Telecommunications Essentials
Chapter 7
Wide Area Networking
WANs
• Networks connected over long distances
• Integrate voice, data, & video
• Can be circuit or packet switched
DDS Equipment• Digital Data Service• Leased lines operate at 56 or 64 kbps (or multiples)• DDS Hub is a digital circuit switch• DSU/CSU acts as a digital modem
WAN Switching
• Circuit Switched• Leased lines• ISDN
• Packet Switched• X.25• Frame Relay• ATM
WAN Equipment
• DSU – Controls the flow between the CPE and CSU
• CSU – Performs the line conditioning
• Mux – Intelligent time division multiplexer
• Routers – Forward the packets
• Backbones – T1/T3 or SONET paths
Point to Point• Circuit switched
• i.e. A head office has links to each subsidiary
• No contention
• Limited expansion capability
Multipoint• Circuit Switched
• A backbone network is shared by all offices
• Competition for resources
WAN Example 1• Enterprise with 4 separate networks
WAN Example 2• Enterprise backbone network
• Requires intelligent multiplexers
Fractional T-1
• Multiple DS-0s can be concatenated
• Supports high speed LAN interconnect
• Supports video conferencing• 384 kbps required for full motion video• Frame rate is lowered to 10-15 fps on lower
speed links
Bandwidth Allocation• Static
• Bandwidth is assigned in 64 kbps chunks
• Dynamic• Bandwidth can be assigned in any increment
ISDN
• Circuit switched• BRI – 2B+D
• Lots of different configurations
• PRI – 23B+D (30B+D in Europe)• LAN/WAN integration
• www.cisco.com
X.25• First generation packet system• A virtual circuit system• Designed for data over analog networks• Packet size: 128 or 256 bytes• Error checking occurs at every intermediate node• www.cisco.com
X.25
• Advantages• Addressing capabilities• Can be statistically multiplexed• Basic congestion control• Error control
• Disadvantages• Queuing delays• Small packet size• No QoS guarantees• Data only
Frame Relay• Second generation packet system• Used by 60,000 enterprises worldwide• Used in burst environments• Supports SVC & PVC services
Frame Relay
• Removes the error correcting from X.25• Digital transmission media is assumed noise free
• The packet is dropped if an error is detected• The end-user application requests a retransmission
• Can carry voice and video• Can encapsulate any type of data into the frame
• Maximum packet size - 4096 bytes• Cannot predict delay/congestion
• Frame Relay Forum
Frame Relay
• Advantages• Cheaper than leased lines• Runs on multiprotocol networks• Bandwidth efficient
• Disadvantages• Variable delay• Assumed high quality digital links
ATM
• Designed to handle data, video, etc.
• Can support voice
• Provides QoS
• 80% of Internet backbones use ATM
• www.cisco.com
ATM Cell
• 5 byte header• 48 byte payload• Connection oriented
• All cells follow the same route as defined by the VPI and VCI
AAL Service Classes• AAL 1 => Service Class A (used for streams)
• AAL 2 => Service Class B
• AAL 3/4 => Service Class C or D
• AAL 5 => Service Class C (used for most other packets)
ATM
• Advantages• Supports bandwidth on demand• Provides QoS• Scales in speed and network size
• Disadvantages• High overhead• High service cost
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