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