1. Backbone Networks, MAN, WAN
PSTN, T-carriers, ATM,
Frame Relay, VPN

2. Key Concepts
Understand various types of backbones and the devices they use
Understand VLANs and the devices they use
Understand various MAN and WAN services, including T-carriers, frame relay, ATM, and High-speed Ethernet
Be familiar with virtual private network services

3. Backbone Networks
Backbone networks are high speed networks that link an organization’s LANs and also provide connections to other backbones, MANs, WANs and the Internet.
Network designers view networks as made of three technology layers:
The access layer which is the technology used in LANs
The distribution layer which is the part of the backbone that connects the LANs together
The core layer connects different backbone networks together, often between buildings

4. Backbone network design layers

5. Collapsed Backbones
Collapsed backbones use a star topology, usually with a high speed switch at the center
Collapsed backbones can use either layer-2 switches or layer-3 routing switches
The two main advantages are:
each connection to the switch becomes a separate point-to-point circuit also giving much higher performance
the network has far fewer devices and so is much simpler to manage
Two minor disadvantages are: 1) use more cable and the cable runs for longer distances, 2) if the central switch fails, the network goes down

6. Rack-mounted collapsed backbone architecture

7. Multi-Switch Ethernet LAN
C3-2D-55-3B-A9-4F
Switch 2, Port 5
A1-44-D5-1F-AA-4C
Switch 1, Port 2
E5-BB D3-56
Switch 3, Port 6
D4-55-C4-B6-9F
Switch 3, Port 2
B2-CD-13-5B-E4-65
Switch 1, Port 7
Switch 1
Switch 2
Switch 3
Port 7 on Switch 2
to Port 4 on Switch 3
Port 5 on Switch 1
to Port 3 on Switch 2

8. Virtual LANs Single-switch VLANs Multiswitch VLANs
VLANs are a new type of LAN architecture using intelligent, high-speed switches
Unlike other LAN types, which physically connect computers to LAN segments, VLANs assign computers to LAN segments by software
VLANs have been standardized as IEEE802.1q and IEEE802.1p
The two basic designs are:
Single-switch VLANs
Multiswitch VLANs

9. Server Broadcasting without VLANS
Client A
Client B
Client C
Server D
Server E
Server
Broadcast
Frame is Broadcast
Goes to all stations
Creates congestion

10. Server Multicasting with VLANS
Multicasting (some), not Broadcasting (all)
Client A
on VLAN1
Client B
on VLAN2
Client C
Server D
Server E
Server
Broadcast

11. VLANs Computers can be assigned to VLANs in four ways:
Port-based VLANs assign computers according to the VLAN switch port to which they are attached
MAC-based VLANs assign computers according to each computer’s data link layer address
IP-based VLANs assign computers using their IP-address
Application-based VLANs assign computers depending on the application that the computer typically uses. This has the advantage of allowing precise allocation of network capacity

12. VLANs Single-Switch or Multiswitch Main advantages Drawbacks
Simpler to manage the broadcast traffic
Precisely allocate resources to different types of traffic
Drawbacks
Cost and management complexity

13. FDDI Topology FDDI operates at 100 Mbps over a fiber optic cable.
FDDI can attach a maximum of 1000 stations over a maximum path of 200 km. A repeater is need every 2 km.
FDDI uses dual counter-rotating rings (called the primary and secondary). Data normally travels on the primary ring.
Stations can be attached to the primary ring as single attachment stations (SAS) or both rings as dual attachment stations (DAS).

14. Optical cable topology for an FDDI LAN

15. FDDI’s Self Healing Rings
Copper Distributed Data Interface (CDDI) is a related protocol using cat 5 twisted wire pairs.
An important feature of FDDI is its ability to handle a breaks in the network by forming a single temporary ring out of the pieces of the primary and secondary rings.
Once the stations detect the break, traffic is rerouted through a new ring formed out of the parts of the primary and secondary rings not affected by the break.
The network then operates over this temporary ring until the break can be repaired.

16. Managing a broken circuit

17. Asynchronous Transfer Mode (ATM)
ATM was originally designed to carry both voice and data traffic over WANs. It is also used in backbone networks.
In the WAN, ATM almost always uses SONET as its hardware layer.
An ATM gateway is needed to convert TCP/IP and Ethernet frames into ATM cells and then converts them back once they have reached their destination network. The translation creates significant delays.

18. ATM Media Access Control
To handle circuit congestion, ATM prioritizes transmissions based on Quality of Service (QoS)
Real time applications, such as voice, get a high priority, since it cannot allow delays.
gets a lower priority, since small delays don’t matter very much.

19. ATM in the backbone

20. Current Backbone Technology Trends
The following trends in backbone technologies have been taking place in recent years:
Organizations are moving to collapsed backbones or VLANs
Gigabit Ethernet use is growing
FDDI seems to be on its way out.
ATM, while still popular in WANs, is losing ground to Gigabit Ethernet as a backbone technology
Taken together, it appears that Ethernet use will dominate both the LAN and backbone environments

21. Effective data rates for backbone technologies
Technology
Effective Data Rate
Full Duplex 1 GbE
1.8 Gbps
Full Duplex 10 GbE
18 Gbps
FDDI
7-70 Mbps depending on traffic
ATM (155 Mbps, Full Duplex)
160 Mbps
ATM (622 Mbps, Full Duplex)
760 Mbps
 Assumes: collapsed backbone connecting Ethernet LANs transmitting mostly large frames

22. Backbone Recommendations
The best practices are recommended for backbones:
Architecture: collapsed backbone or VLAN.
Technology: gigabit Ethernet. ATM and FDDI use has started to fall off over the past year.
The ideal network design combines use of layer-2 and layer-3 Ethernet switches.
The access layer (LANs) uses 10/100 layer-2 switches using cat 5e or cat 6 twisted pair cables (cat 6 is needed for 1000BaseT).
The distribution layer uses layer-3 Ethernet switches that use 1000BaseT or fiber, Cat 6 or Cat 7 TP.
The core layer uses layer-3 Ethernet switches running 10GbE or 40GbE over fiber.
Network reliability is increased using redundant switches and cabling.

23. MAN & WAN
Metropolitan area networks (MANs) typically span from 3 to 30 miles and connect backbone networks (BNs), and LANs.
Wide area networks (WANs) connect BNs and MANs across longer distances, often hundreds of miles or more.
Most organizations cannot afford to build their own MANs and WANs, so they rent or lease circuits from common carriers such as AT&T, BellSouth or SBC.

24. WAN Purposes Link sites (usually) within the same corporation
Provide remote access to individuals who are off-site
Internet access
1. Link Sites 2.
Remote
Access 3.
Internet

25. WANs WAN Technologies Ordinary telephone line and telephone modem
Point-to-Point Leased lines
Public switched data network (PSDN)
Send your data over the Internet securely, using Virtual Private Network (VPN) technology
PSDN
VPN
Point-to-Point

26. Telephone Modem Communication
Need modem at each end up to 33.6 kbps
For 56 kbps download speed server must have a digital connection, not a modem
PSTN
Client A
Server A
Telephone
33.6
kbps
Modem
Binary Data
Analog Modulated
Signal

27. Leased Line Networks Leased Line Point-to-point connection Always on
Usually faster (56 kbps or more)
Usually digital instead of analog
Lower cost per bit transmitted than dial-up service
But speeds are higher, so higher total cost
Must be provisioned (set up)

28. Leased Line Networks Trunk Line-Based Leased Lines
56 kbps Leased Lines
Fractional T1 lines offer low-speed choices between 56 kbps and T1, typically:
128 bps, 256 kbps, 384 kbps, 512 kbps, 768 kbps
T1 Leased Lines (1.544 Mbps)
T3 Leased Lines (44.7 Mbps)
SONET Leased Lines operate at multiples of Mbps
Use either optical fiber or data-grade copper

29. Leased Line Networks Digital Subscriber Lines (DSLs)
Broadband speeds over single pair of voice-grade copper UTP
Less expensive than trunk line-based leased lines
Asymmetric DSL (ADSL)
Downstream (to customer): 256 kbps to over 1.5 Mbps
Upstream (from customer): 64 kbps or higher

30. ASDL with Splitter Data 256 kbps to 1.5 Mbps Subscriber Premises
Telephone Company
End Office Switch
Data
WAN
64 kbps to
256 kbps
ADSL
Modem PC
DSLAM
Splitter
PSTN
Ordinary Telephone
Service
Telephone

31. Leased Line Networks HDSL (High-rate DSL) SHDSL (Super High-rate DSL)
Symmetric speed (768 kbps both ways) over one voice-grade twisted pair
Designed for business use with speed guaranteed
SHDSL (Super High-rate DSL)
Single voice-grade twisted pair; longer distances than ASDL, HSDL
Symmetric, guaranteed speed
Variable speed ranging from 384 kbps to 2 Mbps

32. Cable Modem Services ISP 2. Optical 4. Coaxial Fiber to Cable to
Neighborhood
4. Coaxial
Cable to
Premises 3.
Neighborhood
Splitter
1. Cable
Television
Head End
5. Cable
Modem
Subscriber
Premises PC
6. Requires NIC or USB port

33. Leased Line Networks Cable Modem
Delivered by cable television operator
High asymmetric speed
Up to 10 Mbps downstream
64 kbps to 256 kbps upstream
Speed is shared by people currently downloading in a neighborhood
In practice, medium ADSL speed or higher

34. Leased Line versus Public Switched Data Networks
Multisite Leased Line Mesh Network
Site B
Site A
OC3 Leased Line
T3 Lease
Line
T1 Leased
Line
56 kbps
Leased
Line
56 kbps
Leased
Line
Site C
T1 Leased
Line
56 kbps
Leased
Line
Site D
Site E

35. Leased Line versus Public Switched Data Networks
Public Switched Data Network (PSDN)
Site A
Site B
Public Switched Data
Network (PSDN)
POP
POP
POP
POP
Point of Presence
One leased
line per site
Site D
Site E
Site C

36. Leased Line versus Public Switched Data Networks
Leased Line Network
Many leased lines
Individual leased line spans long distances
Company must plan, buy switching equipment, and operate the network
Public Switched Data Network
Only need one leased line from each site to a POP
Few and short-distance leased lines
PSDN carrier provides planning, switching, and operation of the network

37. Popular PSDN Services Service Typical Speeds Circuit- or Packet-
Switched
Reliable or
Unreliable
Virtual
Circuits?
Relative
Price
X.25
9,600 kbps
to about
40 Mbps
Packet
Reliable
Yes
Moderate
Frame
Relay
56 kbps
to about
40 Mbps
Packet
Unreliable
Yes
Low
X.25 (Obsolete): Slow because of reliability
Frame Relay
Services are offered by all the major carriers

38. Popular PSDN Services Service Typical Speeds Circuit- or Packet-
Switched
Reliable or
Unreliable
Virtual
Circuits?
Relative
Price
ATM
1 Mbps
to about
156 Mbps
Packet
Unreliable
Yes
High
Ethernet
10 Gbps
and
40 Gbps
Packet
Unreliable No
Probably
Low
ATM is faster than Frame Relay
grow in demand as corporate demand outgrows FR
Ethernet MANs are appearing
offer lower prices for comparable speeds

39. Popular PSDN Services Service Typical Speeds Circuit- or Packet-
Switched
Reliable or
Unreliable
Virtual
Circuits?
Relative
Price
ISDN
Two 64 kbps
B channels
One 16 kbps
D channel
Circuit
Unreliable No
Moderate
ISDN
Expensive for its slow speed
Has niche in backup connections because Dial-Up, so only pay for when needed

40. Virtual Private Network1.
Site-to-Site
VPN Server
Tunnel
Internet
VPN Server
Corporate
Site B
Corporate
Site A
3. Remote
Corporate PC
2. Remote
Customer PC
(or site)
Remote
Access for
Intranet
Extranet

41. Virtual Private Network
Virtual Private Network (VPN)
Transmission over the Internet with added security
Why VPNs?
PSDNs are not interconnected
Only good for internal corporate communication
But Internet reaches almost all sites in all firms
Low transmission cost per bit transmitted

42. Virtual Private Network
VPN Problems
Latency and Sound Quality
Internet can be congested
Creates latency, reduces sound quality
Use a single ISP to reduce problems
Security
PPTP for remote access is popular
IPsec for site-to-site transmission is popular

43. ISP-Based PPTP Remote Access VPN
Remote Access VPNs
User dials into a remote access server (RAS)
RAS often checks with RADIUS server for user identification information. Allows or rejects connection
Local
Access
RADIUS
Server
PPTP
RAS
Secure Tunnel
Remote
Corporate PC
Internet
ISP
PPTP
Access
Concentrator
Corporate
Site A

44. Virtual Private Network
Point-to-Point Tunneling Protocol
Available in Windows since Windows 95
No need for added software on clients
Provided by many ISPs
PPTP access concentrator at ISP access point
Some security limitations
No security between user site and ISP
No message-by-message authentication of user
Uses unprotected TCP control channel

45. Hosts Need No Extra Software
IPsec in Tunnel Mode
Tunnel
Mode
IPsec
Server
IPsec
Server
Local
Network
Local
Network
Secure
Tunnel
Tunnel Only
Between Sites
Hosts Need No Extra Software
No Security
In Site Network
No Security
In Site Network

46. Virtual Private Network
IP Security (IPsec)
A network layer, so protects information at higher layers
Transparent: upper layer processes do not have to be modified
HTTP
SMTP
FTP
SNMP
TCP
UDP
Protected
Network Layer with IPsec Protection

47. Virtual Private Network
Security associations:
Agreement on how security options will be implemented
May be different in the two directions
Governed by corporate policies
Security Association (SA1) for Transmissions
From A to B
Security Association (SA2) for Transmissions
From B to A
Party B
Party A

48. MAN/WAN Recommendations
For small networks, POTS may still be reasonable alternative
For moderate volume networks, several choices are popular:
VPNs are a good choice when cost is important and reliability is less of an issue
Frame relay is used when demand is unpredictable
T-Carriers are used if network demand is predictable
For high volume networks Ethernet/IP packet networks are becoming the dominant choice.
Some organizations also may prefer ATM for their high volume networks.