Chapter 13 WAN Technologies and Routing

munchdrabNetworking and Communications

Oct 30, 2013 (3 years and 9 months ago)

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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


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



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


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.


Default route


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)
.


Creation of routing table


static

routing (simple but inflexible)


dynamic

routing (flexible) (RIP/OSPF).



Routing table entries


Destination network


Netmask


Next hop


Cost






Routing Algorithms


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


link state routing (algorithm 13.1)


aka shortest path first (SPF)
(fig 13.9)



OSPF


Example WAN Technologies



ARPANET


based on packet switches connected by leased 56kbps
serial data lines


X.25


popular in Europe, connection
-
oriented


Data link layer of X.25 (ie. LAP B) is responsible for
retransmitted bad frames


ISDN

(Integrated Services Digital Network)


Frame Relay


SMDS (Switched Multi
-
megabit Data Service)


ATM (Asynchronous Transfer Mode)

ISDN


dialed digital connection offered by telephone
companies .


Basic Rate Interface (BRI)


two 64kbps B channels, one 16kbps D (delta) channel.


Primary Rate Interface (PRI)



24 64kbps channels (23 B + 1D) over a T1 line.


TE1 (terminal equipment type 1)



eg. ISDN telephone, ISDN computer, or ISDN FAX


TE2 (terminal equipment type 2)


eg. old analog phone, fax, analog modem

ISDN (cont.)


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.)


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


Inverse multiplexing


allows combining B
-
channels to get speeds greater than
64kbps.


Frame Relay


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)


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)


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.)


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)


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