Chapter 9 – Metropolitan and Wide Area Networks
Dialed Circuit Services
DD – Direct Dialing
WATS – Wide Area Telephone Services
Special rate services that allow dialed circuit calls for both voice communication and data transmission to be purchased in large quantities E.G. 100 hours of usage a month
1-800 numbers
Two main problems
Data transmission rates are slow – 56 kbps
Connections use regular telephone network which means circuits may vary in quality
Dedicated Circuit Services
a connection leased for exclusive use 24/7
voice-grade circuits
- permanent telephone circuit that you don’t need to dial into
- requires modem to connect it to network
wide band analog circuits
- similar to voice-grade circuits but with greater bandwidth
- one 48,000 hertz bandwidth for use with FDM
- requires FDM multiplexer or modem to connect it to network
T-carrier circuits
- Dedicated digtal circuits
- Uses CSU (Channel Service Unit) or DSU (Data Service Unit) to connect it to network
- T-1 circuit
SONET circuits – Synchronous Optical Network – Transport level
- Accepted by ANSI as a standard for optical transmission at gigabit-per-second speeds.
- SONET is considered a subset of SDH – Synchronized Digital Hierarchy
- Add-Drop Multiplexing – lower speed channels can be added or dropped with the use of ADMs
- Uses synchronized multiplexing – which means SONET combines many low speed channels into high speed channels
- Can be used with Synchronized Transfer Mode (STM) and Asynchronized Mode (ATM) signals by using a concept called pointers to deal with frequency and phases changes in the differently synchronized networks
DSL – Digital Subscriber Line circuits
Circuit Switched Services
works like dialed services
buy a connection into a common carrier’s network from the end points of the WAN without specifying all the interconnecting circuits you need
transfer data by requesting a circuit and the common carrier establishes a circuit
all digital
ISDN is based on time division multiplexing (just like a T-1 line)
Narrowband ISDN – Integrated Services Digital Network
combines voice, data and video
two types of service
BRI – Basic Rate Interface (2B+D)
Two 64 KPBS digital transmission channels (B channels) and one 16 KPBS control signaling channel (D channel)
PRI – Primary Rate Interface (23B+D)
23 64 KPBS digital transmission channels and one 16 KPBS control signaling channel
Broadband ISDN – Integrated Services Digital Network – B-ISDN
uses ATM to move data from one end point to the other
B-ISDN encapsulates users’ existing data link layer packets with ATM cells and moves them through an ATM network to the destination
ATM transparent
Three types of services
Full-duplex channel at 155.52 MBPS
Full-duplex channel at 622.08 MBPS
Asymmetrical with two simplex channels: 1 from the subscriber at 155.52 MBPS and 1 from the host to the subscriber at 622.08 MBPS
Packet Switched Services
multiple connections to exist simultaneously between computers
user buys a connection into the common carrier network, pays a fixed fee and is charged for the number of packets transmitted
users connection into network is a PAD (Packet Assembly/disassembly Device)
converts sender’s data into the network layer and data link layer packets used by the packet network
can translate between different data link layer protocols (Ethernet, Token Ring)
can convert codes (ASCII to EBCDIC)
packet switching interleaves bursts of data from many users to maximize use of the shared communication network
two routing methods
datagram – addition of destination address and sequence number to each packet
virtual circuit – establish an end-to-end circuit between sender and receiver
X.25
X.25 Packet Switched networks allow remote devices to communicate with each other across high speed digital links without the expense of individual leased lines
developed by ITU-TSS for use in WANs
Uses fixed-length packets
- End-to-end packet level error control
– checks each packet individually
- Two types of connections
- Switched Virtual Circuits (SVC)
- Permanent Virtual Circuits (PVC)
- Uses a 3 layer protocol
between packet mode DTE and public data network DCE
- Physical – X.21
- Data Link – HDLC LAP-B
- Network – X.25 PLP (Packet Layer Protocol) – handles virtual circuit service, virtual calls, error control, flow control, multiplexing
- PROS
- Speed matching – DTE’s do not have to have to use the same line speed
- CONS
- Store and forward mechanism can cause delays on large block transfers
Frame Relay
users connect to frame relay networks using FRADs (Frame Relay Access Devices)
Uses LAP-F protocol
Uses variable-length packets that adapt to the size of the incoming packet which means its bad for data + voice + video
Node-to-node frame level error control – frames received at nodes with errors are just discarded
One type of connection
DLCI – Data Link Connection Identifier – which needs to be established by network provider in advance and is a PVC
Operates only at the data link layer
Frame Relay does not replace the user’s network and data link layer packets with its own. It encapsulates the entire packet with its own data link layer packet.
No flow control. Frame Relay defines two connection data rates that are negotiated per connection and each virtual circuit as it is established
- CIR – Committed Information Rate
- MAR – Maximum Allowable Rate
Asynchronous Transfer Mode
similar to Frame Relay
no error control
two connection data rates: CIR and MAR
provides QoS information that enables the setting of very precise priorities among different types of transmissions
is scalable, easy to multiplex basic ATM circuits into faster ATM circuits
part of broadband ISDN
connection established before transfer begins
uses fixed length packets of 53 bytes (5 bytes of overhead and 48 bytes of user data)
The ATM process has three layers
ATM Adaptation Layer
- Management of VCC (Virtual Channel Connections) and VPC (Virtual Path Connections)
- Generation of cell header
ATM Cell Switching
- Transmission error management
- Segmentation and re-assembly
- Lost cell conditions
- Flow and timing control
Physical layer
- Data rate 155.52 Mbps and 622.08 Mbps and others
- SONET based – SONET multiplexing and framing
SMDS – Switched Multimegabit Data Service
no error checking
encapsulates incoming packets from the user’s network with ATM-like 53-byte cells, although the address is different than an ATM address.