1. COMPUTER NETWORKS
UNIT - 3

2. Flow Control
Ensuring the sending entity does not overwhelm the receiving entity.
Preventing buffer overflow.
Transmission time:
Time taken to emit all bits into medium.
Propagation time.
Time for a bit to traverse the link.

3. Model of Frame Transmission

4. Stop and Wait Source transmits frame.
Destination receives frame and replies with acknowledgement.
Source waits for ACK before sending next frame.
Destination can stop flow by not send ACK.
Works well for a few large frames.

5. Fragmentation Large block of data may be split into small frames :
Limited buffer size.
Errors detected sooner (when whole frame received).
On error, retransmission of smaller frames is needed.
Prevents one station occupying medium for long periods.
Stop and wait becomes inadequate.

6. Sliding Windows Flow Control
Allow multiple frames to be in transit.
Receiver has buffer W long.
Transmitter can send up to W frames without ACK.
Each frame is numbered.
ACK includes number of next frame expected.
Sequence number bounded by size of field (k).
Frames are numbered modulo 2k.

7. Sliding Window Diagram

8. Example Sliding Window

9. Sliding Window Enhancements
Receiver can acknowledge frames without permitting further transmission (Receive Not Ready).
Must send a normal acknowledge to resume.
If duplex, use piggybacking:
If no data to send, use acknowledgement frame.
If data but no acknowledgement to send, send last acknowledgement number again, or have ACK valid flag (TCP).

10. Error Detection Parity:
Additional bits added by transmitter for error detection code.
Parity:
Value of parity bit is such that character has even (even parity) or odd (odd parity) number of ones.
Even number of bit errors goes undetected.

11. Cyclic Redundancy Check
For a block of k bits transmitter generates n bit sequence.
Transmit k+n bits which is exactly divisible by some number.
Receive divides frame by that number:
If no remainder, assume no error.

12. Error Control Detection and correction of errors. Lost frames.
Damaged frames.
Automatic repeat request:
Error detection.
Positive acknowledgment.
Retransmission after timeout.
Negative acknowledgement and retransmission.

13. Automatic Repeat Request (ARQ)
Stop and wait.
Go back N.
Selective reject (selective retransmission).

14. Stop and Wait Source transmits single frame. Wait for ACK.
If received frame damaged, discard it:
Transmitter has timeout.
If no ACK within timeout, retransmit.
If ACK damaged,transmitter will not recognize it:
Transmitter will retransmit.
Receive gets two copies of frame.
Use ACK0 and ACK1.

15. Stop and Wait -Diagram

16. Stop and Wait – Pros and Cons
Simple.
Inefficient.

17. Go Back N (1) Based on sliding window.
If no error, ACK as usual with next frame expected.
Use window to control number of outstanding frames.
If error, reply with rejection:
Discard that frame and all future frames until error frame received correctly.
Transmitter must go back and retransmit that frame and all subsequent frames.

18. Go Back N - Damaged Frame
Receiver detects error in frame I.
Receiver sends rejection-I.
Transmitter gets rejection-I.
Transmitter retransmits frame i and all subsequent.

19. Go Back N - Lost Frame (1) Frame i lost. Transmitter sends i+1.
Receiver gets frame i+1 out of sequence.
Receiver send reject I.
Transmitter goes back to frame i and retransmits.

20. Go Back N - Lost Frame (2) Frame i lost and no additional frame sent.
Receiver gets nothing and returns neither acknowledgement nor rejection.
Transmitter times out and sends acknowledgement frame with P bit set to 1.
Receiver interprets this as command which it acknowledges with the number of the next frame it expects (frame i ).
Transmitter then retransmits frame I.

21. Go Back N - Damaged Acknowledgement
Receiver gets frame i and send acknowledgement (i+1) which is lost.
Acknowledgements are cumulative, so next acknowledgement (i+n) may arrive before transmitter times out on frame I.
If transmitter times out, it sends acknowledgement with P bit set as before.
This can be repeated a number of times before a reset procedure is initiated.

22. Go Back N - Damaged Rejection
As for lost frame (2)

23. Go Back N - Diagram

24. Selective Reject Also called selective retransmission.
Only rejected frames are retransmitted.
Subsequent frames are accepted by the receiver and buffered.
Minimizes retransmission.
Receiver must maintain large enough buffer.
More complex login in transmitter.

25. Selective Reject - Diagram

26. High Level Data Link Control
HDLC.
ISO 33009, ISO 4335.

27. HDLC Station Types Primary station: Secondary station:
Controls operation of link.
Frames issued are called commands.
Maintains separate logical link to each secondary station.
Secondary station:
Under control of primary station.
Frames issued called responses.
Combined station:
May issue commands and responses.

28. HDLC Link Configurations
Unbalanced:
One primary and one or more secondary stations.
Supports full duplex and half duplex.
Balanced:
Two combined stations.

29. HDLC Transfer Modes (1) Normal Response Mode (NRM).
Unbalanced configuration.
Primary initiates transfer to secondary.
Secondary may only transmit data in response to command from primary.
Used on multi-drop lines.
Host computer as primary.
Terminals as secondary.

30. HDLC Transfer Modes (2) Asynchronous Balanced Mode (ABM):
Balanced configuration.
Either station may initiate transmission without receiving permission.
Most widely used.
No polling overhead.

31. HDLC Transfer Modes (3) Asynchronous Response Mode (ARM):
Unbalanced configuration.
Secondary may initiate transmission without permission form primary.
Primary responsible for line.
rarely used.

32. Frame Structure Synchronous transmission. All transmissions in frames.
Single frame format for all data and control exchanges.

33. Frame Structure Diagram

34. Flag Fields Delimit frame at both ends. 01111110.
May close one frame and open another.
Receiver hunts for flag sequence to synchronize.
Bit stuffing used to avoid confusion with data containing :
0 inserted after every sequence of five 1s
If receiver detects five 1s it checks next bit
If 0, it is deleted
If 1 and seventh bit is 0, accept as flag
If sixth and seventh bits 1, sender is indicating abort

35. Bit Stuffing
Example with possible errors.

36. Address Field
Identifies secondary station that sent or will receive frame.
Usually 8 bits long.
May be extended to multiples of 7 bits
LSB of each octet indicates that it is the last octet (1) or not (0).
All ones ( ) is broadcast.

37. Control Field Different for different frame type:
Information - data to be transmitted to user (next layer up).
Flow and error control piggybacked on information frames.
Supervisory - ARQ when piggyback not used.
Unnumbered - supplementary link control.
First one or two bits of control filed identify frame type.
Remaining bits explained later.

38. Control Field Diagram

39. Poll/Final Bit Use depends on context. Command frame: Response frame:
P bit.
1 to solicit (poll) response from peer.
Response frame:
F bit.
1 indicates response to soliciting command.

40. Information Field Only in information and some unnumbered frames.
Must contain integral number of octets.
Variable length.

41. Frame Check Sequence Field
FCS.
Error detection.

42. HDLC Operation
Exchange of information, supervisory and unnumbered frames.
Three phases:
Initialization.
Data transfer.
Disconnect.

43. Other DLC Protocols (LAPB,LAPD)
Link Access Procedure, Balanced (LAPB):
Part of X.25 (ITU-T).
Subset of HDLC – ABM.
Point to point link between system and packet switching network node.
Link Access Procedure, D-Channel:
ISDN (ITU-D).
ABM.
Always 7-bit sequence numbers (no 3-bit).
16 bit address field contains two sub-addresses.
One for device and one for user (next layer up).

44. Other DLC Protocols (LLC)
Logical Link Control (LLC):
IEEE 802.
Different frame format.
Link control split between medium access layer (MAC) and LLC (on top of MAC).
No primary and secondary - all stations are peers.
Two addresses needed.
Sender and receiver.
Error detection at MAC layer.
32 bit CRC.
Destination and source access points (DSAP, SSAP).

45. Other DLC Protocols (Frame Relay) (1)
Streamlined capability over high speed packet witched networks.
Used in place of X.25.
Uses Link Access Procedure for Frame-Mode Bearer Services (LAPF).
Two protocols:
Control - similar to HDLC.
Core - subset of control.

46. Other DLC Protocols (Frame Relay) (2)
ABM.
7-bit sequence numbers.
16 bit CRC.
2, 3 or 4 octet address field:
Data link connection identifier (DLCI).
Identifies logical connection.
More on frame relay later.

47. Switching Networks
Long distance transmission is typically done over a network of switched nodes
Nodes not concerned with content of data
End devices are stations
Computer, terminal, phone, etc.
A collection of nodes and connections is a communications network
Data routed by being switched from node to node

48. Nodes
Nodes may connect to other nodes only, or to stations and other nodes
Node to node links usually multiplexed
Network is usually partially connected
Some redundant connections are desirable for reliability
Two different switching technologies
Circuit switching
Packet switching

49. Figure 8.2 Taxonomy of switched networks

50. Circuit Switching Dedicated communication path between two stations
Three phases
Establish
Transfer
Disconnect
Must have switching capacity and channel capacity to establish connection
Must have intelligence to work out routing

51. Circuit Switching - Applications
Inefficient
Channel capacity dedicated for duration of connection
If no data, capacity wasted
Set up (connection) takes time
Once connected, transfer is transparent
Developed for voice traffic (phone)

52. Public Circuit Switched Network

53. Circuit Establishment

54. Circuit Switching Principles revisited
Circuit switching designed for voice
Resources dedicated to a particular call
Much of the time a data connection is idle
Data rate is fixed
Both ends must operate at the same rate

55. Call blocking Can’t find a path from input to output Internal blocking
slot in output frame exists, but no path
Output blocking
no slot in output frame is available

56. Message Switching
With message switching there is no need to establish a dedicated path between two stations.
When a station sends a message, the destination address is appended to the message.
The message is then transmitted through the network, in its entirety, from node to node.
Each node receives the entire message, stores it in its entirety on disk, and then transmits the message to the next node.
This type of network is called a store-and-forward network.

57. Message Switching
A message-switching node is typically a general-purpose computer. The device needs sufficient secondary-storage capacity to store the incoming messages, which could be long. A time delay is introduced using this type of scheme due to store- and-forward time, plus the time required to find the next node in the transmission path.

58. Message Switching Advantages:
Channel efficiency can be greater compared to circuit-
switched systems, because more devices are sharing the
channel.
Traffic congestion can be reduced, because messages may be
temporarily stored in route.
Message priorities can be established due to store-and-forward
technique.
Message broadcasting can be achieved with the use of
broadcast address appended in the message.

59. Message Switching Disadvantages
Message switching is not compatible with interactive
applications.
Store-and-forward devices are expensive, because they
must have large disks to hold potentially long messages.

60. Packet Switching: Basic Operation
Data transmitted in small packets
Longer messages split into series of packets
Each packet contains a portion of user data plus some control info
Control info
Routing (addressing) info
Packets are received, stored briefly (buffered) and past on to the next node
Store and forward

61. Packet-Switched Network

62. Use of Packets

63. Advantages Line efficiency Data rate conversion
Single node to node link can be shared by many packets over time
Packets queued and transmitted as fast as possible
Data rate conversion
Each station connects to the local node at its own speed
Nodes buffer data if required to equalize rates
Packets are accepted even when network is busy
Delivery may slow down
Priorities can be used

64. Switching Technique Station breaks long message into packets
Packets sent one at a time to the network
Packets handled in two ways
Datagram
Virtual circuit

65. Datagram Each packet treated independently
Packets can take any practical route
Packets may arrive out of order
Packets may go missing
Up to receiver to re-order packets and recover from missing packets

66. Datagram Diagram

67. Virtual Circuit
In virtual circuit, a preplanned route is established before any packets are sent, then all packets follow the same route.
Each packet contains a virtual circuit identifier instead of destination address, and each node on the preestablished route knows where to forward such packets.
The node need not make a routing decision for each packet.
Example: X.25, Frame Relay, ATM 67 67

68. Virtual Circuit
A route between stations is set up prior to data transfer.
All the data packets then follow the same route.
But there is no dedicated resources reserved for the virtual circuit! Packets need to be stored-and-forwarded. 68 68

69. Virtual Circuits v Datagram
Network can provide sequencing (packets arrive at the same order) and error control (retransmission between two nodes).
Packets are forwarded more quickly
Based on the virtual circuit identifier
No routing decisions to make
Less reliable
If a node fails, all virtual circuits that pass through that node fail.
Datagram
No call setup phase
Good for bursty data, such as Web applications
More flexible
If a node fails, packets may find an alternate route
Routing can be used to avoid congested parts of the network 69 69

71. Comparison of communication switching techniques

72. Classification Packet vs. circuit switches
packets have headers and samples don’t
Connectionless vs. connection oriented
connection oriented switches need a call setup
setup is handled in control plane by switch controller
connectionless switches deal with self-contained datagrams

73. Requirements
Capacity of switch is the maximum rate at which it can move information, assuming all data paths are simultaneously active
Primary goal: maximize capacity
subject to cost and reliability constraints
Circuit switch must reject call if can’t find a path for samples from input to output
goal: minimize call blocking
Packet switch must reject a packet if it can’t find a buffer to store it awaiting access to output trunk
goal: minimize packet loss
Don’t reorder packets

74. Blocking in packet switches
Can have both internal and output blocking
Internal
no path to output
Output
trunk unavailable
Unlike a circuit switch, cannot predict if packets will block (why?)
If packet is blocked, must either buffer or drop it

75. Dealing with blocking Overprovisioning Buffers Backpressure
internal links much faster than inputs
Buffers
at input or output
Backpressure
if switch fabric doesn’t have buffers, prevent packet from entering until path is available
Parallel switch fabrics
increases effective switching capacity

76. What Is ISDN?

77. ISDN Benefits
Carries a variety of user traffic, such as digital video, data, and telephone network services, using the normal phone circuit-switched network
Offers much faster call setup than modems by using out-of-band signaling (D channel)
Often less than one second
Provides a faster data transfer rate than modems by using the 64-kbps bearer channel (B channel)
Can combine multiple B channels to bandwidth of 128 kbps
Can negotiate PPP links

78. ISDN Devices
Terminal Adapter (TA) - Converter device that converts standard electrical signals into the form used by ISDN - allows non-ISDN devices to operate on an ISDN network.
Terminal Equipment Type 1 (TE1) - Compatible with the ISDN network. Example:Telephones, personal computers, fax machine or videoconferencing machine.
Terminal Equipment Type 2 (TE2) - Not compatible with the ISDN network. Example: Analog phone or modem, requires a TA (TE2 connects to TA).
Network termination type 1 & 2 (NT1 and NT2) - A small connection box that physically connects the customer site to the telco local loop, provides a four-wire connection to the customer site and a two-wire connection to the network (PRI – CSU/DSU).

79. ISDN Components and Reference Points

80. ISDN Reference Points
U - Two wire cable that connects the customer’s equipment to the telecommunications provider
R - Point between non-ISDN equipment (TE2) and the TA
S - Four-wire cable from TE1 or TA to the NT1 or NT2
T - Point between NT1 and NT2

81. Analogies NT-1 (Network Terminator-1) TA (Terminal Adapter)
An NT-1 is an interface box that converts ISDN data into something a PC can understand (and vice versa). It works a little like a cable TV descrambler for ISDN signals, and is often built into ISDN adapters.
TA (Terminal Adapter)
This chunk of hardware converts the data it receives over ISDN to a form your computer can understand. Sometimes mistakenly called an ISDN modem or a digital modem, a terminal adapter handles data digitally and does not need to modulate or demodulate an analog signal. Terminal adapters can be an internal board or an external board that connects to the computer through the serial port.

82. ISDN Components and Reference Points #2

83. ISDN Reference Points

84. ISDN and the OSI Reference Model
The ISDN Physical Layer
The ISDN Data Link Layer
The ISDN Network Layer

85. ITU-T Standards of the First Three Layers of ISDN

86. ISDN Protocols
E-series protocols—Telephone network standards for ISDN.
I-series protocols—Specify ISDN concepts and interfaces.
Q-series protocols—Standards for ISDN switching and signaling.
Operate at the physical, data link, and network layers of the OSI reference model
Developed by the ITU-T are organized and grouped into three different series: (Know these – what is on the slide is adequate)
A. E-series protocols – Telephone network standards for ISDN. For example, E.164 specifies international ISDN addressing
B. I-series protocols – Specify ISDN concepts and interfaces. Grouped into four subcategories:
1. I.100 series – Specify ISDN concepts and structures
2. I.200 series – Deals with ISDN service aspects
3. I.300 series – Deals with network layer aspects
4. I.400 series – Deals with the user network interface (UNI)
C. Q-series protocols – Specify standards for ISDN switching and signaling
1. Q.921 describes Link Access Procedure on the D Channel (LAPD).
2. Q.931 defines Layer 3 features
Teaching Tip
A simple way to tell these Q protocols apart is to use the 2nd digit, 2 for Layer 2 and 3 for Layer 3. Cute but effective.
Recognizing the E, I and Q protocols and their general purpose (Q: switching and signaling) is critical for the CCNA exam. The E and I subcategories are not likely but recognizing what you see in C and the Teaching Tip above should be assumed
D. The ISDN protocols operate at the physical, data-link, and network layers of the OSI reference model. Examples:
1. Physical Layer ISDN Protocols – BRI is defined by the ITU-T I.430 protocol (PRI is defined by ITU-T I.431), which describes the physical connections between the ISDN CPE and the ISDN local exchange. I.430 also defines two ISDN physical layer frame formats.
a) Inbound frame formats (frames that travel from the local exchange to the ISDN customer)
b) Outbound frame formats (frames that travel from the ISDN customer to the local exchange)
2. Data Link Layer ISDN protocols – LAPD signaling protocol, defined by ITU-T Q.920 (BRI) and Q.921 (PRI) for transmitting control and signaling information over the D channel between the ISDN CPE and local exchange
a) LAPD frame format is similar to that of the ISO HDLC frame format. Like HDLC and PPP it include a flag, an address, a control, an information (data), and a frame check sequence (FCS) field.
Don’t confuse LAPD with Link Access Procedure, Balanced (LAPB), the data-link layer protocol used in X.25 networks (Chapter 17)
3. Network Layer ISDN protocols - Responsible for specifying the switching and signaling in the end-to-end ISDN communication over the D channel. These protocols include ITU-T I.930) and ITU-T Q.931
There are two useful ISDN debug commands “debug isdn q921” to troubleshoot L2 connectivity to the switch. Also “debug isdn q931” to troubleshoot call processing

87. ISDN Protocol Operating OSI Layers 1 Through 3
Physical layer ISDN protocols
BRI (ITU-T I.430) / PRI (ITU-T I.431)
Defines two ISDN physical layer frame formats
Inbound (local exchange to ISDN customer)
Outbound (ISDN customer to local exchange )
Data link layer ISDN protocols
LAPD signaling protocol (ITU-T Q.920 for BRI and Q.921 for PRI) for transmitting control and signaling information over the D channel
LAPD frame format similar to ISO HDLC frame format
Network layer ISDN protocols
ITU-T I.930 and ITU-T Q.931 defines switching and signaling methods using the D channel.
The ISDN protocols operate at the physical, data-link, and network layers of the OSI reference model. Examples:
1. Physical Layer ISDN Protocols – BRI is defined by the ITU-T I.430 protocol (PRI is defined by ITU-T I.431), which describes the physical connections between the ISDN CPE and the ISDN local exchange. I.430 also defines two ISDN physical layer frame formats.
a) Inbound frame formats (frames that travel from the local exchange to the ISDN customer)
b) Outbound frame formats (frames that travel from the ISDN customer to the local exchange)
2. Data Link Layer ISDN protocols – LAPD signaling protocol, defined by ITU-T Q.920 (BRI) and Q.921 (PRI) for transmitting control and signaling information over the D channel between the ISDN CPE and local exchange
a) LAPD frame format is similar to that of the ISO HDLC frame format. Like HDLC and PPP it include a flag, an address, a control, an information (data), and a frame check sequence (FCS) field.
Don’t confuse LAPD with Link Access Procedure, Balanced (LAPB), the data-link layer protocol used in X.25 networks (Chapter 17)
3. Network Layer ISDN protocols - Responsible for specifying the switching and signaling in the end-to-end ISDN communication over the D channel. These protocols include ITU-T I.930 and ITU-T Q.931
Teaching Tip
There are two useful ISDN debug commands “debug isdn q921” to troubleshoot L2 connectivity to the switch. Also “debug isdn q931” to troubleshoot call processing
Note: With Q.921/Q.931 the second digit indicates the OSI layer.

88. ISDN Physical Layer
ISDN physical-layer frame formats are 48 bits long, of which 36 bits represent data

89. ISDN Data Link Layer
Frame format is very similar to that of HDLC

90. ISDN Network Layer
Two Layer 3 specifications are used for ISDN signaling:
ITU-T I.450 (also known as ITU-T Q.930)
ITU-T I.451 (also known as ITU-T Q.931)
Together, these protocols support:
User-to-user circuit-switched connections
User-to-user packet-switched connections
A variety of standards for:
Call establishment
Call termination

91. ISDN Encapsulation The two most common encapsulations:
PPP
HDLC
ISDN defaults to HDLC.
PPP is much more robust.
Open standard specified by RFC 1661
Supported by most vendors

92. ISDN Uses Remote Access (Telecommuters) Remote Nodes (Voice and Data)
SOHO Connectivity (Small Branches)

93. Remote Access (Telecommuters)

94. Remote Nodes (Voice and Data)

95. SOHO Connectivity (Small Branches)

96. ISDN BRI

97. ISDN Services – BRI Basic Rate Interface (BRI)
Two 64 Kbps B channels, one 16 Kbps D channel, and 48 Kbps worth of framing and synchronization.
Available data bandwidth: 128 Kbps (2 x 64 Kbps)
User bandwidth: 144 Kbps (128 Kbps + a 16 Kbps D channel)
Total line capacity: 192 Kbps (144 Kbps + 48 Kbps framing)
Each B channel can be used for separate applications
Such as Internet and Voice
Allows individual B channels to be aggregated together into a Multilink channel
The quantity and bit capacity of channels in an ISDN line depends on the type of ISDN service implemented. ISDN currently offers the following two services:
1. BRI (Basic Rate Interface) – Consists of two 64 Kbps B channels, one 16 Kbps D channel, and 48 Kbps worth of framing and synchronization.
a) Available data bandwidth: 128 Kbps (2 times 64 Kbps)
b) User bandwidth: 144 Kbps (128 Kbps + a 16 Kbps D channel)
c) Total line capacity: 192 Kbps (144 Kbps + 48 Kbps framing)
Teaching Tip
It is common to refer to ISDN as 144 Kbps (phone company often uses this one), but student should recognize the other two as well.
Insider Information
The significance of the 64 Kbps B channels, which is the same as T1 channels, is the 64 Kbps is the telco standard for voice channels to ensure accurate reproduction of sound. The original purpose of T1 lines was to provide 24 voice lines – a T3 provides 28 times as many (672). If Voice over IP becomes common it will free up tremendous bandwidth because IP technologies compress voice data into 8 Kbps channels using technologies similar to those used on digital movies and music.
2. PRI – Primary Rate Interface. A PRI connection can assign 64 Kbps channels to both ISDN and analog modem connections
a) North America and Japan – PRI service has Kbps B channels, one 64 Kbps D channel, and8 Kbps of synchronization and framing for a total bit rate of up to Mbps (same as T1)
b) Europe, Australia, and other parts of the world – PRI service has Kbps B channels, one 64 Kbps D channel, and 64 Kbps of framing and synchronization for a total bit rate of up to Mbps (same as E1)
3. Both BRI and PRI allow each B channel to be used for separate applications. Example with BRI: the first B channel used for the Internet, while the second B channel is used for phone or FAX service
4. Both BRI and PRI allow individual B channels to be aggregated together into a single logical pipe to increase bandwidth (Multilinking).

98. ISDN Services – PRI Primary Rate Interface (PRI)
A PRI connection can assign various 64 Kbps channels to both ISDN and analog modem connections
North America and Japan – PRI service has Kbps B channels, one 64 Kbps D channel, and 8 Kbps of synchronization and framing for a total bit rate of up to Mbps (same as T1)
Europe, Australia, and other parts of the world – PRI service has Kbps B channels, one 64 Kbps D channel, and 64 Kbps of framing and synchronization for a total bit rate of up to Mbps (same as E1)
Each B channel to be used for separate applications including voice, data and Internet
Multiple B channels can be Multilinked together

99. ISDN BRI Configuration Three Basic Steps
Set the ISDN Switch Type.
Set the SPIDs (If Required).
Set the Encapsulation Protocol.
Basic configuration includes the following:
A. Setting the ISDN Switch Type – Because the various switches operate differently, the router must be configured with the correct switch type function. Switch type can be configured with the global configuration command: Router(config)#isdn switch-type switch-type
1. Switch type configured in global configuration mode applies to all ISDN interfaces. To accommodate multiple switches on different interfaces, the same command can be used in interface configuration mode, as follows: Router(config)#interface bri interface-ID Router(config-if)#isdn switch-type switch-type
2. The BRI interface-ID identifies the interface such as BRI 0 or BRI 1/4
B. Setting the SPID – The CO needs a unique identification number for each BRI B channel that it assigns to its customers. This is called a Service Profile Identifier or SPID – it is sent to the ISDN switch during call setup.
1. Depending on CO switch type the ISDN service provider uses, the customer may be provided with SPIDs to be input into the router. In North America, only the National ISDN-1 and DMS-100 ISDN switches require SPID configuration; the AT&T-5ESS switch does not
2. When assigned SPIDs are used, use the following BRI interface commands: Router(config-if)#isdn spid1 spid# [ldn] Router(config-if)#isdn spid2 spid# [ldn]
a) spid# – String of numbers to identify the ISDN subscriber’s services. One SPID number for each B channel on the BRI interface.
b) ldn (if used) – Local directory (phone-like) number , a seven-digit string assigned by the service provider that is delivered by the ISDN switch in the incoming call setup message
C. Setting the Encapsulation Protocol – You have the option to specify the data-link layer encapsulation protocol that will be used on the ISDN interface.
1. The two most common encapsulations are:
a) High-Level Data-Link Control (HDLC) – the default
b) Point-to-Point Protocol (PPP)
2. Other supported encapsulations include Frame Relay (used if ISDN traffic crosses a Frame Relay network), LAPB (used if ISDN traffic crosses an X.25 network), and LAPD.
3. Recommended encapsulation for ISDN is PPP because it provides features like Challenge Handshake Authentication Protocol (CHAP) authentication, multilink PPP, and standardized support for multiple network-layer protocols. Use the following interface command: Router(config-if)#encapsulation ppp
a) If you choose to use PAP or CHAP, the steps are the same as covered in chapter 18

100. ISDN Global and Interface Configuration Tasks

101. ISDN Physical Interface Diagram

102. ISDN Physical Interface
Connection between terminal equipment (c.f. DTE) and network terminating equipment (c.f. DCE).
ISO 8877.
Cables terminate in matching connectors with 8 contacts.
Transmit/receive carry both data and control.

103. ISDN Electrical Specification
Balanced transmission:
Carried on two lines, e.g. twisted pair.
Signals as currents down one conductor and up the other.
Differential signaling.
Value depends on direction of voltage.
Tolerates more noise and generates less.
(Unbalanced, e.g. RS-232 uses single signal line and ground).
Data encoding depends on data rate.
Basic rate 192kbps uses pseudoternary.
Primary rate uses alternative mark inversion (AMI) and B8ZS or HDB3.