Transcript Chapter 13 WAN Technologies and Routing

Chapter 13 WAN
Technologies and Routing
LAN Limitations

Local Area Network (LAN) spans a single
building or campus.
 Bridged LAN is not considered a Wide Area
technology because bandwidth limitations prevent
bridged LAN from serving arbitrarily many
computers at arbitrarily may sites.
 Limited scalability
Wide Area Network (WAN)

spans sites in multiple cities, countries, continents.
 Scalable
– can grow as needed to connect many sites far away with many
computers at each site.
 high capacity achieved through use of many switches instead of
using a shared medium or single switch to move packets .
 uses packet switching technology where complete packets are
moved from one connection to another.
 Each packet switch is a dedicated computer with memory and I/O
ports to send/receive packets.
 A packet switch is the basic building block of WAN. A WAN is
formed by interconnecting a set of packet switches, and then
connecting computers. Additional switch or interconnections can
be added as needed to increase the capacity of the WAN (figure
13.2).
WAN Characteristics
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shared LAN that allows only one pair of computers to
exchange a frame at a given time
WAN permits many computers to send packets
simultaneously
switched LAN also allow many computers to
communicate simultaneously, but broadcast domain
differ)
Packet switching systems in WAN use store-and-forward
switching. Incoming packets are stored in a buffer queue.
The processor is interrupted to forward (queue) the
packet to the proper outgoing port.
This technique allows a packet switch to buffer a short
burst of packets that arrive simultaneously.
Physical Addressing in A WAN
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Many WANs use a hierarchical addressing
scheme that makes forwarding more
efficient.
 Hierarchical address (figure 13.3)is divided
into two parts
– switch#
– port#
Routing
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aka Next-Hop Forwarding
a packet switch keeps a routing table of the next place
(hop) to send a packet so the packet will eventually reach
its destination (figure 13.4)
When forwarding a packet, a packet switch only needs to
examine the first part of a hierarchical address.
routing table can be kept to a minimal size
Values in a routing table must guarantee
– universal routing where each possible destination has a next-
hop route
– optimal routes where next-hop value will take the packet closer
to its destination.
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Default route
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Source Independence:
– next-hp forwarding does not depend on packet’s
original source; instead the next hop to which a packet
is sent is a function of the packet’s destination address
only (fig 13.6) (fig 13.7).
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Creation of routing table
– static routing (simple but inflexible)
– dynamic routing (flexible) (RIP/OSPF).
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Routing table entries
–
–
–
–
Destination network
Netmask
Next hop
Cost
Routing Algorithms
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vector-distance algorithm (algorithm 13.2)
– requires messages to be sent from one packet
switch to another switch that contains pairs of
values which specify a destination and a
distance to that destination.
– RIP
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link state routing (algorithm 13.1)
– aka shortest path first (SPF)(fig 13.9)
– OSPF
Example WAN Technologies
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ARPANET
– based on packet switches connected by leased 56kbps
serial data lines
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X.25
– popular in Europe, connection-oriented
– Data link layer of X.25 (ie. LAP B) is responsible for
retransmitted bad frames
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ISDN (Integrated Services Digital Network)
 Frame Relay
 SMDS (Switched Multi-megabit Data Service)
 ATM (Asynchronous Transfer Mode)
ISDN
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dialed digital connection offered by telephone
companies .
 Basic Rate Interface (BRI)
– two 64kbps B channels, one 16kbps D (delta) channel.
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Primary Rate Interface (PRI)
– 24 64kbps channels (23 B + 1D) over a T1 line.
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TE1 (terminal equipment type 1)
– eg. ISDN telephone, ISDN computer, or ISDN FAX
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TE2 (terminal equipment type 2)
– eg. old analog phone, fax, analog modem
ISDN (cont.)
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NT1 (network Termination type 1)
– provides a connection (U-interface containing 1
twisted-pair copper on RJ-11) to phone company and a
separate connection to your house’s ISDN network
(S/T interface bus containing 4wire on 8-pin RJ-45
operating at 192kbps to accommodate 2B +D + 48bps
overhead). NT1 requires external power supply: if
power is down, you can’t dial out; advisable to provide
UPS or install separate analog phone line.
ISDN (cont.)
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TA (terminal Adapter)
– aka ISDN modem. A protocol converter that contains
interfaces for connecting TE2 equipment to NT1 via
S/T interface
– Eg. TE1 – NT1 – phone company
– Eg. TE2 – TA – NT1 – phone company
– Eg. Ascend Pipeline 25 has Ethernet connector, 2
analog RJ-11 POTS, 1 ISDN BRI S/T or U interface
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Inverse multiplexing
– allows combining B-channels to get speeds greater than
64kbps.
Frame Relay
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a link layer protocol occupying layer 2 (Data link) of
the OSI model
 Bad frames are discarded by frame relay
 retransmission is done by layer 4 (transport)
 Frame structure
– Flag ( 1 byte)
– Data Link Connection ID (2 bytes)
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no notion of source and destination addresses found in other
protocols.
Each DLCI identifies a virtual circuit from one location to a remote
location.
– Data field(up to 4096 bytes)
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may contain a Network Level Protocol ID (NLPID) header to indicate
whether data is IP or IPX or Decnet, 2 octet CRC, and a 1 octet flag.
Frame Relay (cont.)
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A physical link between to physical locations may
contain multiple permanent virtual circuits (PVC)
via multiplexing
 Committed Information Rate (CIR)
– data rate that is guaranteed on a particutlar DLCI.
– CIR is defined as a committed bust size of Bc bits over
time T .
– Excess burst size Be bits are delivered on a best effort
basis. Bits over Bc + Be during time T may be
immediately discarded.
Asynchronous Transfer Mode
(ATM)
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designed for voice, video and data services
that require low delay and low jitter
(variance in delay) and high speed.
 All ATM cells are 53-octets long
 Layer 2