Transcript Chapter 2

Chapter 2
Wide Area Network Technologies
What is a WAN ?
• The set of connecting links between local area
networks
• These links can be made using:
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Public switched telephone network-PSTN
Leased telephone lines (T1-1.5 Mbps, T3-44.7 Mbps)
Fiber-optic cable
Satellite links
Packet radio waves
Microwaves
Internet
What is a WAN ? – Continued
• Most WANS are private and owned by the
company that uses them
• Internet is the largest WAN in the world
• Companies are forming private WANs through
encrypted communications over the Internet
• WANs suffer from extremely limited bandwidths –
many times slower than LANs
• WAN links are good for short messages
Some PSTN Services and
Transmission Rates
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X.25
56Kpbs
T1
1.5Mbps
T3 44.7Mbps
ATM 44.7Mbps
SLIP
• Serial Line Internet Protocol
• Common protocol used to transmit IP packets over
serial lines and telephone connections
• Designed to handle just TCP/IP traffic – not other
protocols
• Must know your IP address and the IP address of
the remote computer
• SLIP does not provide for data compression
PPP
• Point-to-Point Protocol
• Common protocol used to transmit IP packets over
serial lines and telephone connections
• PPP can transmit TCP/IP traffic as well as IPX,
AppleTalk, etc. simultaneously
• Offers data compression
• Offers enhanced security
• PPP has slowly replaced SLIP
How IP Datagrams are
Encapsulated by a Windows 2000
that Uses WAN Technology
• Encapsulation done at Data Link Layer
prior to being sent onto the physical
medium
• Delimiters – distinguishes each frame at the
Data Link Layer as well as frame’s payload
from header and trailer
How IP Datagrams are
Encapsulated – Cont
• Protocol Identification – Used to distinguish
different protocols on the WAN link such as
TCP/IP, IPX, or AppleTalk
• Addressing – Destination must be identified
if multiple destinations are supported
• Bit-level integrity check – Checksum
process used
Categories of WAN
Encapsulations for IP Datagrams
• Point-to-point links
– Maximum of two nodes
– No Data Link Layer addressing needed
– Examples include analog phone lines, ISDN lines, T-1,
T-3
• Non-Broadcast Multiple Access (NBMA)
– Segment has more than 2 nodes
– No facility to broadcast a single IP datagram to multiple
locations
– X.25, ATM
SLIP Encapsulation
• Provides only frame delimitation services
• Does not provide protocol identification or bitlevel integrity verification
• Uses a special character called the END character
(0xC0)
• END is placed at the beginning and the end of
each IP datagram
• Character stuffing technique used to prevent the
occurrence of the END character within the IP
datagram
• See Figure 2-1 for SLIP encapsulation
Character Stuffing
• Any occurrence of the END character inside
the datagram is replaced with a sequence
beginning with another special character
called ESC (0xDB)
• END is replaced with 0xDB-DC
• ESC in original datagram is replaced by
0xDB-DD
• See Figure 2-2 for example
PPP Encapsulation
• Flag – Indicates start and end of a PPP frame (1
byte)
• Address – Used as a destination address on a
multi-point network. On point-to-point links, the
destination is the other node. The value of 0xFF is
used here – the broadcast network (1 byte)
• Control – always set to the same value (0x03) to
indicate an unnumbered frame (1 byte)
• Protocol – Used to identify the upper layer
protocol of the frame (2 bytes)
– 0x00-21 indicates IP datagram
– 0x00-2B indicates IPX datagram
PPP Encapsulation – Cont.
• IP datagram – data from Network Layer
• Frame Check Sequence – 2 byte FCS used
for error detection. PPP frame is silently
discarded if the sent FCS does not equal
calculated FCS