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:– 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

• 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 bit-level

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