  • Each VC assigned a specific class of service with a priority

Best Practice Backbone Design

• Architectures
  • Performance and cost à Collapsed backbone
• VLANs closer; but not mature enough
• Efficiency of data rates
  • Data Link Protocol Efficiency
• FDDI with 99%: Overhead 29 bytes; up to 4500 byte data
• ATM with about 87%: Overhead: 5 bytes over 53 byte cell
  • MAC Efficiency

Improving Backbone Performance

• Improve computer and device performance
  • Upgrade them to faster devices
  • Use faster routing protocols
    • Static routing is faster for small networks
  • Use gigabit Ethernet as BB (eliminate translations)
  • Increase memory in devices
• Improve circuit capacity
  • Upgrade to a faster circuit; Add additional circuits
  • Replace shared circuit BB with a switched BB
• Reduce network demand
  • Restrict applications that use a lot of network capacity
  • Reduce broadcast messages (placing filters at switches)

(c)Implications for Management

• Increased traffic at backbone due to faster technologies
  • May requires that BN be replaced
    • Design BN to be easily upgradeable
• FDDI and ATM becoming as legacy technologies
  • Vendors stopping the production of these
  • Begin to invest more funds to replace these
• Ethernet moving into Backbone extensively
  • One standard technology used for both LANs and BN
  • Cheaper equipment; Easier management