Transcript What is a Computer Network?

Networking in the K-12
Community
Rick Bagwell
Nichols Research
Alabama Research and Education Network
(Formerly the Alabama Supercomputer Network)
AREN
Local Area Network Options (LAN)
• Ethernet
– Shared, Switched, Fast, Gigabit
• Token Ring
• FDDI (Fiber Distributed Data Interface)
• ATM (Asynchronous Transfer Mode)
AREN
Ethernet Types
• Ethernet
–
–
–
–
10Base-T (100m, Unshielded Twisted Pair = UTP)
10Base-2 (~200m, Coax)
10Base-5 (500m, Coax)
10Base-FL (2000m=2km, Multimode Fiber)
• Fast Ethernet
– 100Base-TX (100m over CAT5 UTP)
– 100Base-FX (2000m=2km over MM Fiber)
• Gigabit Ethernet
– 1000Base-SX (300m over MM Fiber)
– 1000Base-LX (550m over MM Fiber, 3000m over SM Fiber)
AREN
Ethernet
• 10 Mbps
• CSMA/CD
– Carrier Sense Multiple Access with Collision Detection
• Can be configured with cable, hubs, or
switches
AREN
Common Ethernet Topologies (non-segmented)
Hub
IBM Compatible
IBM Compatible
Server
Ethernet
Hub
Hub
Workstation
Computer
Computer
Computer
Computer
Stackable Hubs
AREN
Computer
Computer
Computer
Computer
Computer
Computer
Mac II
Limitations of Cascaded Hub Topology
• Distance limitations of UTP or Fiber
• Network size limited by the 5-4-3 rule
– just remember hosts cannot talk to each other if they are connected
through more than 4 hubs or repeaters
• One Broadcast Domain
– collisions likely as number of hosts increases
• Users generally don’t know about the 5-4-3 rule
and just keep on adding hubs at random points
AREN
Limitations of Stacked Hub Topology
• All hosts must be within 100m of the stack when
using UTP
• Using fiber to overcome distance limitations is
useful for MDF to IDF connections but is cost
prohibitive to the desktop
• “Stackables” use proprietary interconnects
• One broadcast domain
AREN
Limitations of 10Base-2 and 10Base-5
• Difficult to troubleshoot
– Terminations, Terminations, Terminations
• 5-4-3 rule must be strictly enforced
• Cable infrastructure does not support
upgrades to Fast or Gigabit Ethernet.
• Difficult to troubleshoot
– Terminations, Terminations, Terminations
AREN
Ethernet: The Next Generation
• Layer 2 switches (Ethernet, FastE, GigE)
–
–
–
–
Switching decisions are made based on MAC address
Segments network into separate traffic domains
Broadcasts are still heard by all hosts
Does not affect IP subnetting
Ethernet Switch
Ethernet Hub
129.66.32.4
129.66.32.1
AREN
129.66.32.3
129.66.32.2
Ethernet: The Next Generation (cont.)
• Layer 3 switches (Ethernet, FastE, GigE)
– Switching decisions are made based on MAC and IP addresses
– High-speed ASIC-based routing
– Can usually be configured as Layer2/Layer3 switches on a per-port
basis
– Layer 3 switching takes place between subnets
Ethernet Switching Router
Ethernet Hub
199.88.72.15
AREN
129.66.32.1 129.66.32.2
207.157.18.2
Ethernet: The Next Generation (cont. again)
• Layer 4 “switches” (Ethernet, FastE, GigE)
– Switching decisions are based on Application, MAC and IP
addresses
– Can usually be configured as Layer2/3/4 switches on a per-port
basis
– Layer 3 switching takes place between subnets
– Layer 4 “switching” is used to provide QoS and multimedia
services
Ethernet Switching Router
Ethernet Hub
199.88.17.2
207.157.13.9
AREN
129.66.32.1 129.66.32.2
Ethernet Network Design Pointers
• Use Ethernet switching as much as possible
– Limit the number of hosts sharing a switched port
– Switched to the desktop is nice but expensive
• Use Layer 3 switching (routing) for large (>200 hosts)
networks and high-performance.
• Use Fiber between MDF and IDFs when distance exceeds
100m (300ft to be conservative)
• Do NOT use Fiber to the desktop unless you are made of
money
AREN
How do I connect my schools to each other
and to the Internet?
• The same general answer applies for almost every choice
of WAN connection technology.
– Routers at each school are connected via some WAN technology to
the other routers in the school system
– Schools are on different IP subnets
• Internet access
– Usually obtained for the school system by connecting a single
location to the Internet with a high-speed connection.
– Other schools in the system can access the Internet by routing
through the central location.
– Can be purchased on a school by school basis
• This option can be MUCH more expensive and likely gains little, if
anything, in the way of performance.
AREN
Typical School System Network
Internet
CSU/DSU
CSU/DSU
CSU/DSU
CSU/DSU
CSU/DSU
Core Router
CSU/DSU
Router
IBM Compatible
Ethernet
IBM Compatible
AREN
IBM Compatible
Wide Area Network Options (WAN)
–
–
–
–
–
–
–
–
AREN
DS1 (a.k.a T1)
Fractional DS1 (Frac. T1)
Frame Relay
Cable Modem
ISDN
xDSL
Private/Leased Fiber (or copper)
Wireless
What is a “T1” and how does it work ??
• High speed “phone” line
– digital
– 1.544Mbps (if all 24 channels are used)
– 24 “phone” lines
• Used by phone companies between COs
• Used by businesses between offices
• Used by Networking folks for Data
AREN
Advantages and Disadvantages of T1 Lines
for School System Networking
• Advantages:
– Reasonable WAN speed (1.5Mbps)
– An educational tariff is available for T1s in Alabama.
– Full T1’s are almost always less expensive than Frame Relay (due
to the edu. Tariff)
– Telco is responsible for insuring that the line stays operational
• Disadvantages:
– Bandwidth is limited (1.5Mbps)
– Requires more router ports at central site than Frame Relay
• but large systems can use Channelized DS3
AREN
What is Frame Relay and how does it work ??
• Circuit switched network technology
• You purchase access to the “cloud”
– usually a T1 or fractional T1
• Virtual connections are made across the cloud to
connect schools (PVCs)
• A Committed Information Rate (CIR) can be
purchased that guarantees a level of performance
AREN
Frame Relay Example
System
Central
Office
Frame Cloud
T1
School A
AREN
School B
T1
Advantages and Disadvantages of Frame Relay for School
System Networking
• Advantages:
– Reasonable WAN speed (64kbps -- 1.5Mbps)
• When T1 or Fract. T1 is the transport
– Buy bandwidth only when you need it ****
– One serial port at the central site can support multiple incoming PVCs
– Telco is responsible for insuring connectivity
• Disadvantages:
– No educational tariff is currently available in Alabama.
– Full T1’s are almost always less expensive than Frame Relay (due to the
edu. Tariff)
• This is true even when low CIRs or no CIRs are compared to T1s
AREN
Cable Modems and xDSL
• High-speed (256kbps -- 10Mbps) wide area connectivity
that can be used to connect schools or for Internet access
• Usually only available in the larger cities (Huntsville,
Birmingham, Montgomery, Mobile)
• Too early to recommend or dismiss as a technology for
school system use
– Both Cable Modem systems and xDSL can and often are oversubscribed
– Some sort of service level agreement should be obtained before
either of these technologies are considered
AREN
ISDN (Integrated Services Digital Network)
• Designed to be a digital phone service
– BRI: 2B+D
– PRI: 23B+D
(Basic Rate Interface)
(Primary Rate Interface)
• Telco’s frown on “nailing up” ISDN connections
• Limited bandwidth (64-128kbps for BRI)
• Good solution for “dial backup”
– if you can afford dial backup
AREN
Private or Leased Fiber Optic Lines
• My personal favorite
• Extremely high bandwidth available (1Gbps is easily
obtained)
• Can economically use Fast Ethernet over such and
infrastructure (100Mbps)
• No recurring costs (T1 min. is $240.00 per month)
• Must either obtain right-of-way or have existing fiber
donated by telco or power company
AREN
Wireless WAN/MAN
• Good solution in Flat areas or when a mountain is nearby
– line of sight is required !!!
•
•
•
•
•
High Bandwidth solutions are available (2Mbps--155Mbps)
10Mbps is now very economical
No recurring cost (unless leased)
Leasing provides insurance against malfunction or the weather
My second choice if fiber is unavailable
AREN
Unconventional Wisdom
(at least some people would say so)
•
Never use managed Ethernet hubs
–
–
–
•
•
•
•
•
most of the functionality is never used
much more expensive than “dumb” hubs
exception to this rule: VLANs on hubs
The only use for Fiber Optics is between IDFs and the MDF when the distance
is over 300ft
Almost never buy Frame Relay for connecting schools in a single system to
each other (Educational T1s are cheaper).
Always compare all WAN options (T1, Frame, Fiber, Wireless, etc.)
Use network appliances and/or LINUX systems for Internet servers.
Having a trained Network Manager makes ALL the difference
– Don’t have to rely completely on vendors and contractors
•
Get network advice from someone other than the vendor before purchasing
AREN
Contact Information
Rick Bagwell
Network Engineer
Nichols Research Corporation
(Contractor for AREN)
Voice: (256) 971-7432
FAX: (256) 971-7491
Email: [email protected] -or- [email protected]
AREN